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Crawford, W. C. & Webb, S. C. Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°-10° N. Earth Planet. Sci. Lett. 203, 117-130
Abstract
Measurements of the seafloor deformation under ocean waves (compliance) reveal an asymmetric lower crustal partial melt zone (shear velocity less than 1.8 km/s) beneath the East Pacific Rise axis between 9° and 10°N. At 9°48′N, the zone is less than 8 km wide and is centered beneath the rise axis. The zone shifts west of the rise axis as the rise approaches the westward-stepping 9°N overlapping spreading center discontinuity and is anomalously wide at the northern tip of the discontinuity. The ratio of the compliance determined shear velocity to the compressional velocities (estimated by seismic tomography) suggests that the melt is well-connected in high-aspect ratio cracks rather than in isolated sills. The shear and compressional velocities indicate less than 18% melt in the lower crust on average. The compliance measurements also reveal a separate lower crustal partial melt zone 10 km east of the rise axis at 9°48′N and isolated melt bodies near the Moho beneath four of the 39 measurement sites (three on-axis and one off-axis). The offset of the central melt zone from the rise axis correlates strongly with the offset of the overlying axial melt lens and the inferred center of mantle melting, but its shape appears to be controlled by crustal processes.
... Similar on-axis SAML observations have since been made elsewhere along the East Pacific Rise and Juan de Fuca ridges (e.g., Arnoux et al., 2019;Carbotte et al., 2020Carbotte et al., , 2021, and appear to be a consistent feature of fast-intermediate spreading ridges. Mid-crustal and lower-crustal melt lenses have also been imaged throughout the lower crust up to 10 km away from the ridge axis on the East Pacific Rise and Juan de Fuca ridges (e.g., Canales et al., 2009Canales et al., , 2012Crawford & Webb, 2002). ...
... Additional geophysical evidence for the presence, and vertical movement, of partial melt in the lower oceanic crust comes from seismic compliance measurements that constrain S-wave velocity (V S ) models before and after eruptions (Crawford et al., 1999;Crawford & Webb, 2002;Zha et al., 2014). Compliance measurements taken in 1999, before eruption of the EPR 9°-10°N segment, are most consistent with models of low V S throughout the lower crust, indicating the presence of partial melt at mid and lower crustal depths over widths up to 4 km away from the ridge axis. ...
... For the Wadi Khafifah section studied here, Ca in olivine closure temperatures and cooling rates provide a reliable estimate of the time that the Mush Zone persists beneath the ridge axis at magmatically robust regions of fast-spreading crust for comparison with geophysical estimates from similar sections of the East Pacific Rise, for example, 9°-10°N (e.g., Crawford & Webb, 2002;Dunn et al., 2000;Zha et al., 2014). ...
Quantifying the timescales of magma solidification and eruption beneath mid‐ocean ridges is critical to understanding how melts are emplaced and crystallized, and how long these magmas and their crystals remain in a semi‐molten “mush‐like” state. Here, we present a new method to quantify the crystallization and solidification temperatures of individual gabbros in the lower crust of the Wadi Tayin massif (Samail ophiolite). Cotectic crystallization temperatures of olivine + plagioclase + clinopyroxene are constrained by our clinopyroxene Mg# thermometer, and supported by clinopyroxene‐olivine Fe‐Mg equilibration temperatures. We find that lower crustal gabbros reach the cotectic at approximately 1,160 ± 16°C, and are fully solidified between 781 and 1011°C, depending on cooling rate. Our results show that lower crustal gabbros spend, on average, 7,500–11,500 years in a partially molten state with no systematic change in duration with depth in the crust. Samples with very fast cooling rates result in solidification only a few tens of meters off‐axis from their emplacement location and likely indicate solidification near the distal edge of the on‐axis melt lens or in off‐axis melt lenses. Incorporating a uniform 1 km wide emplacement region directly on‐axis, as inferred geophysically, and a half‐spreading rate of 55 mm/yr, our results suggest that the Mush Zone at fast‐spreading ridges remains partially molten for 0–3 km away from the ridge axis, but up to 4–7 km considering all possible sources of uncertainty. Both geophysical and geochemical estimates of the Mush Zone are inconsistent with conductive cooling of the lower oceanic crust.
... Additional vertically stacked subaxial magma lenses in the lower crust Marjanović et al., 2014] and the Moho transition zone (MTZ) have been imaged within the axial low-velocity zone and are believed to also contribute to axial crustal accretion. However, in addition to on-axis magmatism, there is a growing body of evidence for off-axis magmatism related to magma ascent at mid-ocean ridges and therefore continuation of crustal accretion away from the narrow axial zone [e.g., Perfit et al., 1994;Goldstein et al., 1994;Crawford and Webb, 2002;Sims et al., 2003;Nedimović et al., 2005;Durant and Toomey, 2009;Canales et al., 2012]. For example, Durant and Toomey [2009] attributed their observations of P wave diffractions, seismic attenuation, and P-to-S wave converted phases about 20 km east of the East Pacific Rise (EPR) axis at 9°20 0 N to the presence of a melt lens about 2 km beneath the seafloor that is underlain by low-velocity, high attenuation crust. ...
... For example, Durant and Toomey [2009] attributed their observations of P wave diffractions, seismic attenuation, and P-to-S wave converted phases about 20 km east of the East Pacific Rise (EPR) axis at 9°20 0 N to the presence of a melt lens about 2 km beneath the seafloor that is underlain by low-velocity, high attenuation crust. Results from seafloor compliance measurements at the EPR show low-velocity partial melt zones in the lower crust extending into the uppermost mantle, from~4.5 to~6.5 km below seafloor (bsf), 10 km east of the ridge axis at 9°48 0 N, and MTZ melt sills (~9 km bsf) about 2.5 km east of the ridge axis at 9°08 0 N [Crawford and Webb, 2002]. On the EPR ridge flanks at 9°30 0 -9°32 0 N and 9°48 0 -9°52 0 N, petrological studies identify anomalously young basalts AGHAEI ET AL. ...
... Seismic and compliance studies at the EPR indicate the presence of a broad subcrustal melt reservoir 10-20 km wide and centered about the ridge axis with 3-11% melt fraction within and beneath the MTZ, possibly distributed in sills ( Figure 9) [Garmany, 1989;Dunn et al., 2000;Crawford and Webb, 2002;Toomey et al., 2007]. This subcrustal melt reservoir is likely a general feature of intermediate-to-fast spreading centers, with its melt content being a function of magma supply to the ridge . ...
... The crystal mush zone (also called the magma plumbing system, by some authors) is likely subjected to a wide variety of spatially diverse combined crystallization, mixing, and reaction processes (O'Hara & Fry, 1996;O'Hara & Mathews, 1981;Sinton & Detrick, 1992). Despite the complex characteristics of such processes, the control on size, shape, and evolution of the crystal mush region is a function of the crust forming process and magma supply (Crawford & Webb, 2002). The melt fraction present in the mush is likely variable but generally increases toward the AML (Zha et al., 2014). ...
... The melt fraction present in the mush is likely variable but generally increases toward the AML (Zha et al., 2014). While seismic tomography models estimate the amount of melt present at the base of the crust to only a few percent (Dunn et al., 2000), models using seafloor compliance measurements find a larger and more variable melt fraction of 5%-24% (Crawford & Webb, 2002;Crawford et al., 1999;Zha et al., 2014). This melt is likely stored in connected films or sills rather than in isolated pockets and has a residence time on a decadal time scale at fast spreading rates (Zha et al., 2014). ...
... Consequently, a large amount of melt will crystallize if the temperature is reduced by only some 10°C below the plagioclase liquidus (Kelemen & Aharonov, 1998). Although enhanced water content of the parental melt will lower the plagioclase saturation temperature, the presence of a widely molten magma chamber within the lower crust is not observed by crustal tomography (e.g., Dunn et al., 2000) or compliance modeling (e.g., Crawford & Webb, 2002;Zha et al., 2014) Therefore, melt intruding the lower crust either has to move rapidly toward the AML before becoming saturated in plagioclase, or it will start to crystallize and generate latent heat that has to be removed efficiently to avoid forming a molten lower crustal magma reservoir. Field, petrographic and geochemical evidence for the existence of a high-temperature hydrothermal system active in the lower crust is numerous (e.g., Bosch et al., 2004;Currin et al., 2018;Dygert et al., 2017;Koepke et al., 2005aKoepke et al., , 2005bKoepke, Mueller, et al., 2014;Nicolas & Mainprice, 2005;Wolff et al., 2013;Zihlmann et al., 2018). ...
Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast‐spreading oceanic crust. Incompatible trace elements of co‐existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid‐ocean‐ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady‐state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open‐system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE‐in‐plagioclase‐clinopyroxene thermometry compared to and supported by anorthite‐in‐plagioclase thermometry reveals a relationship of TREEAn ${T}_{\text{REE}}^{\text{An}}$ [°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast‐spread crust.
... Additional vertically stacked subaxial magma lenses in the lower crust Marjanović et al., 2014] and the Moho transition zone (MTZ) have been imaged within the axial low-velocity zone and are believed to also contribute to axial crustal accretion. However, in addition to on-axis magmatism, there is a growing body of evidence for off-axis magmatism related to magma ascent at mid-ocean ridges and therefore continuation of crustal accretion away from the narrow axial zone [e.g., Perfit et al., 1994;Goldstein et al., 1994;Crawford and Webb, 2002;Sims et al., 2003;Nedimović et al., 2005;Durant and Toomey, 2009;Canales et al., 2012]. For example, Durant and Toomey [2009] attributed their observations of P wave diffractions, seismic attenuation, and P-to-S wave converted phases about 20 km east of the East Pacific Rise (EPR) axis at 9°20 0 N to the presence of a melt lens about 2 km beneath the seafloor that is underlain by low-velocity, high attenuation crust. ...
... For example, Durant and Toomey [2009] attributed their observations of P wave diffractions, seismic attenuation, and P-to-S wave converted phases about 20 km east of the East Pacific Rise (EPR) axis at 9°20 0 N to the presence of a melt lens about 2 km beneath the seafloor that is underlain by low-velocity, high attenuation crust. Results from seafloor compliance measurements at the EPR show low-velocity partial melt zones in the lower crust extending into the uppermost mantle, from~4.5 to~6.5 km below seafloor (bsf), 10 km east of the ridge axis at 9°48 0 N, and MTZ melt sills (~9 km bsf) about 2.5 km east of the ridge axis at 9°08 0 N [Crawford and Webb, 2002]. On the EPR ridge flanks at 9°30 0 -9°32 0 N and 9°48 0 -9°52 0 N, petrological studies identify anomalously young basalts AGHAEI ET AL. ...
... Seismic and compliance studies at the EPR indicate the presence of a broad subcrustal melt reservoir 10-20 km wide and centered about the ridge axis with 3-11% melt fraction within and beneath the MTZ, possibly distributed in sills ( Figure 9) [Garmany, 1989;Dunn et al., 2000;Crawford and Webb, 2002;Toomey et al., 2007]. This subcrustal melt reservoir is likely a general feature of intermediate-to-fast spreading centers, with its melt content being a function of magma supply to the ridge . ...
We use 3-D multichannel seismic data to form partial angle P wave stacks and applyamplitude variation with angle (AVA) crossplotting to assess melt content and melt distribution withintwo large midcrustal off-axis magma lenses (OAMLs) found along the East Pacific Rise from 9°37.50Nto9°570N. The signal envelope of the partial angle stacks suggests that both OAMLs are partially moltenwith higher average melt content and more uniform melt distribution in the southern OAML than in thenorthern OAML. For AVA crossplotting, the OAMLs are subdivided into seven ~1 km2analysis windows.The AVA crossplotting results indicate that the OAMLs contain a smaller amount of melt than theaxial magma lens (AML). For both OAMLs, a higher melt fraction is detected within analysis windowslocated close to the ridge axis than within the most distant windows. The highest average meltconcentration is interpreted for the central sections of the OAMLs. The overall low OAML melt contentcould be indicative of melt lost due to recent off-axis eruptions, drainage to the AML, or limited mantlemelt supply. Based on the results of this and earlier bathymetric, morphological, geochemical, andgeophysical investigations, we propose that the melt-poor OAML state is largely the result of limited meltsupply from the underlying mantle source reservoir with smaller contribution attributed to melt leakageto the AML. We hypothesize that the investigated OAMLs have a longer period of melt replenishment,lower eruption recurrence rates, and lower eruption volumes than the AML, though some could be singleintrusion events.
... In contrast to the variable width and depth of the melt sill beneath the east limb, the sill imaged beneath the west limb is centered below the ridge axis and varies little in width or depth within the area studied [Kent et al., 2000]. Although no melt sill was mapped beneath the OSC basin, midcrustal [Bazin et al., 2003] and lower crustal [Crawford and Webb, 2002] melt have been identified beneath the northern part of the basin, and an upper mantle low velocity zone, suggesting a few percent melt, cuts diagonally across the basin from the west to the east limb [Dunn et al., 2001;Toomey et al., 2007]. ...
... Data from the ARAD seismic tomography experiment: melt sill (shaded gray) [Kent et al., 2000], low-velocity anomaly maxima at 2.3 km bsf interpreted as melt (red dashed lines) [Bazin et al., 2003]. Lower crustal melt maxium based on compliance measurements (green dashed lines) [Crawford and Webb, 2002], axial magma chamber seismic reflection profile (thick dashed black line) , upper mantle low velocity zones at 9 km bsf (dashed blue lines; darkest blue indicates lowest P wave velocity) [Toomey et al., 2007]. The northern extent of the ARAD study is indicated by thin black dotted line; extensions of the melt sill, lower crust seismic anomaly and compliance data to the north of this line represent our interpretation. ...
... [61] The finding of abundant crustal melt at the 9N OSC [e.g., Kent et al., 2000;Crawford and Webb, 2002;Bazin et al., 2003;Singh et al., 2006] and normal, albeit relatively evolved, magma geochemistry [Wanless et al., 2012] argues against the idea that this discontinuity experiences reduced magma supply and migrates in response to alongaxis pulses of magma from a robust segment center. Nevertheless, the presence of abundant crustal melt, particularly associated with the propagating east limb, encourages the view that more local magma supply leads to propagation of the east limb and the overall southward migration of the OSC. ...
... A recent study of cooling rates from the upper section of the lower crust at the East Pacific Rise (EPR) reports near conductive cooling for the first few hundreds of meters of lower oceanic crust [Faak et al., 2015]. Although some of these observations seem to favor conductive cooling of the lower crust, field evidence from Oman [Bosch et al., 2004;Nicolas and Mainprice, 2005] and seismic methods [Crawford and Webb, 2002;Dunn et al., 2000] support deep hydrothermal circulation [Gillis et al., 2012]. In addition, a hybrid shallow on-axis and deep off-axis hydrothermal flow scheme that is found in 3-D simulations can reconcile some of these observations [Hasenclever et al., 2014], which further support the concept of pervasive crustal-scale fluid flow. ...
... A study from the Eastern Lau Spreading Centre reveals a similar thermal structure for the upper 5 km of the crust [Dunn et al., 2013]. Based on seafloor compliance measurements, Crawford and Webb [2002] proposed that the lower crustal melt zone centered at the ridge axis is less than 8 km wide. The findings of both methods (tomography and compliance measurements) are typically interpreted in that hydrothermal circulation throughout the lower oceanic crust is responsible for the much narrower zone of partial melts In addition to the competing styles of hydrothermal circulation, also the process of oceanic crust formation remains hotly debated [Gillis et al., 2014;VanTongeren et al., 2015]. ...
... In addition, we compare the different modes of crustal accretion in two different hydrothermal cooling setups. In the first set of models, the permeability structure has been adjusted so that the calculated steady state thermal structure closely matches the seismically inferred one for the EPR at 9°N [Crawford and Webb, 2002;Dunn et al., 2000], which requires elevated permeability values along the side of the lower crustal magmatic system. In the second set, hydrothermal circulation only occurs in the upper crust, so that the lower crust is cooled by thermal diffusion only. ...
Hydrothermal convection at mid-ocean ridges links the ocean's long term chemical evolution to solid earth processes, forms hydrothermal ore deposits, and sustains the unique chemosynthetic vent fauna. Yet, the depth-extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes, remain poorly constrained. Here, based on coupled models of crustal accretion and hydrothermal circulation, we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast spreading ridges. We integrate numerical models with observations of melt lens depth, thermal structure, and melt fraction. Models matching all these observations always require a deep crustal-scale hydrothermal flow component and less than 50% of the lower crust crystallizing in-situ.
... In this particularly well instrumented 9°-10°N domain, melt accumulations at Moho depth have been detected by seismic refraction (Garmany, 1989) and reflection (Nedimovic et al., 2005;Singh et al., 2006;Carbotte et al., 2013;Fig. 2b) and by compliance measurements (Crawford et al., 1999;Crawford and Webb, 2002). Seismic refraction suggests that partially molten sills may extend as far as 22 km from the ridge axis in the transverse direction, probably being divided into distinct sills (Garmany, 1989). ...
... (c) Compliance measurements at 9°48ʹ for the EPR. The low-velocity zones are interpreted as melt-rich mantle upwelling, and correspond approximately to the Crustal Melt Zones (CMZ) recognized near the Moho by seafloor seismic and compliance measurements (Crawford and Webb, 2002). White squares inside the large insert in Fig. 2a locate compliance measurements where CMZ have been detected. ...
... The MTZ described in Oman corresponds to the level of melt sills (CMZ or MLVZ) imaged at the EPR at 9°N by seismic (Garmany, 1989;Toomey et al., 2007) and compliance studies (Crawford et al., 1999;Crawford and Webb, 2002). The CMZ lens contains a few tens per cent melt beneath the Moho and is a melt reservoir large enough to feed the upper melt lens. ...
A comprehensive model for the activity of the elementary accretion segment at fast-spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis.This article is protected by copyright. All rights reserved.
... However, as this melt rises from below, melt distribution near the Moho may be the real controlling factor of the tectonic and magmatic segmentation (Carbotte et al., 2013). Seismic imaging (Barth et al., 1991), compliance studies (Crawford and Webb, 2002), and tomographic imaging (Toomey et al., 2007) point to melt accumulation in the upper mantle, which appears to be segmented, and not always centered beneath the ridge. Despite recent progress in multichannel seismic techniques (Aghaei et al., 2014), it remains difficult to image the Moho structure and melt distribution at this depth, in as much detail as the upper-crust axial melt lens. ...
... s beneath the Moho; the consistent dip towards the ridge of the most outward reflectors may correspond to the zone where the MTZ thins over a 1-3 km horizontal distance. Melt-rich zones found at greater distances, at or above the Moho (Crawford and Webb, 2002) are more probably related to the "wehrlite" (MTZ-like) intrusions in the crust, and may explain the surprising crustal thickening observed on the ridge flanks, 5 to 10 km from the ridge axis ( Fig. 7d in Aghaei et al., 2014). The abrupt thinning of the MTZ suggests that no detectable trace of the MTZ should remain tens of kilometers away from the ridge. ...
Our knowledge of melt distribution in the lower crust and upper mantle at oceanic fast spreading centers is very limited. Evidence of melt accumulation, sometimes away from the axis, has been imaged and interpreted at the Moho of the East Pacific Rise; but the detailed structures of these deep magma lenses remains much more difficult to unveil than that of the shallow axial melt lens at the top of the plutonic crust. Ophiolites offer on-land sections of oceanic lithosphere that can complement marine geological and geophysical observations. We present results of a geological survey of the Moho transition zone at a paleospreading center in the Oman ophiolite. We find that the thickness of this dunite-rich horizontal layer increases from a few meters at the axis to hundreds of meters 6 km away from the axis, and is reduced to a few meters 2–3 km further away. The base of the Moho transition zone contains dunite and very depleted harzburgite with isotropic plagioclase and clinopyroxene impregnations, and stockwork-like magmatic breccia, indicative of episodic high melt fractions. We conclude that the melt-free dunite horizontal layer may stop the progression of ascending melt; this lead to melt accumulation within the uppermost harzburgite beneath the Moho transition zone and forms the isotropic impregnations. As the melt dissolves the harzburgite orhtopyroxene, and is flushed to the top of the MTZ though the breccia, it leaves new dunite at the base of the Moho transition zone. Repetition of this process renders the Moho transition zone thicker as it moves away from the ridge axis, until it leaves the main area of mantle melt delivery. Then, tectonic thinning and intrusion of parts of the MTZ into the lower crust reduce the MTZ thickness. These processes seem coherent with several marine geophysical observations.
... Subsequent freezing and down-and outward movement of the crystallized residue constructs the lower crust [Henstock et al., 1993;Phipps-Morgan and Chen, 1993;Quick and Delinger, 1993]. The more recent "multiple sill" model proposes magma injection in multiple lenses throughout the crustal section including near the Moho [Crawford and Webb, 2002;Kelemen and Aharonov, 1998;MacLeod and Yaouancq, 2000]. At present neither model appears to completely satisfy all the available geological and geophysical evidence. ...
... A similar process may operate from a magma chamber at the base of the crust. (Right) In the 'many sills' model the crust is formed by emplacement of thin melt lenses into the crust at different levels between the Moho and a high-level magma chamber [Crawford and Webb, 2002;Kelemen and Aharonov, 1998]. These magma chambers may be emplaced independently [Kelemen et al., 2000], or as lateral extensions from a magmatic system feeding a high-level magma chamber from the upper mantle [MacLeod and Yaouancq, 2000]. ...
... Subsequent freezing and down-and outward movement of the crystallized residue constructs the lower crust [Henstock et al., 1993;Phipps-Morgan and Chen, 1993;Quick and Delinger, 1993]. The more recent "multiple sill" model proposes magma injection in multiple lenses throughout the crustal section including near the Moho [Crawford and Webb, 2002;Kelemen and Aharonov, 1998;MacLeod and Yaouancq, 2000]. At present neither model appears to completely satisfy all the available geological and geophysical evidence. ...
... A similar process may operate from a magma chamber at the base of the crust. (Right) In the 'many sills' model the crust is formed by emplacement of thin melt lenses into the crust at different levels between the Moho and a high-level magma chamber [Crawford and Webb, 2002;Kelemen and Aharonov, 1998]. These magma chambers may be emplaced independently [Kelemen et al., 2000], or as lateral extensions from a magmatic system feeding a high-level magma chamber from the upper mantle [MacLeod and Yaouancq, 2000]. ...
... Detrick et al., 1990;Singh et al., 2006a;Desissa et al., 2013;Combier et al., 2015) as the processes at fast-spreading ridges (e.g. Crawford & Webb, 2002;Nedimovic et al., 2005;Singh et al., 2006b;Boudier & Nicolas, 2011;Arnulf et al., 2014;Xu et al., 2014). Specifically, crustal melt lenses commonly observed at fast-spreading centers based on seismic reflection studies are rare at slow-spreading centers. ...
... The magmatic processes associated with oceanic crustal accretion at slow-spreading ridges are more poorly understood than those at fast-spreading ridges (Detrick et al., 1990;Coogan et al., 2000Coogan et al., , 2002Crawford & Webb, 2002;Nedimovic et al., 2005;Singh et al., 2006aSingh et al., , 2006bBoudier & Nicolas, 2011;Desissa et al., 2013;Arnulf et al., 2014;Coogan, 2014;Xu et al., 2014;Combier et al., 2015). Seismic wave velocities have revealed that the oceanic crust has a layered structure that includes layer-1 (sediments), layer-2 (basalts and dolerite dikes) and layer-3 (gabbros). ...
The magmatic processes associated with oceanic crustal accretion at slow-spreading ridges are less well understood compared to those at fast-spreading ridges. The zoned plagioclase in the basalt might record the magmatic processes due to its very slow intra-crystal diffusion of CaAl-NaSi. Plagioclase phenocrysts in the plagioclase phyric basalt from Hole U1433B of IODP Expedition 349 in the South China Sea show complex zoning patterns (e.g., normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magmatic dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithologic units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core-rim zonation with high-An cores (An = ~85 %; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti but enriched in Sr and Eu, which are interpreted as resulting from the dissolution-crystallization processes during the hot melt reacting with preexisting plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show core-mantle-rim normal zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magmatic mixing and differentiation. Reverse zoning plagioclases (Pl-C) are sparsely present throughout the basalts but mostly occur in the lower part of the hole. The cores of euhedral Pl-C phenocrysts are compositional comparable to the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma.
Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava but rarely in the pillow lava, while Pl-B phenocrysts present dominantly in the pillow lava, which reflects the reducing melt-rock interaction and enhancing magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magmatic mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt-rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes of magmatic dynamics of the mid-ocean ridge basalt (MORB) through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, due to a low melt supply, the primitive melt percolates through and interacts with the mush zone as porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution-crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains the crystal cargos with melt inclusion-bearing high-An plagioclases. At the major stage of extension, due to a relatively high melt supply, the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zonings of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension accompanied with oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of oceanic crust.
... The gabbro lenses in the MTZ are indistinguishable from layered gabbros in the lower crust; they have the same type of modal layering and chemical composition (Kelemen et al., 1997a), which suggests that they formed in the same way. The presence of melt at the MTZ is now recognized by a growing number of marine geophysical studies at fast and intermediate spreading ridges (Crawford and Webb, 2002;Garmany, 1989;Nedimovic et al., 2005), underscoring the importance of understanding the structure of the MTZ. This paper presents new field observations and a microstructural study of gabbroic rocks in the MTZ associated with the well-known Maqsad mantle diapir . ...
... Seamount volcanism points to outliers that must be taken into account in a comprehensive melt circulation model. The discovery of melt sills in the lower crust at the crust-mantle interface, located as far as 20 kilometers (Garmany, 1987;Crawford et al., 1999Crawford et al., , 2002 from the rise, also shows that not all melt emplacement is focused beneath the ridge. In addition, a recent tomographic study at the East Pacific Rise (EPR) demonstrates that melt upwelling in the mantle does not systematically occur directly beneath the axis but also 5 to 20 kilometers away from it . ...
L'ophiolite d'Oman permet d'observer les roches mantelliques inaccessibles aux dorsales. Avec cinq diapirs à l'axe (dont Maqsad) et un diapir hors-axe (Mansah), elle permet d'étudier les processus magmatiques en jeu dans les diapirs au niveau de la Zone de Transition au Moho, qui est différente entre les deux types de diapirs avec hors-axe des pyroxénites dans la dunite à la place de gabbros lités à l'axe. Le diapir de Mansah est entouré d'intrusions gabbroïques dans le manteau et la croûte. Les roches hors-axe ont des valeurs d'eNd plus faibles que celles à l'axe qui ont des compositions similaires aux MORB, suggérant une source des liquides plus riche en veines de pyroxénite hors-axe. Les eNd montrent aussi que les pyroxénites et les intrusions gabbroïques sont une suite magmatique. L'abondance de clinopyroxenes hors-axe provient de la réaction entre des liquides provenant de la fusion de veines de pyroxénites avec la harzburgite appauvrie de la lithosphère, ce qui donne à ces clinopyroxènes des compositions appauvries en éléments traces. La présence d'eau provenant de la lithosphère hydratée favorise la cristallisation de ces clinopyroxènes à la place du plagioclase qui doit normalement apparaître à cette profondeur dans la MTZ, ce qui est le cas à l'axe. Le diapir hors-axe pourrait fournir une analogie aux seamounts que l'on trouve actuellement à proximité des dorsales rapides et faire la lumière sur les interactions entre la lithosphère appauvrie et le matériel ascendant hors-axe, ainsi que sur les structures internes de ces seamounts. Ce travail offre des évidences probantes pour l'existence de veines de pyroxénites dans le manteau asthénosphérique sous les dorsales
... Detrick et al., 1987; the so-called axial magma lens or AML). This AML overlies a much larger region of lower crust that contains a mixture of crystals and melt (Dunn et al., 2000;Crawford & Webb, 2002). Because MORB erupted on-axis must pass through the mush zone and AML in transit from the mantle to the surface this further confirms the role of magma chambers in controlling MORB compositions. ...
... Fast-spreading mid-ocean ridges have near steadystate melt bodies close to the base of the sheeted dike complex, underlain (and probably surrounded) by a partially molten mush zone (Detrick et al., 1987;Kent et al., 1993;Dunn et al., 2000;Crawford & Webb, 2002;Carbotte et al., 2013). The small size of the AML (roughly tens of meters high and 500-1000 m wide) and the high heat output from the roof of this body into the overlying hydrothermal system demonstrate that replenishment must occur on a decadal timescale to prevent it freezing (e.g. ...
At fast-spreading mid-ocean ridges the existence of a near steady-state axial magma lens indicates that melt differentiation is an open-system process. Field relations in ophiolites and tectonic windows at fast-spreading ridges, together with some chemical characteristics of mid-ocean ridge basalts (MORB) and oceanic plutonic rocks, indicate that assimilation is a common process in and around the axial magma lens. Magma and mush zone mixing and mingling is indicated by the petrology of MORB and oceanic plutonic rocks; mush disaggregation provides an efficient mechanism for return of interstitial melt to an eruptible reservoir—a form of in situ crystallization. Despite such copious evidence to the contrary, MORB differentiation is generally modeled assuming perfect fractional crystallization (Rayleigh distillation). Here we present a simple open-system model for MORB differentiation that includes assimilation and in situ crystallization that can be used to generate synthetic basalt datasets to compare with natural sample suites. Inversion of the model allows the parental melt compositions to be estimated quantitatively. We use a numerical Bayesian inversion scheme to determine the parental melt compositions for three large (>150 samples in each) normal-MORB suites from the East Pacific Rise. The parental melt compositions determined this way differ significantly from those that would be calculated assuming closed-system fractional crystallization. Parental MORB are more depleted than commonly assumed, suggesting that the upper mantle is more depleted than generally believed and/or that the extent of melting is larger (for example, with melts poorly focused to the ridge axis). The more depleted character of parental than erupted melts has important implications for using basalt trace element systematics in chemical geodynamic models. For example, the Sm/Nd of parental MORB are significantly lower than those of erupted MORB and this needs considering in models of the Nd-isotope evolution of the mantle.
... Chilled margins against the underlying dike screens precludes segregating a crystal residue that subsides to form the lower crust as in the gabbro glacier model, and its overall fractionated composition requires that crystals have been segregated elsewhere. This implies that sills or other bodies containing cumulate materials must exist deeper in the crust and/or below the crust/mantle boundary, consistent with recent models based on lower crustal sections of ophiolites (e.g., Boudier et al., 1996;Kelemen et al., 1997;MacLeod and Yaouancq, 2000) and some marine geophysical experiments (Crawford and Webb, 2002;Dunn et al., 2001;Garmany, 1989;Nedimovic et al., 2005;Canales et al., 2009). However, the gabbro glacier mode of accretion or a combination of the gabbro glacier and sheeted sills models cannot yet be rejected, as fractionated gabbros in the dikegabbro transition zone are not unexpected, and the predicted region of cumulate rocks could still exist just below the present maximum depth of Hole 1256D. ...
... Simple gabbro glacier models suggest much slower rates of cooling for the lower crust (~0.02°C/y) than those required to match recent seismic tomographic models and compliance results from the EPR (Crawford and Webb, 2002;Crawford et al., 1999;Dunn et al., 2001). Multiple sills models require deep nearaxis hydrothermal cooling and rapid cooling rates (~0.1°C/y); ...
... For example, this was the starting point for a recent model of hydrothermal circulation developed by Hasenclever et al. (2014). However, compliance data suggest that the crust is partially molten to >5 km off-axis (Crawford and Webb, 2002) in the region studied by Dunn et al. (2000), inconsistent with the thermal structure proposed by Dunn et al. (2000). Likewise, Han et al. (2014) show that at 9 • 37 -40 • N on the EPR there are off-axis melt bodies that exist up to 10 km off-axis. ...
... The differential equation (Eq. (A.2)) can be solved numerically for one-dimensional diffusion in a plate by applying the method of central finite differences (Crank, 1975;Costa et al., 2008): ...
... Although there are currently no mid-ocean ridges on Earth with spreading rates as high as the Pacific-Cocos ridge during the Miocene, seismic data from the fast-spreading East Pacific Rise provide constraints on the width of the magmatic system beneath the ridge axis. Seismic tomography and seafloor compliance data have shown that a low-velocity zone, comprising a region of partial melt, occupies most of the lower crust (Crawford et al., 1999;Crawford & Webb, 2002;Dunn et al., 2000). It is generally ∼8-10 km wide, and is overlain by a narrower lens with higher melt proportions (Sinton & Detrick, 1992). ...
Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere.
... Marjanovic et al., 2014). In addition, melt in the lower crust and/or the crust-mantle transition zone was also inferred by seafloor compliance studies (Crawford and Webb, 2002). Thus, there is an increasing set of arguments for a multi-level complex of melt lenses beneath the ridge axis (Marjanovic et al., 2014). ...
We analyzed high-quality seismic reflection profiles across the ocean-continent transition in the Enderby Basin between the Kerguelen Plateau and the Antarctic margin. There, we observe numerous high-amplitude dipping reflections in the lower oceanic crust which was accreted at a magmatic spreading center as testified by the almost uniform 6.4-7 km thick crust and its unfaulted, flat top basement. The deep reflections are rooting onto the Moho and are dipping both ridgeward and continentward. They occur in dense networks in mature oceanic crust as well as close to the continentward termination of oceanic crust and in the ocean-continent transition zone. The comparison with field observations in the Oman ophiolite suggests that these lower crustal dipping reflectors could correspond to syn-magmatic faults. In Oman, very high temperature (up to syn-magmatic), high temperature (sub-solidus plastic deformation) and low temperature (brittle) deformation coexist along the same fault over distances of a few hundred meters at Moho level. This very high temperature gradient may be explained by the sudden and intense interaction between crystallizing magmas and hydrothermal fluids induced by the episodic nucleation of faults in a context of continuous magmatic spreading. The igneous layering becomes extremely irregular compared to its monotonous sub-horizontal orientation away from the faults which, together with enhanced hydrothermal alteration restricted to the fault zones, might change the physical properties (velocity, density) and increase the reflectivity of syn-magmatic faults. We further speculate that these processes could explain the brightness of the lower crustal dipping reflectors observed in our seismic reflection data. Both the seismic reflection profiles of the Enderby Basin and the Oman ophiolite show evidence for syn-accretion tectonism at depth together with the systematic rotation of originally horizontal lava flows or originally vertical dikes, pre-dating cessation of magmatic activity. This indicates ubiquitous deformation processes within the axial zone of magmatic spreading centers.
... Seafloor compliance is a measure of the deformation of the oceanic crust in response to loading from infragravity waves at periods longer than ≥30 s (e.g. Crawford et al. 1991;Crawford & Webb 2002;Doran & Laske 2016). The applicable period range is limited by the water depth at short periods. ...
We present models of crustal and uppermost mantle structure beneath the Hawaiian Swell and surrounding region. The models were derived from ambient-noise intermediate-period Rayleigh-wave phase velocities and from seafloor compliance that were estimated from continuous seismic and pressure recordings collected during the Hawaiian Plume-Lithosphere Undersea Mantle Experiment (PLUME). We jointly inverted these data at the locations of over 50 ocean-bottom instruments, after accounting for variations in local bathymetry and sediment properties. Our results suggest that the crystalline crust is up to 15 km thick beneath the swell and up to 23 km thick closer to the islands. Anomalously thick crust extends toward the older seamounts, downstream of Hawaii. In a second region, anomalies immediately to the south of Hawaii may be associated with the leading edge of the shallow Hawaiian magma conduit. In a third region, thickened crust to the immediate west of Hawaii may be related to Cretaceous seamounts. Low seismic velocities identified in the uppermost mantle to the northeast of Hawaii may be linked to the Molokai Fracture Zone and may be manifest of complex non-vertical pathways of melt through the upper lithosphere. Velocity anomalies decrease in amplitude towards the surface, suggesting that melt becomes focused into conduits at depths between 20 and 40 km that escape the resolution capabilities of our dataset.
... The internal structure of this region, such as the distribution of melt and its geometry, is not well constrained because of, for example, the uncertainties of converting compressional wave velocity with varying melt distribution. Locally, melt has been shown to pool at or below the Mohorovicic discontinuity (Moho), both on-and off-axis, and also within the lower crust off-axis (Garmany, 1989;Crawford and Webb, 2002;Durant and Toomey, 2009;Canales et al., 2009). ...
... However, studies on both MORB and plutonic rocks from the lower oceanic crust have provided evidence suggesting that the crustal evolution of MORB is complex, involving more than fractional crystallization alone (e.g., Lissenberg & MacLeod, 2016 and reference therein). Seismic studies indicate that mid-ocean ridges are mostly underlain by crystal mush (Canales et al., 2000;Carbotte et al., 2013;Crawford & Webb, 2002;Crawford et al., 1999;Dunn et al., 2000;Seher et al., 2010;Vera et al., 1990). Hence, melts and crystals coexist throughout the mid-ocean ridge magma plumbing system, providing two possible mechanisms for reactions to occur between melts and crystals. ...
Reaction between mid‐ocean ridge basalt (MORB) and crystal mush in the lower oceanic crust has been invoked to explain chemical variations of both MORB and minerals in the lower oceanic crust. Nonetheless, such reactions have been little studied experimentally. We conducted experiments to investigate the mechanisms and chemical consequences of melt‐mush interaction by reacting molten MORB with troctolite at 0.5 GPa. Isothermal experiments demonstrate that melt infiltrates into troctolite with dissolution of plagioclase and olivine. The reacted melts have higher MgO and Al2O3 and lower TiO2 and Na2O contents and crystallize more primitive olivine and plagioclase compared to those crystallized from the unreacted melts, suggesting melt‐mush reaction could result in the formation of high‐Al basalt. The melt compositional variations induced by reaction also significantly affect the calculated pressures for MORB fractionation, indicating that major element‐based barometers for MORB fractionation can only be used reliably if reaction can be ruled out. After reaction, the troctolite contains olivine with plagioclase inclusions and poikilitic clinopyroxene with partially resorbed olivine and plagioclase chadacrysts, indicating that melt‐mush interaction occurs through dissolution‐reprecipitation mechanisms. Clinopyroxene has high Mg# (>83) and elevated Na2O and TiO2 contents, and olivine has different Fo versus Ni correlations from fractional crystallization models, which provide testable parameters for the effect of melt‐mush reaction in the rock record. By comparison with samples from lower oceanic crust and layered intrusions, we propose that melt‐mush reaction plays an important role during magma transport in the crystal mush in both oceanic and continental magma systems.
... This region of low seismic velocity is assumed to be a crystal-rich mush zone (i.e. a framework of touching crystals with interstitial melt; Fig. 1a; e.g. Dunn et al., 2000;Han et al., 2014), an interpretation supported by seafloor compliance surveys (Crawford & Webb, 2002). Fast-spread lower ocean crust forms from the solidification of melt and mush, however, the mechanisms operating within that melt-mush framework remain debated (e.g. ...
The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples from the uppermost ∼800 m of plutonic crust exposed in the Pito Deep Rift. We integrate two- and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96–724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below ∼180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.
... Nevertheless, here we have demonstrated that an accurate 3-D P wave velocity model coupled with good seafloor instrumentation has led to the first geophysical evidence of a conjugate network of inward and outward dipping faults being simultaneously seismically active beneath a caldera and that the seismicity pattern could be more precisely located even outside of the station network. To further improve the seismicity distribution beneath Axial Seamount, future research should now focus on improving the shear wave velocity structure within the volcano using a joint hypocenter and shear wave velocity inversion (e.g., Baillard et al., 2017) or seafloor compliance measurements (e.g., Crawford & Webb, 2002). Our work should also help motivate deploying more cabled or temporary seismometers to the north and south of the current OOI-network before the next eruption at Axial Seamount (see . ...
Axial Seamount is the most volcanically active site of the northeast Pacific, and it has been monitored with a growing set of observations and sensors during the last two decades. Accurate imaging of the internal structure of volcanic systems is critical to better understand magma storage processes and to quantify mass and energy transport mechanisms in the crust. To improve the three-dimensional velocity structure of Axial Seamount, we combined 469,891 new traveltime arrivals, from 12 downward extrapolated seismic profiles, with 3,962 existing ocean-bottom-seismometers traveltime arrivals, into a joint tomographic inversion. Our approach reveals two elongated magma reservoirs, with melt fraction up to 65%, representing an unusually large volume of melt (26–60 km³), which is likely the result of enhanced magma supply from the juxtaposition of the Cobb hot spot plume (0.26–0.53 m³/s) and the Axial spreading segment (0.79–1.06 m³/s). The tomographic model also resolves a subsided caldera floor that provides an effective trap for ponding lava flows, via a “trapdoor” mechanism. Our model also shows that Axial's extrusive section is thinnest beneath the elevated volcano, where anomalously thick (11 km) oceanic crust is present. We therefore suggest that focused and enhanced melt supply predominantly thickens the crust beneath Axial Seamount through diking accretion and gabbro crystallization. Lastly, we demonstrate that our three-dimensional velocity model provides a more realistic starting point for relocating the local seismicity, better resolving a network of conjugate outward and inward dipping faults beneath the caldera walls.
... The variation in crystallinity is most probably the result of the very short time scales (a few decades) of cooling of melt to mush in an axial melt lens (Singh et al., 1998). The melt lens overlies a large region characterized by low seismic velocities interpreted as a crystal mush with a small (<15%) average proportion of melt (Crawford et al., 1999;Vera et al., 1990;Dunn et al., 2000;Crawford & Webb, 2002), which contains sills with higher melt fractions Marjanovic et al., 2014). ...
Mid-ocean ridge basalts (MORB) provide fundamental information about the composition and melting processes in the Earth's upper mantle. To use MORB to further our understanding of the mantle, is imperative that their crustal evolution is well understood and can thus be accounted for when estimating primary melt compositions. Here, we present the evidence for the occurrence of reactive porous flow, whereby migrating melts react with a crystal mush in mid-ocean ridge magma chambers. This evidence comprises both the textures and mineral major and trace element geochemistry of rocks recovered from the lower oceanic crust, and occurs on a range of scales. Reaction textures include dissolution fronts in minerals, ragged grain boundaries between different phases and clinopyroxene–brown amphibole symplectites. However, an important finding is that reaction, even when pervasive, can equally leave no textural evidence. Geochemically, reactive porous flow leads to shifts in mineral modes (e.g. the net replacement of olivine by clinopyroxene) and compositions (e.g. clinopyroxene Mg–Ti–Cr relationships) away from those predicted by fractional crystallization. Furthermore, clinopyroxene trace elements record a progressive core–rim over-enrichment (relative to fractional crystallization) of more-to-less incompatible elements as a result of reactive porous flow. The fact that this over-enrichment occurs over a distance of up to 8 mm, and that clinopyroxenes showing this signature preserve zoning in Fe–Mg, rules out a diffusion control on trace element distributions. Instead, it can be explained by crystal–melt reactions in a crystal mush. The data indicate that reactive flow occurs not only on a grain scale, but also on a sample scale, where it can transform one rock type into another [e.g. troctolite to olivine gabbro, olivine gabbro to (oxide) gabbro], and extends to the scale of the entire lower oceanic crust. Melts undergoing these reactive processes change in composition, which can explain both the major element and trace element arrays of MORB compositions. In particular, reactive porous flow can account for the MORB MgO–CaO–Al 2 O 3 relationships that have previously been interpreted as a result of high-pressure (up to $8 kbar) crystal fractionation, and for over-enrichment in incompatible elements when compared with the effects of fractional crystallization. The finding of a significant role for reactive porous flow in mid-ocean ridge magma chambers fits very well with the geophys-ical evidence that these magma chambers are dominated by crystal mush even at the fastest spreading rates, and with model predictions of the behaviour of crystal mushes. Together, these observations indicate that reactive porous flow is a common, if not ubiquitous, process inherent to mushy magma chambers, and that it has a significant control on mid-ocean ridge magmatic evolution.
... The MTZ concept has been developed in the fast-spreading Oman ophio lites (Boudier and Nicolas, 1995), and it is now described in the East Pacific Rise (EPR) (Crawford and Webb, 2002;Carbotte et al., 2013). The MTZ is locally much thicker in slow-spreading ophiolites than in fast-spreading ophiolites, despite a much larger heat supply within it. ...
The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a common horizontal Jurassic–Lower Cretaceous sedimentary cover showing that they were not deeply and intrinsically affected by later Alpine thrusting. The western massifs of Mirdita, first oceanic core complex (OCC) and detachment shear zone described in ophiolites, compare with OCCs in slow-spreading ridges and provide continuous exposure of the deep internal structure of this system, revealing its kinematics, thanks to detailed structural mapping in peridotites and gabbros. The Mirdita detachments root in the Moho transition zone (MTZ), a weak zone at the top of asthenospheric mantle, where basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ∼400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho beneath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, despite being deeply affected by hydrothermal alteration. From there, the partially molten MTZ was detached as a shear zone, mixing with lower gabbros. The OCC emerged, migrating upsection and eastward over 5 km. Finally, the OCC front is observed in hornblende-rich syntectonic mylonites derived from upper gabbros and from the overlying former lid. Serpentinization is static within these mylonites. A low-temperature detachment fault is expressed as a sheared antigoritic mélange at the margin of the mylonitic shear zone. Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin has been preserved below the OCC rooting. The dominant asthenospheric flow direction trends parallel to the ridge axis. This mantle flow rotates over 200 m into the low-temperature mylonitic detachments, where OCC motion turns transversal to the ridge. Crystal preferred orientation measurements on six samples point to brown hornblende crystal growth during mylonitic flow and illustrate the change of olivine intra-crystalline slip system in mylonites compared to porphyroclastic harzburgite.
... Episodic sill formation by melt ponding and crystallization at the Moho has been suggested as a method of lower oceanic crustal accretion . Evidence for melt ponding near the Moho includes seismic studies of mid-ocean ridges which have detected a significant fraction of melt (2.5-17%) in the lower oceanic crust (Crawford and Webb, 2002;Singh et al., 2006;Canales et al., 2009Canales et al., , 2012Marjanovic et al., 2014;Zha et al., 2014). Compliance modeling of seismic data at a fast spreading ridge suggests that melt in the lower oceanic crust resides in interconnected melt-filled sills or cracks (Zha et al., 2014). ...
The Taney Seamounts are a NW-SE trending linear, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300 km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, is defined by four well-exposed calderas, which expose previously infilled lavas. The calderas can be differentiated in time by their cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle (ROV) to obtain stratigraphically-controlled samples, a unique aspect of this study.
... Toomey et al. (2007), mantle melt upwelling was found 5 to 20 km from the axis over nearly half of a 200 km first order segment of the East Pacific Rise (EPR). The discovery of melt lenses in the lower crust, up to 20 km from the rise (Crawford and Webb, 2002;Crawford et al., 1999;Garmany, 1989), also shows that not all melt is focused beneath the ridge. Furthermore, seamount volcanism provides direct evidence of off-axis magmatic processes. ...
The Oman ophiolite includes both a fossil fast spreading axis, defined by five mantle diapirs, and an off-axis mantle diapir emplaced 30 km from the axis, providing a natural laboratory for the study of off-axis magmatic processes. We compare field and petrological observations coupled with geochemical and isotopic analyses of samples from the off-axis diapir with those of the nearest on-axis diapir, with a particular focus on the Moho Transition Zone (MTZ). Both diapirs are defined by the presence of steeply plunging lineations, but in the on-axis case, these lineations rotate gradually into parallelism with the horizontal magmatic lineations of the overlying crust, while in the off-axis case, a shear zone separates the steeply plunging lineations from the horizontal lineations of the surrounding mantle. In the on-axis diapir, the MTZ is 50 to 500 m thick and composed of dunite with layered gabbro lenses whereas in the off-axis diapir, the MTZ is thicker and composed of dunite with massive (∼20% of MTZ) clinopyroxenite lenses and a notable absence of plagioclase. Moreover, the off-axis diapir is associated with amphibole-bearing intrusions, consisting of Mg-rich gabbroic sills in the mantle peripheral to the diapir, and microgabbroic lenses of broadly basaltic composition in the overlying crust. The εNd values of the pyroxenites in the MTZ of the off-axis diapir fully overlap with those of the intrusions in the surrounding mantle and crust, suggesting that they are genetically related. Calculated rare earth element (REE) abundances of liquids in equilibrium with clinopyroxene imply that the magmas that traversed the MTZ of the off-axis diapir were more depleted in highly incompatible elements than their counterparts in the MTZ of the on-axis diapir. On the other hand, Nd isotopic compositions of the off-axis samples ( in 18 of 19 samples) indicate derivation of their parental magmas from a less depleted source than that which produced the magma associated with the on-axis gabbro ( , 10 analyses).
... Isotherms constrained by cooling rates obtained from the HDR plutonics are broadly spaced in the off-axis environment, reflecting the near-conductive cooling of the lower crust that facilitates the preservation of off-axis magma lenses. This does not preclude the presence of magma lenses beneath the axial magma lenses and near the Moho, as evident in seismic reflection data (Marjanović et al., 2014) and from modeling of compliance (e.g., Crawford and Webb, 2002) data. Moreover, the spacing of the isotherms does not preclude limited near-axis hydrothermal cooling. ...
The cooling history of the lowermost ocean crust formed at the East Pacific Rise and exposed at the Hess Deep Rift is constrained using fresh primitive olivine gabbro and troctolitic cores recently recovered by IODP Expedition 345. The Mg-in-plagioclase and Ca-in-olivine geospeedometers constrain the initial subsolidus cooling rates, down to closure temperatures in the range of 900–750 °C. These independent geospeedometers yield similar slow cooling rates, 0.005–0.0001 °Cyr-1, with a mean value of 0.0011 °Cyr-1, making these results robust despite the uncertainties inherent in the methods. The slow cooling rates, derived using natural samples, provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling. The cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex. This implies that lower crust formed at fast spreading rates remains hot in the near axis environment, indicating limited hydrothermal fluid ingress and reaction.
... For the initial plagioclase composition, we use lower oceanic crust cumulate plagioclase from a troctolite sampled at Atlantis Massif, Mid-Atlantic Ridge (Drouin et al., 2009) with 177 ppm Sr and an anorthite content of An 82 . Seismic studies of melt distribution at the Moho beneath the East Pacific Rise at 9-10 N suggest that melt porosities range from 2Á5 to 17% (Dunn et al., 2000;Crawford & Webb, 2002); therefore, we perform model runs at 0Á025/, 0Á05/ and 0Á1/. Studies of the crustmantle transition zone in ophiolites constrain the grain size of plagioclase phases at 0Á5-2 mm (Garrido et al., 2001). ...
Seamounts formed adjacent to mid-ocean ridges are the most abundant on Earth, numbering several orders of magnitude higher than hotspot-related seamounts. The Taney Seamounts are a linear NW–SE-trending, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300 km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, has four well-defined calderas. These calderas have clear cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle to obtain stratigraphically controlled samples, a unique aspect of this study. Changes in lava geochemistry are consistent with an open-system sub-caldera reservoir that undergoes periodic collapse, replenishment, shallow crystallization, and eruption. Replenishing magmas contain large, anorthite-rich plagioclase crystals that exhibit sieve textures and zoning indicating interaction with percolating melt. The enrichment of elements in the lavas that are incorporated in plagioclase (e.g. aluminum, strontium) provides chemical evidence for the interaction between mantle-derived melts and plagioclase cumulates in the lower oceanic crust or upper mantle (8–12 km), prior to magmas entering the sub-caldera plumbing system. Based on trace element variations, the erupted lavas vary from typical peridotite-derived normal mid-ocean ridge basalt (N-MORB) compositions to those with an apparent residual garnet signature. Geochemical and thermodynamic modeling shows that decompression melting of a MORB mantle peridotite re-fertilized by garnet pyroxenite partial melts can reproduce the garnet signature observed in the Taney-A edifice lavas. Hence the magmatic architecture of Taney Seamount-A is characterized by the melting of a mixed lithology mantle, melt–rock interaction in the upper mantle to lower oceanic crust, and open-system evolution in a sub-caldera magma reservoir.
... High fractionation pressures at the EPR are unexpected, since the thermal boundary layer at fast fast-spreading rates is predicted to lie at shallow levels, which would prohibit fractionation of rising melts in the mantle. Moreover, since the lower crust underneath the EPR is comprised of a crystal mush (e.g.: Crawford and Webb, 2002; Dunn et al., 2000), and it is capped by a melt lens (e.g.: Kent et al., 1993), melts fed from the mantle are likely to be trapped in this magma chamber prior to eruption. This would inevitably lead to mixing and most likely also to partial crystallization, thus destroying the highpressure signal melts may have had after possible fractionation increments in the mantle. ...
Editor: C.P. Jaupart Keywords: mid-ocean ridge basalt lower oceanic crust Mid-Atlantic Ridge melt–rock reaction geochemistry Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host rock, and olivine is notably absent as a chadacryst despite being abundant in the host rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such reactions are enriched in Al 2 O 3 and MgO and depleted in CaO and SiO 2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al 2 O 3 –MgO–SiO 2 relationships of MORB may result from melt–rock reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–rock reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB.
... When these magma bodies assume a crystalline form greater than about 50 %, rheologically they behave much like a solid. The presence of melts in the lower crust and also in the upper mantle is suggested by active source tomography (Toomey et al., 1990;Dunn and Toomey, 1997) and compliance techniques (Crawford et al., 1991;Crawford et al., 1998;Crawford and Webb, 2002). In the latter technique, the pressure field caused by the long-period ocean waves that deform the sea floor is studied. ...
Deciphering the seismic character of the young
lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In
this study, we determine the seismic structure of the oceanic plate near the
MORs using the P-to-S conversions isolated from quality data recorded
at five broadband seismological stations situated on ocean islands in their
vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations
reveal that the Moho depth varies between ~ 10 ± 1 km and
~ 20 ± 1 km with the depths of the
lithosphere–asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an
additional low-velocity layer below the expected LAB depths at stations on
Ascension, São Jorge and Easter islands. The layer probably relates to the
presence of a hot spot corresponding to a magma chamber. Further, thinning of
the upper mantle transition zone suggests a hotter mantle transition zone
due to the possible presence of plumes in the mantle beneath the stations.
... Carbotte et al., 2006Carbotte et al., , 2008Nedimović et al., 2005Nedimović et al., , 2008Van Ark et al., 2007;Newman et al., 2011). At the EPR, new data were acquired in 2008 as part of the first academic multisource and multistreamer three-dimensional (3D) MCS study ever conducted (Mutter et al., 2009;Canales et al., in press (Detrick et al., 1987;Vera et al., 1990;Kent et al., 1993aKent et al., ,b, 2000Harding et al., 1993;Singh et al., 2006) and geophysical studies using other techniques (e.g., Crawford et al., 1999Crawford et al., , 2002Dunn et al., 1997Dunn et al., , 2000Toomey et al., 2007). New studies employing these techniques are underway at the Endeavour and EPR sites (Key and Constable, 2005;Toomey et al., 2010;Zha et al., 2010), and exciting new results are anticipated in the coming years. ...
As part of the suite of multidisciplinary investigations undertaken by the Ridge 2000 Program, new multichannel seismic studies of crustal structure were conducted at the East Pacific Rise (EPR) 8°20'-10°10'N and Endeavour Segment of the Juan de Fuca Ridge. These studies provide important insights into magmatic systems and hydrothermal flow in these regions, with broader implications for fastand intermediate-spreading mid-ocean ridges. A mid-crust magma body is imaged beneath Endeavour Segment underlying all known vent fields, suggesting that prior notions of a tectonically driven hydrothermal system at this site can be ruled out. There is evidence at both sites that the axial magma body is segmented on a similar 5-20 km length scale, with implications for the geometry of high-temperature axial hydrothermal flow and for lava geochemistry. The new data provide the first seismic reflection images of magma sills in the crust away from the axial melt lens. These offaxis magma reservoirs are the likely source of more-evolved lavas typically sampled on the ridge flanks and may be associated with off-axis hydrothermal venting, which has recently been discovered within the EPR site. Clusters of seismic reflection events at the base of the crust are observed, and localized regions of thick Moho Transition Zone, with frozen or partially molten gabbro lenses embedded within mantle rocks, are inferred. Studies of the upper crust on the flanks of Endeavour Segment provide new insights into the low-temperature hydrothermal flow that continues long after crustal formation. Precipitation of alteration minerals due to fluid flow leads to changes in P-wave velocities within seismic Layer 2A (the uppermost layer of the oceanic crust) that vary markedly with extent of sediment blanketing the crust. In addition, intermediate-scale variations in the structure of Layers 2A and 2B with local topography are observed that may result from topographically driven fluid upflow and downflow on the ridge flanks.
... Singh et al. 2006a;Combier et al. 2008). Seafloor compliance and seismic tomography studies indicate melt volumes in the crust and uppermost mantle under the ridge axis do not diminish toward the 98N OSC (Crawford & Webb 2002;Toomey et al. 2007). Indeed, a local region of higher, not lower, melt content is inferred in the shallow mantle beneath the 98N OSC (Dunn et al. 2001;Toomey et al. 2007). ...
Mid-ocean ridges display tectonic segmentation defined by discontinuities of the axial zone, and geophysical and geochemical observations suggest segmentation of the underlying magmatic plumbing system. Here, observations of tectonic and magmatic segmentation at ridges spreading from fast to ultraslow rates are reviewed in light of influential concepts of ridge segmentation, including the notion of hierarchical segmentation, spreading cells and centralized v. multiple supply of mantle melts. The observations support the concept of quasi-regularly spaced principal magmatic segments, which are 30-50 km long on average at fast- to slow-spreading ridges and fed by melt accumulations in the shallow asthenosphere. Changes in ridge properties approaching or crossing transform faults are often comparable with those observed at smaller offsets, and even very small discontinuities can be major boundaries in ridge properties. Thus, hierarchical segmentation models that suggest large-scale transform fault-bounded segmentation arises from deeper level processes in the asthenosphere than the finer-scale segmentation are not generally supported. The boundaries between some but not all principal magmatic segments defined by ridge axis geophysical properties coincide with geochemical boundaries reflecting changes in source composition or melting processes. Where geochemical boundaries occur, they can coincide with discontinuities of a wide range of scales.
Volcanotectonics is comparatively new scientific field that combines various methods and techniques of geology and physics so as to understand the structure and behaviour of polygenetic (central) volcanoes and the conditions for their eruptions. More specifically, volcanotectonics uses the techniques and methods of tectonics, structural geology, geophysics, and physics to collect data on volcanoes, as well as to analyse and interpret the physical processes that generate those data. The focus is on processes responsible for periods of volcanic unrest, caldera collapses, and eruptions.
For basic science, one principal aim of volcanotectonics is to develop methods for reliable forecasting of eruptions. Accurate forecasting as regards the location, time, and magnitude of eruptions has long been a major goal in volcanology. Volcanotectonics provides a theoretical framework and understanding of the physical processes that take place inside volcanoes prior to eruptions, thereby offering methods and techniques that allow us to use data obtained during unrest periods to forecast eruptions. For applied science and human society, one principal aim of volcanotectonics is to develop methods for preventing very large eruptions. This second aim – namely methods that allow us to prevent very large eruptions - may come as a surprise to some, but is of fundamental importance for the future of human civilisation. Very large eruptions, whose eruptive volumes may be of the order of hundreds or thousands of cubic kilometres, provide existential threat to human civilisation.
The purpose of this book is to provide an overview of the scientific field of volcanotectonics. The book is primarily aimed at, first, undergraduate and graduate students in geology, geophysics, and geochemistry and, second, civil authorities, scientists, engineers, and other professionals who deal with volcanoes and the associated hazards in their work. The book has be designed so that it can be used (1) for an independent study, (2) as a textbook for a course on volcanotectonics, and (3) as a supplementary text for general courses on volcanology, structural geology, geology, geophysics, geothermics, and natural hazards.
Each chapter begins with an overview of the aims and ends with a summary of the main topics discussed. In addition there is a list of symbols used in the chapter. Important concepts and conclusions are in bold face. In volcanotectonics the focus is on quantitative results. This is reflected in the 68 worked examples (solved probems) most of which include calculations. In addition there are 253 exercises (supplementary problems), many of which also require calculations. The examples and exercises are meant to provide a deeper understanding of the basic principles of volcanotectonics and their use for understanding the formation of volcanoes, the physical processes that maintain their activities, and providing reliable eruption forecasts. While volcanic activity cannot be understood or forecasted without basic knowledge of the relevant physics, the physics presented in the book is mostly elementary and explained in detail. The only exception is part of Chapter 10, where more advanced physics is introduced to explain the propagation paths of magma-driven fractures.
I have taught much of the material in the book at various universities over the past 20 years to earth-science students in Norway, Germany, and England. In particular, many of the chapters form the basis of an undergraduate course on volcanology which I have taught in the past six years in England. Based on this experience, most of the material in the book should be suitable for earth-science students with a very modest knowledge of mathematics and physics.
Contents
Chapter 1. Introduction
Chapter 2. Volcanotectonic structures
Chapter 3. Volcanotectonic deformation
Chapter 4. Volcanic earthquakes
Chapter 5. Volcanotectonic processes
Chapter 6. Formation and dynamics of magma chambers and reservoirs
Chapter 7. Magma movement through the crust: dike paths
Chapter 8. Dynamics of volcanic eruptions
Chapter 9. Formation and evolution of volcanoes
Chapter 10. Understanding unrest and forecasting eruptions
Appendix A. Units, dimensions, and prefixes
Appendix B. The Greek alphabet
Appendix C. Some mathematical and physical constants
Appendix D. Elastic constants
Appendix E. Properties of some common crustal materials
Appendix F. Physical properties of lavas and magmas
We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.
Abstract Differential pressure gauges (DPGs) are a standard component of modern broadband ocean‐bottom seismometer instruments and have proven useful for observing a wide range of seismic and oceanographic phenomena. However, the response function of the DPG remains poorly known, limiting our ability to recover amplitude and phase information from seafloor pressure signals with high fidelity. The sensitivity and long‐period response are difficult to calibrate in the lab, as they are known to vary with temperature and pressure and perhaps between sensors of the same design. We present the results of a field experiment designed to determine empirical response functions in situ by inducing a pre‐defined pressure offset on a deployed instrument. The results compare favorably with calibrations estimated independently through post‐deployment data analyses. Our study demonstrates that observed response functions can deviate from the nominal response by a factor of two or greater with regards to both the sensitivity and the time constant. Incorporating calibration devices such as those described here into future deployments may prove to be a cost‐effective way to improve the accuracy and utility of differential pressure data.
Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. © 2018 The Author(s) Published by the Royal Society. All rights reserved.
We present models of compressional and shear velocity structure of the oceanic sediments and upper crust surrounding the Hawaiian islands. The models were derived from analysis of seafloor compliance data and measurements of Ps converted phases originating at the sediment‐bedrock interface. These data were estimated from continuous broadband ocean bottom seismometer acceleration and pressure records collected during the Plume‐Lithosphere Undersea Mantle Experiment, an amphibious array of wideband and broadband instruments with an aperture of over 1,000 km. Our images result from a joint inversion of compliance and Ps delay data using a nonlinear inversion scheme whereby deviation from a priori constraints is minimized. In our final model, sediment thickness increases from 50 m at distal sites to over 1.5 km immediately adjacent to the islands. The sedimentary shear velocity profiles exhibit large regional variations. While sedimentary structure accounts for the majority of the compliance signal, we infer variations in shear velocity in the uppermost bedrock on the order of ±5%. We also require relatively high values of Poisson's ratio in the uppermost crust. Lower crustal velocities are generally seen to the north and west of the islands but do not appear well correlated with the Hawaiian Swell bathymetry. A region of strong low velocity anomalies to the northeast of Hawaii may be associated with the Molokai fracture zone.
We desire increased useful bandwidth in our seismic surveys: higher frequencies to increase resolution, and lower frequencies for deeper penetration and in particular to enable better velocity-model-building by full-waveform inversion. To design an acquisition strategy to achieve these goals we must understand both the signal produced by our sources and the noise to be overcome. What matters is signal-to-noise, not signal or noise alone. We examine two ocean-bottom node datasets from the Atlantis field in the deep water Gulf of Mexico to learn more.
We find that the primary source of noise above about 4 Hz is seismic interference. The signal and noise are of similar character and the signal-to-noise ratio is thus approximately constant with frequency. Instead of increasing our signal, we may instead use the predictable nature of the noise to better attenuate it. Below 2 Hz the noise is dominated by the natural microseismic background of the earth, which rapidly increases in amplitude with decreasing frequency. In contrast, below about 7 Hz airguns decrease in amplitude at about 16 dB / octave. Below 2 Hz the signal-to-noise ratio thus declines precipitously with decreasing frequency. New acquisition paradigms may be required to meet the low-frequency challenge.
Presentation Date: Tuesday, October 18, 2016
Start Time: 3:45:00 PM
Location: 163/165
Presentation Type: ORAL
We designed a thermo-mechanical model for fast spreading mid-ocean ridge with variable viscosity, hydrothermal cooling, latent heat release, sheeted dyke layer, and variable melt intrusion possibilities. The model allows to take into account several accretion possibilities as: the "gabbro glacier" (G), the "sheeted sills" (S) or the "mixed shallow and MTZ lenses" (M). Viscosity contrasts of 2 to 3 orders of magnitude between the hot and cold phases have been tested. We also explored hydrothermal cooling according to various cracking temperatures for crustal rocks. Hence, the model allows exploring various near ridge motions and thermal patterns that induce various cooling histories for gabbros. According to the assumed opening-closure temperature range, the cooling rates sample the near-ridge structure or record areas farther from the ridge. As an analogy to experimental petrology we called ICR the cooling rates sampled near the ridge and SRC the cooling rates sampled far from the ridge where the flow tends to laminar and conductive patterns. The results emphasize that the cooling rates may significantly depend on the choice of this opening-closure temperature range. The results show that numerical modeling of thermo-mechanical properties of the lower crust's may bring information to study the hypotheses related to the ridge accretion structure, hydrothermal cooling and thermal state at the fast-spreading ridges.
The origin of mid-ocean ridge segmentation - the systematic along-axis variation in tectonic and magmatic processes - remains controversial. It is commonly assumed that mantle flow is a passive response to plate divergence and that between transform faults magma supply controls segmentation. Using seismic tomography, we constrain the geometry of mantle flow and the distribution of mantle melt beneath the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge. Our results, in combination with prior studies, establish a systematic skew between the mantle-divergence and plate-spreading directions. In all three cases studied, mantle divergence is advanced with respect to recent changes in the plate-spreading direction and the extent to which the flow field is advanced increases with decreasing spreading rate. Furthermore, seismic images show that large-offset, non-transform discontinuities are regions of enhanced mantle melt retention. We propose that oblique mantle flow beneath mid-ocean ridges is a driving force for the reorientation of spreading segments and the formation of ridge-axis discontinuities. The resulting tectonic discontinuities decrease the efficiency of upward melt transport, thus defining segment-scale variations in magmatic processes. We predict that across spreading rates mid-ocean ridge segmentation is controlled by evolving patterns in asthenospheric flow and the dynamics of lithospheric rifting.
Seafloor morphology and crustal structure vary significantly in the Lau back-arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low-velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5–0.8 km/s slower than crust produced at the present-day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.
As part of the suite of multidisciplinary investigations undertaken by the Ridge 2000 Program, new multichannel seismic studies of crustal structure were conducted at the East Pacific Rise (EPR) 8 degrees 20'-10 degrees 10'N and Endeavour Segment of the Juan de Fuca Ridge. These studies provide important insights into magmatic systems and hydrothermal flow in these regions, with broader implications for fast- and intermediate-spreading mid-ocean ridges. A mid-crust magma body is imaged beneath Endeavour Segment underlying all known vent fields, suggesting that prior notions of a tectonically driven hydrothermal system at this site can be ruled out. There is evidence at both sites that the axial magma body is segmented on a similar 5-20 km length scale, with implications for the geometry of high-temperature axial hydrothermal flow and for lava geochemistry. The new data provide the first seismic reflection images of magma sills in the crust away from the axial melt lens. These off-axis magma reservoirs are the likely source of more-evolved lavas typically sampled on the ridge flanks and may be associated with off-axis hydrothermal venting, which has recently been discovered within the EPR site. Clusters of seismic reflection events at the base of the crust are observed, and localized regions of thick Moho Transition Zone, with frozen or partially molten gabbro lenses embedded within mantle rocks, are inferred. Studies of the upper crust on the flanks of Endeavour Segment provide new insights into the low-temperature hydrothermal flow that continues long after crustal formation. Precipitation of alteration minerals due to fluid flow leads to changes in P-wave velocities within seismic Layer 2A (the uppermost layer of the oceanic crust) that vary markedly with extent of sediment blanketing the crust. In addition, intermediate-scale variations in the structure of Layers 2A and 2B with local topography are observed that may result from topographically driven fluid upflow and downflow on the ridge flanks.
Oceanic crust is created as magma rises to fill the gap between diverging tectonic plates and is consumed in subduction zones. It is geologically young, with a mean age of 60. Ma, and is thin, averaging 6.5. km in thickness. Oceanic crust consists almost exclusively of extrusive basalt and its intrusive equivalents. This chapter focuses on the roughly 1.5-2. km thick 'volcanic layer' consisting of lava flows that overlie the feeder dikes that make up the sheeted dike complex. The basalts of the oceanic crust, referred to as mid-ocean ridge basalts (MORB), are dominantly tholeiitic and are, on average, depleted in incompatible trace elements compared to basalts erupted in other tectonic environments. Isotopic compositions and incompatible trace element concentrations and ratios suggest that their depleted character is inherited from their mantle source and that this source varies in composition both locally and on the scale of ocean basins. Their major element chemistry appears to be controlled primarily by the temperature of the underlying mantle, which determines the extent and pressure of melting, and, consequently, the thickness of the oceanic crust and the depth of the ridge axis. Basalts erupted at back-arc spreading centers, called back-arc basin basalts, are compositionally similar to MORB, but have some compositional features suggesting incorporation of one or multiple subduction-related components in their source.
The world's mid-ocean ridges form a single, connected global ridge system that is part of every ocean, and is the longest mountain range in the world. Geologically active, mid-ocean ridges are key sites of tectonic movement, intimately involved in seafloor spreading. This coursebook presents a multidisciplinary approach to the science of mid-ocean ridges – essential for a complete understanding of global tectonics and geodynamics. Designed for graduate and advanced undergraduate students, it will also provide a valuable reference for professionals in relevant fields. Background chapters provide a historical introduction and an overview of research techniques, with succeeding chapters covering the structure of the lithosphere and crust, and volcanic, tectonic and hydrothermal processes. A summary and synthesis chapter recaps essential points to consolidate new learning. Accessible to students and professionals working in marine geology, plate tectonics, geophysics, geodynamics, volcanism and oceanography, this is the ideal introduction to a key global phenomenon.
Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.
The lower oceanic crust forms when melts that are generated within the mantle beneath mid-ocean ridges cool and crystallize at depth below the seafloor. The compositions of mid-ocean ridge basalts (MORBs), such as their low Mg# (Mg/(Mg+Fe2+)), show that they have been significantly modified after separation from the mantle by partial crystallization within the lower oceanic crust.There are currently only a handful of locations in the modern ocean basins where significant sampling of the lower oceanic crust has occurred. Trace element and isotopic data from these oceanic plutonic rocks suggest that the parental melts were relatively homogeneous. In turn, this suggests either that melt extraction from the mantle efficiently mixes melts generated under different conditions or that crustal level homogenization is very efficient. Several lines of evidence, including disequilibrium between the crystals and host basalt in many MORBs, suggest that magma mixing is an important process within the lower oceanic crust. The plutonic rocks record evidence for extensive melt-rock reaction within crystal mush zones that fractionates incompatible trace elements from one another more than predicted by fractional crystallization. The bulk composition of the lower oceanic crust and, hence, of Moho-crossing melts is poorly constrained due to inadequate sampling. However, there is evidence for depletion of incompatible elements within the lower crust compared to the overlying upper crust in all areas studied, with the extent of depletion proportional to the element's partition coefficient.Spreading rate plays an important role in controlling lower crustal processes through its impact on the thermal structure near the ridge axis. At fast-spreading ridges, the flux of magma (and hence specific and latent heat) into the crust is large; however, geophysical surveys show that there is only a small melt sill at the base of the sheeted dike complex; this sill is underlain by a larger mush zone. The lack of a large molten region requires efficient hydrothermal heat extraction from the lower crust. Several lines of evidence suggest that much of the latent heat of crystallization is extracted from the lower crust through the roof of the small sill, with crystals subsiding down and out from this body to form the lower crust. Residual interstitial melt is largely compacted out of the crystal mush zone. At slow-spreading ridges, the magma flux into the crust can be an order of magnitude lower than at fast-spreading ridges and steady-state magma chambers do not form. Instead, lower crustal magma chambers are transient features in which crystallization leads to a wide range of cumulate compositions forming during the final stages of solidification.Several characteristics of oceanic gabbros and MORB differentiation trends at slow-spreading ridges have been interpreted as requiring crystallization at elevated pressure. However, there are alternative explanations for all of the data, and it is unlikely that substantial amounts (>10%) of crystallization occurs at high pressure (>0.3. GPa). Despite the limited heat supply at slow-spreading ridges, extensive crystallization at >0.3. GPa would require efficient hydrothermal heat extraction from this depth for which there is little evidence.
Geophysical measurements and models constrain the total rate of production of crustal material and the flux of thermal energy over the global ridge system. Flux estimates based on basin-scale compilations of heat-flow measurements or on 1-dimensional geodynamic models of melt generation and plate cooling provide a useful, but only a partial view of the crustal thermal regime. The rate of heat supply to the crust depends both on complex patterns of flow in the mantle, and on how this flow supplies magma (a major carrier of advective heat flux) to the crust. Much progress is still needed if we are to understand the thermal regime at and close to the crust-mantle boundary, and hence the extent to which segmentation and other variations in crustal structure may be inherited from the mantle. Within the lower and middle crust, the thermal regime is dominated by the presence (or otherwise) of crustal magma chambers. Over the last decade, geophysical data have provided a progressively more sophisticated understanding of these features, at all spreading rates. Correlations and quantitative links between new models of magma chamber structure and what is known from other disciplines about the overlying hydrothermal circulation system remain weak. Significant unknowns also still remain regarding the patterns and pathways of hydrothermal circulation within the crust. High resolution geophysical data are now beginning to provide quantitative constraints on the physical structure (overall porosity, and interconnectedness of pore spaces) of the permeable crust. The same observations and methods are also beginning to allow us to detect in situ variations in the properties of the fluids themselves, to depths equivalent to the base of layer 2, and on horizontal scales of several kilometres. In one case this has provided glimpses of what may be two-layer hydrothermal convection, related to phase separation. How flow patterns are influenced by key tectonic parameters such as spreading rate and ridge morphology remains an open issue. Also unknown is the extent to which shallow circulation may be driven by newly injected dikes, and the spatial and temporal scales of the resultant thermal perturbations. Lastly, we must consider the case where high and low temperature hydrothermal circulation is occurring in the absence of any significant crustal magma body. Are such systems related to the cooling of rocks that have recently crystallized from basaltic magmas? Do serpentinization reactions play a significant role? And how widespread are such circulation regimes?.
The differentiation of mid-ocean ridge basalt (MORB) is investigated with a focus on intermediate- to fast-spreading ridges and two recently proposed differentiation mechanisms: (i) differentiation in replenished-tapped-crystallizing (RTX) magma chambers, and (ii) chromatographic element separation during melt-rock reaction in the lower crust. There is compelling evidence in the petrology and geochemistry of MORB indicating that magma chambers at mid-ocean ridges behave as open systems, as required on thermal grounds in locations where a steady-state magma chamber exists. It has recently been suggested that the commonly observed over-enrichment of more-to-less incompatible elements during MORB differentiation can be explained by such an RTX model. However, the petrology of samples from the lower oceanic crust suggests an alternative mechanism could produce this over-enrichment. Clinopyroxene crystals in oceanic gabbros are commonly strongly zoned in incompatible elements with crystal rims apparently having grown from melts with very high incompatible element abundances. Elevated Zr/LREE in clinopyroxene rims, which has been interpreted as indicating growth from a melt in which these elements had been fractionated from one another by melt-rock reaction (chromatographic separation), is shown to be more simply explained by post-crystallization diffusive fractionation. However, the high incompatible element abundances in crystal rims demonstrates that the interstitial melt in crystal mush zones becomes highly differentiated. Disaggregation of such mush zones is indicated by the crystal cargo of MORB and must be accompanied by the return of interstitial melt to the eruptible reservoir – a form of in situ crystallization. Both a magma chamber undergoing closed system in situ crystallization, and a RTX magma chamber in which crystallization occurs in situ, are shown to be capable of reproducing the differentiation trends observed in MORB. Simple stochastic models of the latter process suggest that significant variations of incompatible element abundances and ratios, at a constant MgO content, are likely to be generated from a single parental melt compositions. Additionally, parental melt compositions will generally be substantially more depleted than would be suggested if only fractional crystallization is considered. This has important implications for understanding the composition of the upper mantle. For example, the Sm/Nd of MORB are likely to be significantly lower than that of the Moho-crossing melt complicating analysis of the Nd-isotopic evolution of the upper mantle.
In this study we construct a thermal and mechanical model for the genesis of oceanic crust. Magma is halted in its ascent within the oceanic crust when it reaches a freezing horizon, where the dilational volume change associated with magma freezing leads to viscous stresses that favor magma ponding near the freezing horizon. To model the steady state thermal impact of crustal accretion via dike injection and pillow flows, we treat all crustal accretion in rocks cooler than a magma “solidus” to occur in a narrow 250-m-wide dike-like region centered about the ridge axis. The rest of the oceanic crust is modeled to be emplaced as a steady state magma lens directly beneath the “solidus” freezing horizon where the steady state emplacement rate is determined by the constraint that this lens supply all crust that is not emplaced through diking/extrusion above the magma lens. If hydrothermal heat transport within crustal rocks cooler than 600°C removes heat 8 times as efficiently as heat conduction, then we find that a steady state magma lens will only exist within the crust for ridges spreading faster than a 25 mm/yr half rate. The depth dependence of the magma lens with spreading rate is in good agreement with seismic observations. These results suggest that a fairly delicate balance between magmatic heat injection during crustal accretion and hydrothermal heat removal leads to a strongly different Crustal thermal structure at fast and slow spreading ridge axes. Our results support the hypothesis that median valley topography is due to extension of strong ridge axis lithosphere; it is the difference in thermal regime that is directly responsible for the striking difference between the typical median valley seen at slow spreading ridges (e.g., Mid-Atlantic Ridge) and the axial high seen at fast spreading ridges (e.g., East Pacific Rise). This paradigm for the origin of a median valley at a slow spreading ridge predicts that along-axis variations in median valley topography of a slow spreading center reflect variations in recent magmatic heat input along a segment, that is, that the axial topography is a good time-averaged indicator of the relative importance of hydrothermal cooling and magmatic injection along a given section of a ridge segment. We determine the accumulated crustal strain associated with lower crustal flow which supports the hypothesis that the Oman Ophiolite crust was created at a paleo-analogue to a fast spreading ridge and also suggests that crustal strain, and not cumulate layering, may be the dominant physical process that generates “layered gabbros” within the Oman Ophiolite.
Mid-ocean ridge volcanism is largely concentrated within a few
kilometers of the spreading axis. Geophysical models of upwelling
induced melting beneath a ridge axis predict significant melting within
a ˜100 kilometer wide region beneath a ridge axis. This provides a
strong constraint for testing models of melt migration beneath a
spreading center since a successful melt migration mechanism must be
able to focus melt from a broad region of melt production to the narrow
emplacement zone at the ridge axis. Tensile dike propagation transport
of melt within the mantle would move melt away from the ridge axis and
thus does not satisfy this constraint. However, porous flow of melt
within a viscously deforming asthenosphere is a viable possibility.
Models of porous flow are constructed which consider the effects of
mantle flow-induced pressure gradients and strain-induced anisotropic
permeability on porous melt migration. In order for flow-induced
pressure gradients to shape melt migration the sub-ridge asthenospheric
viscosity must be 1021 Pa-s, two orders of magnitude larger
than currently accepted values. Mantle strain-induced anisotropic
permeability variations can explain the focussing of melt towards the
ridge axis if the ratio of the directional anisotropic permeabilities is
as small as 3. Furthermore, mantle strain-induced anisotropy in rock
permeability may be an important mechanism shaping melt migration within
the mantle.
We have applied waveform inversion to multichannel seismic reflection data collected at the East Pacific Rise at 9°40′N in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection. Our analysis supports the idea of a molten sill as previously suggested from forward modeling of seismic data from this location. Our inverted solution has a 30-m-thick sill with a P wave seismic velocity of 2.6 km s−1. Although not well constrained by the data we believe that the S wave velocity in the sill is not significantly different from 0.0 km s−1. The low P- and S wave velocities in the sill imply that it contains less than 30% crystals. The molten sill is underlain by a velocity gradient in which the P wave velocity increases from 2.6 to 3.5 km s−1 over a vertical distance of 50-m. The shape of our velocity-depth profile implies that accretion of material to the roof of the sill is minor compared to accretion to the floor. The underlying velocity gradient zone may represent crystal settling under gravity. We suggest that only material from the 30-m-thick layer can erupt.
A wide-aperture profile along the ridge axis from 14ø29'S to 13ø39'S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 rn thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13øN on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is real rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along-axis flow.
Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1-2km wide) melt lens overlying a zone of crystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt. These results are consistent with a model that effectively separates the processes of magma mixing and fractionation into different parts of a composite magma chamber. -from Authors
A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50' N and 13°30' N is interpreted to arise from the top of a crustal magma chamber located 1.2-2.4 km below the sea floor. The magma chamber is quite narrow (
We explore two-dimensional, steady state, flow and thermal models of oceanic spreading center structure. These models include the effects of hydrothermal heat transport and crustal accretion for two basic accretion geometries: (1) a lens model where all crust below the sheeted dike layer passes through a magma lens at the base of the dike layer before subsiding and flowing to deeper levels; and (2) a dike model where crust is emplaced in a vertical slot from the surface to Moho and subsequently moves horizontally (rigidly) away from the axis of accretion. The axial heat flux resulting from these two end-member accretion geometries is compared, and an assessment is made of the feasibility of using various observations to differentiate between these two accretion geometries. We find that the steady state axial heat flux is predominantly influenced by three factors: the spreading rate, the magmatic budget (crustal thickness) and the efficiency of hydrothermal cooling. The total steady state heat flux from the neovolcanic zone (2 km wide) increases almost linearly with the spreading rate for both the dike and lens accretion geometries. The major difference between the dike and the lens models is nonthermal: they predict different accumulated strain distributions within off-axis crust. Crustal flow due to crustal accretion within a crustal-height ``dike'' leads to little accumulated strain, while intense crustal strain results from crustal subsidence and flow below a steady state magma lens. Ophiolite and marine seismic observations of crustal layering appear to be the strongest observational tests to discriminate between these two accretion geometries; they currently favor a lens-like model of lower crustal accretion.
The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9°30'N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three-dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low-velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the crust (5-7 km) and broad in the mantle (~18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise-parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9°28'N and 9°35'N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along-axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9°28'N and 9°35'N devals, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid-state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment-scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.
The nature and morphological characteristics of axial summit troughs on fast (-90- 130 mm/yr 'l full spreading rate) and superfast spreading (> 130 mm/yr 'l) mid-ocean ridge crests reflect the time-integrated effects of long-term magmatic cycles, short-term volcanic episodicity, and the tensional stress regime imposed on young ocean crust. Two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid-ocean ridge crests. (1) Narrow axial troughs,
Geophysical and petrological boundaries on mid-ocean ridges provide ideal locations to study the relationships between magmatic, tectonic, and hydrothermal processes. Alvin-based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the East Pacific Rise (EPR) crest between ∼9°36′N and 9°38′N. This ridge-crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough (ASCT) that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area. The ASCTs overlap by ∼3 km and are offset in a right-lateral sense by 0.45 km. Near-bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval. When combined with the inferred age and structure of the seafloor, basalt geochemistry, and hydrothermal fluid chemistry, these temporal changes suggest active southward propagation of the eastern ASCT and show that the western ASCT was unaffected by the recent magmatic event. Numerous extinct hydrothermal vents and older-looking lava flow surfaces suggest waning of magmatic activity in the western ASCT.Young-looking lava flows within or proximal to the eastern ASCT have anomalously high Mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9°46′-52′N EPR region. We propose that the young-looking lava flows in the eastern ASCT are related to the 1991 eruption. Data show that the 9°37′N axial discontinuity marks a magmatic and hydrothermal boundary along the EPR ridge-crest, and we argue that it be classified as a third-order discontinuity. This result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9°37′N discontinuity.
Ocean surface waves with periods longer than 30 s create periodic, horizontally propagating pressure fields at the deep seafloor. Seafloor displacements resulting from these pressure fields depend on the density and elastic parameters of the oceanic crust. The displacement to pressure transfer function, the seafloor compliance, provides information about ocean crustal density and elasticity, and we outline a linearized inversion method to determine ocean crustal shear velocity from the compliance. By computing compliance partial differences with respect to changes in ocean crust shear velocity, we provide estimates of inversion stability and of the compliance sensitivity to crustal properties. Seafloor compliance, measured from pressure and acceleration spectra, is presented for two different sites: Axial Seamount on the Juan de Fuca Ridge and the West Cortez Basin in the California continental borderlands. The compliances and inverted structure for these two sites show significant differences; in particular, a zone of low shear strength is observed at depth within Axial Seamount, suggesting the presence of at least 3% partial melt within the upper 2500 meters of the ediface. These results suggest that the method provides a useful new geophysical prospecting tool.
High-resolution images covering large areas of the seafloor reveal numerous discontinuities along the mid-ocean ridge. These discontinuities occur at a range of scales (10−1,000 km) and define a fundamental segmentation of seafloor spreading centres. Some are transient; others persist for millions of years, migrating along the mid-ocean ridge and disrupting the structural and geochemical character of approximately 20% of the oceanic lithosphere.
Using the near-bottom ARGO imaging system, we visually and acoustically surveyed the narrow ( < 200 m wide) axial zone of the fast-spreading East Pacific Rise (EPR) along 83 km of its length (9°09′–54′N), discovered the Venture Hydrothermal Fields, and systematically mapped the distribution of hundreds of hydrothermal features relative to other fine-scale volcanic and tectonic features of the ridge crest. The survey encompasses most of a 2nd order ridge segment and includes at least ten 4th order (5–15 km) segments defined by bends or small lateral offsets of the ridge crest or axis (Devals). 4th order segmentation of the ridge crest is clearly expressed in the high-resolution ARGO data by the fine-scale behavior of the ridge axis and by changes in the characteristics of the axial zone (axial lava age, extent of fissuring, axial morphology and structure, etc.) across segment boundaries. The distribution and along-strike variability of hydrothermal features corresponds closely to the morphotectonic/structural segmentation of the ridge. On the 2nd order scale, we find that high T hydrothermal activity correlates with: (1) shallowing of the axial magma chamber (AMC) reflector to depths < 1.7 km beneath the ridge axis; and, (2) with the presence of a well-developed axial summit caldera (ASC). Previous work refers to this feature as an axial summit graben (ASG); however, the extent of volcanic collapse along the ASG revealed by the ARGO survey adds to evidence that on fast-spreading ridges it is an elongate volcanic caldera rather than a tectonic graben, and supports the introduction of “axial summit caldera” as a more accurate descriptor. All but 1 of the 45 active high T vent features identified with ARGO are located within 20 m of the margins of the ASC. Despite the significant extent of our track coverage outside the ASC, no important signs of venting were seen beyond the axial zone. On the 4th order scale, the abundance and distribution of hydrothermal features changes across 4th order segment boundaries. We find that high T vents are most abundant where: (1) the ASC is very narrow (40–70 m), (2) the AMC reflector is most shallow ( < 1.55 km beneath the axial zone), and (3) the axial lavas are youngest and least fissured. To explain the observed distribution of vent activity, a two-layer model of ridge crest hydrothermal flow is proposed in which 3-D circulation at lower T in the volcanic section is superimposed on top of axis-parallel high T circulation through the sheeted dike complex. In the model, circulation parallel to the ridge axis is segmented at the 4th order scale by variations in thermal structure and crustal permeability which are directly associated with the spacing of recent dike intrusions along strike and with cracking down into the sheeted dikes, especially along the margins of the ASC. Based on ratios between numbers of active high T vents and inactive sulfide deposits along particular 4th order segments, and on corresponding volcanic and tectonic characteristics of these segments, we suggest that the individual 4th order segments are in different phases of a volcanic-hydrothermal-tectonic cycle that begins with fissure eruptions, soon followed by magma drainback/drainage and accompanying gravitational collapse, possible development of an ASC, and onset of hydrothermal activity. The hydrothermal activity may wax and continue for up to several hundred years where an ASC is present. The latest phase in the cycle is extensive tectonic fissuring, widening of the ASC by mass wasting along its margins, and waning of hydrothermal activity. In the ARGO area, where full spreading rates are 11 cm/yr, the entire cycle takes less than ∼ 1000 years, and the tectonic phase does not develop where the time interval between eruptions is significantly less than 1000 years.
Dynamic models are presented to investigate the consequences of melting and melt transport for stable trace element geochemistry in open systems. These models show that including explicit melt transport in 2-D adds non-trivial behaviour because melts and residues can travel and mix along very different trajectories. Calculations are presented for both equilibrium and disequilibrium transport, and passive and active mid-ocean ridge flows. These calculations demonstrate that trace elements are sensitive to mantle dynamics and can readily distinguish between different end-member flow fields. Passive, plate-driven flow with strong melt focusing produces enrichments of incompatible elements. Active small-scale solid convection within the partially molten region, however, can lead to extreme dilution of incompatible elements, suggesting that this form of convection may not be significant beneath normal ridges. These calculations provide additional predictions about across-axis trends of geochemical variability and estimate the variation in concentrations that can occur even for a constant source. Many of these results are not seen in geochemical models that neglect melt transport and we discuss how this new behaviour affects the inferences drawn from simpler models.
The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties. Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14:00S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming th
In order to investigate the impact of off-axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust (layer 2A), we performed an extensive geophysical survey on the eastern flank of the East Pacific Rise at 14°S. Seismic refraction and heat flow data were obtained along a 720-km-long and 25 to 40-km wide corridor, covering thinly sedimented seafloor created since 8.5 Ma. The seismic data yield a seismic velocity of ~2.9 km/s at the top of 0.5-m.y.-old basement rocks. Within about 8 m.y. the velocity increases gradually to a value of mature oceanic crust (~4.3 km/s). Heat flow data, derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements, suggest that an open hydrothermal circulation system persists for at least 6-7 m.y. In crust older than 7 Ma, regional heat flow is close to values predicted by plate cooling models, suggesting that hydrothermal circulation is going to cease. Considering published dating of alteration minerals, it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10-15 m.y. This idea may explain why seismic velocities in the Pacific ocean have not changed significantly in igneous crust older than 8-10 Ma. In regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different; velocities increase rapidly and reach values of mature oceanic crust within 1-2 m.y. We therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust [Stephen and Harding, 1983; Rohr, 1994].
A wide-aperture profile along the ridge axis from 14°29′S to 13°39′S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 m thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13°N on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is real rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along-axis flow.
There must be a transition from continuous porous flow to transient
formation of melt-filled fractures in the region of magma transport
below oceanic spreading centers. This transition may occur at
permeability barriers that impede porous flow of melt, at the base of
the oceanic crust and within the lower crust. We summarize evidence for
formation of melt-filled lenses at the base of the crust in the Oman
ophiolite, and evidence indicating that melt is very efficiently
extracted from these lenses, probably in fractures. We also discuss the
possible formation of melt lenses elsewhere in the oceanic lower crust.
We then present a simple physical model for the periodic formation of
melt-filled fractures originating in a melt lens beneath a permeability
barrier. Finally, quantitative models show how modal layering in lower
crustal gabbros can form as a result of the periodic pressure changes
associated with fracture formation. An ancillary result of chemical
modeling is the quantification of a thermal gradient in the lower crust
of the Oman ophiolite during igneous accretion beneath a spreading
center, from 1165 to 1195°C near the dike/gabbro transition to
˜1240°C near the crust/mantle transition. This and other data
for the Oman gabbros support models in which much of the lower crust
forms by crystallization in sills at a variety of depths, from the
dike/gabbro transition to the base of the crust.
Six cross-axis and three along-axis common depth point (CDP) profiles near the 9°03′N overlapping spreading center (OSC) were reprocessed to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Travel time modeling of the AMC reflector reveals an asymmetric distribution of melt across the 9°03′N OSC. The asymmetric pattern of melt is attributed to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust. In this model, melt ascends upward (buoyancy forces) until deflected by the impermeable sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovolcanic zone where fissuring produces a temporary conduit for emplacement. Discrete jumps in modeled AMC width toward the overlap basin represent a further displacement/defocusing of melt supply (western AMC edge) relative to the neovolcanic zone (eastern AMC edge). The asymmetric pattern of melt therefore represents a gradual, en-echelon accommodation of melt supply across the 9 km of ridge axis offset at 9°03′N. -from Authors
An attempt is made to reconcile the field observations in ophiolites, which seemingly require large magma chambers, with oceanographic requirements for a smaller magma chambers. For this purpose, the geology of the gabbroic rocks of the Samail ophiolite, which is considered to have formed by fast spreading, is compared with the results of a finite element analysis of the Sinton-Detrick (1992) model for fast spreading centers. Results of the comparison demonstrate that many of the characteristics of the Samail gabbros may indeed be reconciled with the Sinton-Detrick geometry.
Many Alpine ophiolite complexes characteristically display a pseudostratiform sequence of ultramafics, gabbro, diabase, pillow lava and deep-sea sediments. These masses resemble the known rock suite from the ocean floor. They are either fragments of old oceanic crust and mantle caught up in deformed belts, or results of diapiric emplacement of partly molten mantle material on or near the sea bottom. Such complexes are widespread in the Tethyan mountain system and have been recognized also from the circum-Pacific region. The Troodos Massif, Cyprus, consists of a pseudostratiform mass of harzburgite, dunite, pyroxenite, gabbro, quartz diorite, diabase and pillow lava arranged in a dome-like manner. The diabase forms a remarkable dyke swarm, trending mostly north-south in which 100 km of extension is indicated over 100 km of exposure. Such a feature suggests formation by sea-floor spreading. Layering of pyroxenite, harzburgite and dunite generally is perpendicular to subhorizontal rock unit contacts. The harzburgite and dunite are tectonites and probably represent uppermost mantle. Pyroxenite, gabbro, quartz diorite and diabase may represent the products of partial fusion of mantle material or of fractional crystallization of such partial fusion products. Chemical compositions of mafic intrusive and extrusive rocks do not fit well with oceanic tholeiite compositions, but resemble greenstones and associated rocks recently reported from the oceans. The massif probably formed about an old Tethyan ridge. Some pillow lavas may be crust added after the main spreading episode. A fault zone active during emplacement of the lower units of the complex may represent a fossil transform fault. Complex chilled margins in the dyke swarms and mutually contradictory cross-cutting relations between dykes and plutonic mafic rock suggest formation of ocean crust by multiple intrusion of small portions of liquid. Uneven top surface of the dyke swarm and some conjugate dyke systems suggest independently varying rates of magma supply and extension. Other Tethyan ophiolites, particularly in Greece and Italy, exhibit internal structure parallel to, rather than perpendicular to, major rock units, and some show much less diversity in mafic rock type. If these masses are fragments of ocean floor and mantle, such differences in internal structure may be due to differences in spreading processes-perhaps differences in spreading rate.
Within the last decade a new picture of the oceanic crust has emerged from advances in seismic experimental design, instrumentation, and analysis techniques. In this new picture, layer 2 is a region in which velocity increases rapidly with depth. While there is evidence of finer structure within layer 2, the exact nature of this structure is still poorly resolved. Layer 3 is much more homogeneous vertically than layer 2 and appears to have gentle vertical velocity gradients and occasional low-velocity zones in νp and νs. Although it has been observed at several sites, the widespread existence of a high-velocity basal crustal layer is in doubt. The thickness of the crust-mantle transition has been observed to vary between 0 and 2 km from site to site, and even at flat lying, uncomplicated sites, seismic evidence for lateral heterogeneities on a scale of a few kilometers can be found. This seismic picture of the crust is in good agreement with the seismic velocities of rocks from ophiolite complexes and is consistent with the theoretically expected behavior of seismic velocities in porous, water-saturated rocks at elevated pressures. A review of velocity results obtained from use of synthetic seismogram modeling techniques is given, and the types of synthetic techniques suitable for marine work are described.
This thesis discusses the results of seismic tomography experiments investigating the internal structure of the East Pacific Rise (EPR) and its discontinuities. The EPR is not a linear feature but is partitioned by transform faults and non-transform offsets. Within the group of ridge-axis offsets, overlapping spreading centers (OSC) are of particular interest as they are restricted to fast-spreading mid-ocean ridges such as the EPR. Three seismic refraction surveys centered at an axial discontinuity---the 9°03'N overlapping spreading center, at the center of the fastest spreading segment at 17°30'S, and on the flanks of the same segment, are used to investigate the structure of the EPR and its variability with along-axis segmentation. Counter to earlier predictions, we observe that the 9°03'N overlapping spreading center at the distal end of the 9°N segment does not have a reduced magma supply.
Tectonic and magmatic segmentation of ridges occurs at several scales that overlap or form a continuum. A good working hypothesis is that segmentation is hierarchical and that magmatic and tectonic segmentation are linked and related by mantle flow and upwelling patterns. The largest scale of magmatic segmentation is isotopic, reflecting differences in mantle history and composition spanning a range of scales as large as individual ocean basins or larger. Superimposed, is the most conspicuous scale of segmentation, defined by large transforms and/or non-transform offsets. Magmatically, this scale of segmentation seems to reflect differences in the melting process together with the effects of smaller scale mantle heterogeneity. There appear to be differences between slow- and fast-spreading magmatic segments, although other factors such as magma supply, plume influence and ridge obliquity may also have important effects. Recurring patterns at individual segments, at least partly independent of spreading rate, include: constancy v. variability in inferred melting parameters, regular v. irregular spatial distribution of enriched basalts, MgO values, and other parameters, and correlation vs. no correlation of chemistry with axial depth.
The smallest scales of magmatic segmentation require the most closely spaced and detailed sampling. Since there have been relatively few studies of this type, there are many questions remaining. It is likely that this scale of segmentation is controlled by deep crustal and shallow upper mantle processes. However, the superimposed effects of mantle heterogeneity and time-dependent processes such as volcanic-magmatic cycles complicate the picture.
Times of vein mineral deposition in ocean crust have been determined both Rb-Sr isochron ages of vein smectites and by comparison of 87Sr/86Sr ratios of vein calcites with the known variations of seawater 87Sr/86Sr ratio with time. Results from drill sites 105, 332B and 418A, Atlantic Ocean, which have basement formation ages of 155 m.y., 3.5 m.y., and 110 m.y., respectively, show that vein deposition is essentially complete within 5-10 m.y. after formation of a basaltic crust. This provides direct evidence that hydrothermal circulation of seawater through the oceanic curst is an important process for only 5-10 m.y. after crust formation.
Multichannel seismic reflection data acquired between 8°50' and
9°50'N and between 12°30' and 13°30'N along the East Pacific
Rise provide a three-dimensional view of the young oceanic crust.
Seafloor-to-Moho reflection travel times vary by up to 0.9 s within our
study areas; the total range of crustal travel times in the 9°N area
is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05
s. The variation is systematic, indicating thinner crust locally
associated with overlapping spreading centers (OSCs) and, in the 9°N
area, segment-scale variation along crustal isochrons. Crustal travel
time is found to be a valid proxy for oceanic crustal thickness. Outside
of the axial low-velocity volume, thickness can be calculated from time
to ˜500 m. Even in the axial region thickness can be calculated to
<1 km, if low-velocity zone position is known. Crustal thicknesses
calculated from travel times vary by 2.6 km in the 9°N area, and by
1.5 km in the 13°N area. The majority of this variation is
attributed to seismic layer 3 (the lower crust). Segment-scale variation
of ˜1.8 km (˜5.5 to 7.3 km thickness) is observed in the
9°N area, with thinnest crust formed between ˜9°40' and
9°50'N and thickest formed between ˜9°15 and 9°20'N.
Results imply a three-dimensional pattern of magma supply to the 9°N
segment. The OSC at 9°03'N is associated with major disruptions of
the segment-scale pattern, in the form of local thin areas within the
discordant zone; the smaller OSC at 12°54'N is not associated with
dramatic changes in thickness of the surrounding crust. In the absence
of OSCs, the process of crustal formation displays more temporal
uniformity along flow lines than spatial uniformity along isochrons
within a segment. Thicker crust does not always correlate with shallower
ridge bathymetry, broader axial cross section, or more negative mantle
Bouguer or subcrustal gravity anomaly. Variable thickness of the
crust-mantle transition region as well as crustal flow in the axial
region may be responsible for this unexpected result. We hypothesize
that the geophysical signature of diapiric mantle upwelling beneath a
fast spreading ridge is relatively thin crust associated with a thick
Moho transition zone and a subcrustal gravity low. Such a diapiric
upwelling center appears to be now located beneath the East Pacific Rise
near 9°40' to 9°50'N.
Reprocessed multichannel seismic profiles from the 9°N segment of
the East Pacific Rise reveal prominent shallow subbasement events. These
events are identified as wide-angle reflections from the base of seismic
layer 2A, based upon modeling of expanding spread profile data and
velocity functions. The layer 2A reflections typically increase from
0.15 s after the seafloor reflection at the rise axis to 0.3-0.45 s
within 1-2 km of the axis, corresponding to an increase in layer
thickness of 200-600 m. No further systematic increase in layer
thickness is observed, although lateral variability of the order of a
few hundred meters in thickness is observed at greater offsets from the
rise axis. However, the intermittent character of the imaged layer 2A
reflection is attributed to focusing and defocusing of energy by the
seafloor bathymetry rather than necessarily to intrinsic lateral
variability at the base of the layer. The base of layer 2A is
interpreted as corresponding to the transition between the extrusive
section, pillow basalts and sheet flows, and a sheeted dike complex. The
rapid thickening of the layer near the rise axis is attributed to
successive lava flows burying the initially shallow top of the sheeted
dike complex as the layer passes through the neovolcanic zone. Lateral
variability of layer 2A can significantly affect the imaging of the
underlying axial magma chambers as average velocities within layer 2A
are approximately half that of layer 2B. For an along-axis profile,
apparent along-axis variability in the depth of the axial magma chamber
is traced to variability in the thickness of layer 2A caused by
wandering of the profile relative to axis. Within the resolution of the
data, the time delay of the magma chamber reflection relative to the
base of layer 2A is constant.
We have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates. The ridge-propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other. The ridge-propagation force increases as ridge ends approach but then declines sharply as the ends pass, so ridge segments may overlap somewhat. The sign of the path factor changes as ridge ends approach and pass, so the overlapping ridge ends may diverge and then converge following a hook-shaped path. The magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting. For different loading conditions simulating ridge push, plate pull, and ridge suction, we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated. For all conditions, the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap; thus crust in this zone may fracture and weaken in preparation for the formation of a through-going transform fault. The calculated shear planes rotate toward an orientation about 90° from the strike of ridges as the ends pass, thus favoring the orthogonal arrangement of ridges and transforms. The magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated. The mean stress is tensile in the interaction zone, so basins may form there, except in the case of ridge push loading. The planes across which the maximum tension acts are oblique to ridges, thus favoring obliquely oriented normal faults bounding the transform valley.
Describes a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9°32′N on the East Pacific Rise. Off axis, Q values in the upper 1 km average 35-50, while at depths greater than 2-3 km Q is at least 500-1000. The high levels of attenuation in the uppermost crust probably result from the combined effect of frictional, fluid flow, and scattering mechanisms. Within 1-3 km of the rise axis, Q increases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ~300-m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, then Q in layer 2A must be about 10-20. No measurements of Q are obtained in the immediate vicinity of the 1.6-km-deep axial magma lens because no wave paths cross the rise axis through this region. Along-axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along-axis variations in magma supply. -from Authors
The work presented in this dissertation addresses mid-ocean ridge processes which govern the formation of oceanic lithosphere, including: the nature of mantle flow and melt delivery to the oceanic crust, the storage of melt within the crust, crustal formation, and the penetration of seawater into the crust for hydrothermal cooling of the magmatic system. This study was made possible through the development of new marine seismic techniques, including tomographic methods that were expanded to include secondary seismic arrivals (Chapter II), seismic anisotropy (Chapters II--IV), new analysis methods (Chapter II), and parallel computation (Appendix A). The results, when combined with those of previous reports, yield an improved view of axial magmatic and hydrothermal systems, one that relates the physical characteristics of these systems to the ascent and aggregation of melt at mantle depths and its subsequent delivery to the crust. Using P wave data collected by a seismic experiment near 9°30'N on the East Pacific Rise, Chapters II--IV present tomographic images of the crust and shallowmost mantle seismic structure. The data set analyzed here has allowed, for the first time, the determination of the three-dimensional seismic structure of the lower crust and shallow mantle. The seismic structure of the rise is characterized by a low velocity volume (LVV; DeltaVp < -0.2) that extends from 1.4 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the lower crust (4--8 km) and broad in the mantle (18 km). Along the rise, the LVV varies in magnitude with the lowest velocities located between minor rise-axis discontinuities at 9°28N and 9°35' N. Shallow crustal anisotropy is 4% from 0.5--1 km depth below the sea floor, 2% from 1--2 km depth, and 0% below; the fast direction of P wave propagation is along the rise axis. Mantle anisotropy is 6--8%; the fast direction of P wave propagation is across the rise. The results indicate that (1) seismically-detectable, rise-parallel cracks form on or near the rise axis and penetrate to ˜2 km, (2) off-axis heat removal is relatively efficient throughout the crust near the rise and inefficient at Moho and mantle depths, (3) melt accumulates in two axial reservoirs, one near the top of the magmatic system and one near the Moho transition zone, (4) along-axis variations in the estimated melt content in the crust are similar to that in the mantle, implying that the mantle midway between the 9°28' and 9°35' discontinuities is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south, and (5) mantle flow is uniformly diverging from the spreading axis; no evidence for diapiric mantle flow was found in the data. Chapters II--IV were/will be published elsewhere and include co-authored material.
A magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface. The excess pressure results partly from overburden pressure caused by the presence of high- and low-density rocks (dikes and extrusives, respectively) above the lens. The thicker the pile of low-density extrusives, the lower the average overburden density. Thus, extrusion above a lens should be self-regulating, in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions. Flexural stresses may affect extrusion by altering the pressure on a magma chamber. For ridges lacking an axial valley, we predict that deeper magma lenses should correlate with thicker extrusive layers, consistent with recent observations.
We have reprocessed seven cross-axis common depth point (CDP) profiles between the Clipperton transform and the 9°17'N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross-axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9°53'N-9°35'N Devals, the 9°35'N-9°17'N Devals, and south of the 9°17'N Deval were
We systematically investigated near-axis subsidence on the ridge flanks of intermediate and fast spreading mid-ocean ridges using bathymetric data from well-surveyed portions of the Southeast Indian Ridge (spreading at 72-76 mm/yr), the northern East Pacific Rise (91-96 mm/yr), and the southern East Pacific Rise (144 mm/yr). In all three regions, the mean subsidence rate of young (
The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower crustal gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The gabbroic sills and the layered gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid-ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the gabbro sills indicate that they formed from small, open-system, melt-filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (
With complete SeaBeam and SeaMARCII bathymetry and nearly complete SeaMARCII side scan coverage of the northern East Pacific Rise and its flanks from 8° to 17°N, the entire population of seamounts on young seafloor (0 to ~2 Ma) formed along this fast spreading center is characterized. There are 179 seamounts in this area taller than 200 m, most of which belong to one of 21 seamount chains. These chains are oriented between the relative and absolute plate motion directions of the Pacific and Cocos Plates. Observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near-axis seamounts are formed in a narrow zone on crust aged ~0.1-0.3 Ma with significant growth in a zone several times wider. A compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge. -from Authors
Seafloor compliance measurements across the East Pacific Rise at 9°48′N reveal low shear velocities throughout the crust and at the crust-mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9°48′N, the shear velocity is less than 20 m/s in the shallow on-axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on-axis melt lens 5.4 ± 1 km beneath the seafloor, with shear velocities slower than 50 m/s. This “deep” melt lens may be created by melt pooling at a permeability or density barrier at the crust-mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5–18% melt. Melt persists in the lower crust to at least 10 km off-axis, where the top of the lower crustal low-velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on-axis to 0.58 by 10 km off-axis, indicating that there are abundant thin cracks in the sheeted dikes on-axis and that these cracks close away from the rise axis. High on-axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.
The deep seafloor deforms under the pressure loading of linear ocean surface gravity (water) waves at low frequencies (0.003 to 0.04 Hz). The ratio of seafloor displacement to pressure loading as a function of frequency, known as the seafloor compliance function, depends on the shear velocity structure of the oceanic crust and upper mantle. Compliance measurements are used to estimate oceanic crustal structure, particularly within low shear velocity regions such as sediments, fractured rock, and partial melt. Compliance calculated from laterally homogeneous (one-dimensional, l-D) crustal models shows that a buffed low-velocity zone (LVZ) causes a peak in the compliance function at wavelengths 4 to 6 times longer than the LVZ depth, and that the compliance amplitude depends primarily on the LVZ shear velocity. A new numerical code allows forward modeling of compliance for two-dimensional oceanic crustal models. The new code demonstrates that the peak in the compliance function directly over a finite width LVZ reaches a maximum value at higher frequency, and is of smaller amplitude, than predicted from 1-D modeling. The compliance maximum persists outside of the region underlain by the LVZ but diminishes in amplitude and shifts to lower frequencies with increasing distance from the LVZ. The numerical models indicate that significant peaks in the compliance function indicate crustal LVZs, but that multiple compliance measurements are necessary to independently constrain the depth, location, and shear velocity of these features.
MCKENZIE'S model of crustal creation at the ocean ridges1,2 and its derivatives3,4 predicts such features as the topography and high heat flow of the ridges. In spite of this success there are some unsatisfactory aspects of the model; for example, the arbitrary temperature distribution in the intrusive zone gives rise to infinite heat generation and the lithospheric thickness is a free parameter not determined by the physics. We offer here a simple refinement of McKenzie's model that overcomes these difficulties. The essential difference stems from the inclusion of terms in the boundary conditions to account for the evolution of latent heat in places where the plate is growing. We first describe the physical basis of the model.
Three-dimensional images of crustal seismic structure beneath the East Pacific Rise show pronounced axial heterogeneity over distances of a few kilometres. A linear high-velocity anomaly, approximately 1–2 km in width and restricted to the uppermost 1 km of the crust, is centred on the rise axis. An axial low-velocity anomaly at depths of 1–3 km varies in amplitude along the axis, consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis. This apparent thermal segmentation along axis is consistent with injection of mantle-derived melt midway along a locally linear, 12-km-long segment of the rise.
Recent models of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section of the oceanic crust crystallizes from a thin melt lens located just below the sheeted dike complex. The shape of the lens has been deduced from seismic reflection data at fast-spreading ridges. On the basis of structural studies in the Oman ophiolite, we suggest that the accretion of the lower crust may not proceed entirely in this way. We emphasize the contrast between: (1) upper level gabbros characterized by a magmatic foliation which, from a flat attitude at depth, rapidly steepen upward and tend to become oriented parallel to the sheeted dikes; and (2) lower gabbros, flat-lying, magmatically deformed, and more or less strongly layered. Wehrlite layers and lenses which contribute to the layering of these gabbros have previously been interpreted as sills. We suggest here that the modally graded bedding, which is an important feature of the lower layered gabbros, may have similarly originated as sills. This is deduced from the fact that, above mantle diapirs, the several hundred metre thickness of the transition zone contains sills of layered gabbros, commonly organized in modally graded sequences. These sills, which are interlayered with dunite or harzburgite, contain gabbros which are shown here to be structurally similar to those in the layered gabbro unit at all scales. If this interpretation is correct, the gabbro section of the oceanic crust in Oman is built up by crystallization, both along the walls and the floor of the perched magma lens, followed by subsidence, and also in sills intruded either in the subsiding foliated gabbros or in the mantle dunites of the Moho transition zone. Supply from the perched melt lens generates the upper foliated gabbros, and supply by sills emplaced near Moho level gives rise to the basal layered gabbros and the gabbro-troctolite lenses of the transition zone. Feeding of the perched melt lens by vertical dikes and feeding of the Moho horizon by sills may correspond to successive stages of a basaltic melt injection episode.
Although considerable progress has been made in the study of fast-spreading, mid-ocean ridge magma chambers over the past fifteen years, the fraction of melt present in the chamber remains poorly constrained and controversial. We present new constraints obtained by modelling the seismic properties of partially molten gabbros at the ridge axis. P-wave velocities at low frequencies are calculated in the foliation=lineation reference frame using a differential effective medium technique. The model takes into account the lattice preferred orientation of the crystalline phase and the average shape of the melt phase. The structural parameters are obtained from the Oman ophiolite. The structural reference frame is given by the general trend of the gabbro foliation and the melt fraction and shape are estimated using the textures of nine upper gabbro samples. The estimated melt fraction and shape depend on the assumptions regarding which part of the observed textures represent the melt in the gabbroic mush of the magma chamber. However, we can put limits on the reasonable values for the melt fraction and shape. Our results are consistent with a melt fraction of the order of 10 to 20% in the Low-Velocity Zone (i.e. the magma chamber), which is anisotropically distributed with the melt pockets preferentially aligned parallel to the foliation and approximated by oblate ellipsoids with approximate dimensions of 4 : 4 : 1. These results are also consistent with the seismic structure of the East Pacific rise at 9º300. The anisotropic melt distribution can, at least partially, explain the vertical velocity gradient described in the LVZ.
The inversion of electromagnetic sounding data does not yield a unique solution, but inevitably a single model to interpret the observations is sought. We recommend that this model be as simple, or smooth, as possible, in order to reduce the temptation to overinterpret the data and to eliminate arbitrary discontinuities in simple layered models. The inversion of both magnetotelluric and Schlumberger sounding field data, and a joint magnetotelluric-resistivity inversion, domonstrate the method and show it to have practical application.-from Authors
The hydrothermal processes at ridge crests have been extensively studied during the last two decades. Nevertheless, the reasons why hydrothermal fields are only occasionally found along some ridge segments remain a matter of debate. In the present study we relate this observation to the mineral precipitation induced by hydrothermal circulation. Our study is based on numerical models of convection inside a porous slot 1.5 km high, 2.25 km long and 120 m wide, where seawater is free to enter and exit at its top while the bottom is held at a constant temperature of 420°C. Since the fluid circulation is slow and the fissures in which seawater circulates are narrow, the reactions between seawater and the crust achieve local equilibrium. The rate of mineral precipitation or dissolution is proportional to the total derivative of the temperature with respect to time. Precipitation of minerals reduces the width of the fissures and thus percolation. Using conventional permeability versus porosity laws, we evaluate the evolution of the permeability field during the hydrothermal circulation. Our computations begin with a uniform permeability and a conductive thermal profile. After imposing a small random perturbation on the initial thermal field, the circulation adopts a finger-like structure, typical of convection in vertical porous slots thermally influenced by surrounding walls. Due to the strong temperature dependence of the fluid viscosity and thermal expansion, the hot rising fingers are strongly buoyant and collide with the top cold stagnant water layer. At the interface of the cold and hot layers, a horizontal boundary layer develops causing massive precipitation. This precipitation front produces a barrier to the hydrothermal flow. Consequently, the flow becomes layered on both sides of the front. The fluid temperature at the top of the layer remains quite low: it never exceeds a temperature of 80°C, well below the exit temperature of hot vent sites observed at black or white ‘smokers’. We show that the development of this front is independent of the Rayleigh number of the hydrothermal flow, indicating that the mineral precipitation causes cold, diffusive vents. Finally, we present a model suggesting that the development of smokers is possible when successive tectonic/volcanic events produce a network of new permeable fissures that can overcome the permeability decrease caused by mineral precipitation. Such a model is consistent with recent seismic data showing hydrothermal vents located at seismologically active ridge segments.
Gabbroic sills intruding dunite in the crust-mantle transition zone (MTZ) of the Oman ophiolite have textures and compositions very similar to those in modally layered gabbros that form the lower part of the gabbro section in the ophiolite, and different from those in non-layered gabbros near the dike-gabbro transition. The presence of gabbroic sills in the MTZ indicates that modally layered gabbros can form far below the level of magmatic neutral buoyancy and far below the dike-gabbro transition. Minerals in the sills and lower, layered gabbros are in FeMg and trace element exchange equilibrium with liquids identical to those that formed the sheeted dikes and lavas in the ophiolite. In contrast, many of the upper, non-layered gabbros resemble crystallized liquid compositions, similar to the dikes and lavas. The lower, layered gabbros probably formed in sills similar to those in the MTZ. Mantle-derived magmas cooled in these sills, where they crystallized from a few percent to 50% of their mass. Residual liquids then rose to form upper gabbros, dikes and lavas. Sills may form beneath permeability barriers created by the crystallization of cooling liquid migrating by porous flow. Once permeability barriers are present, however, porous flow becomes a less important mode of magma ascent, compared to ponding in sills, gradual increase in magma pressure, and periodic ascent in hydrofractures. Thus, gabbroic sills in the MTZ may represent the transition in fast-spreading ridge environments from continuous porous flow in the mantle to periodic diking in the crust.
With the aim of a simultaneous interpretation of elastic, anelastic and electric in situ data from the asthenosphere a comprehensive set of numerical models is developed for partial melt in different geometrical configurations. For the elastic and anelastic modulus use is made throughout of the melt squirt mechanism. Frequency dependence is not treated in detail but estimated from the limiting cases of the relaxed and unrelaxed modulus. This has the advantage that quantitative values of viscocity and flow path dimensions are not required. In the models melt can be assumed to occur in the form of tubes, films, and triaxial ellipsoidal inclusions of arbitrary aspect ratio. The conditions in which the solutions for triaxial ellipsoidal inclusions can be approximated by simpler ones for spheroidal inclusions are discussed. It is then shown up to which aspect ratio a published model on melt films is applicable. The problem of interconnection of inclusions is treated with a statistical numerical approach. It is found that a reduced degree of interconnection may have a significant influence on anelastic relaxation at melt fractions corresponding to a moderate modulus decrease. A useful representation of the anelastic melt models is introduced by plotting the relaxation strength against the effective modulus, both of which depend on the state of melting. Such diagrams allow a clear distinction between the different melt geometries and may be used for the interpretation of observed data. Finally, different melt geometries are superimposed and it is found that under certain conditions bulk dissipation may reach the order of that for shear.
The Rock Physics Handbook addresses the relationships between geophysical observations and the underlying physical properties of rocks. It distills a vast quantity of background theory and laboratory results into a series of concise chapters that provide practical solutions to problems in geophysical data interpretation. This expanded second edition presents major new chapters on statistical rock physics and velocity-porosity-clay models for clastic sediments. Other new and expanded topics include anisotropic seismic signatures, borehole waves, models for fractured media, poroelastic models, and attenuation models. This new edition also provides an enhanced set of appendices with key empirical results, data tables, and an atlas of reservoir rock properties – extended to include carbonates, clays, gas hydrates, and heavy oils. Supported by a website hosting MATLAB routines for implementing the various rock physics formulas, this book is a vital resource for advanced students and university faculty, as well as petroleum industry geophysicists and engineers.
Interpretation of seismic velocity and attenuation in partially molten rocks has been limited, with few exceptions, to models that assume the melt to be distributed either as spheres or as thin films. However, other melt phase geometries, such as interconnected tubes along grain edges, might equally well account for seismic observations if there is a much larger fraction of melt. Seismic velocity and attenuation are estimated in rocks in which the melt phase has the tube geometry, and the results are compared with results expected for the more familiar film model under similar conditions. For a given melt fraction, tubes are found to give moduli intermediate between moduli for rigid spherical inclusions and compliant films. For example, in polycrystalline olivine at 20 kbar the model predicts a decrease in Vs of 10% and a decrease in Vp of 5% at 0.05 melt fraction, without considering inelastic relaxation. Shear attenuation appears to be dominated by viscous flow of melt between the tubes and/or films.
The topographic features known as abyssal hills characterize >30% of
the ocean floor, and yet their origin has been the subject of vigorous
debate for over 40 years. Submersible-based investigations show that
Pacific abyssal hills are created on the flanks of the East Pacific Rise
as horsts and grabens which lengthen with time. Hills are bounded on one
side by ridge-facing scarps produced by normal faulting, and on the
other by more gentle slopes produced by volcanic growth faulting.