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Mid-Ocean Ridge Magma Chambers
Abstract
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
... The strength of young oceanic lithosphere is largely influenced by the thermal regime of mid-ocean ridges (MORs) (1)(2)(3), which is shaped by a dynamic balance between heat supplied by magma cooling and crystallization, and heat lost through hydrothermal circulation (4)(5)(6)(7). Seismically imaged axial melt lenses (AMLs) and/or low P wave velocity anomalies (LVAs) are strong indicators of the thermal structure of MORs, as they constitute proxies for the basaltic solidus (~1,000 °C) that encloses crystal mush zones at the ridge axis ( Fig. 1 A and B) (4,8,9). Standard thermal models (4,5) predict MOR thermal regimes that dramatically cool as the full spreading rate decreases below 60 km/My, such that a steady-state AML cannot exist within the crust at spreading rates below ~40 km/ My (Fig. 1C). ...
... The top boundary (i.e., seafloor) temperature (T 0 ) is set to 0 °C, and the bottom boundary temperature (T b ) is imposed at 1,300 °C. The basaltic solidus (T S ) and the liquidus (T M ) are set to 1,000 and 1,200 °C, respectively, based on the average composition of mid-ocean ridge basalts (8). The initial temperature field is set to linearly increase from 0 °C to 1,000 °C between the seafloor and 1.5 km depth and to linearly increase from 1,000 °C to 1,300 °C between 1.5 km and the bottom of the domain. ...
... We also consider that the amount of melt at every injection (dike plus AML) is constant at W AML ⋅ H AML [1]. Combining [1] to [7], we get: [8] where W AML and H AML are fixed, and τ can be calculated from U and Hc according to Eq. 1. H dike is measured as Z AML if the AML lies at crustal depths, F dike is a parameter to be chosen, and H AML ' can be calculated once F dike is given. ...
The thermal state of mid-ocean ridges exerts a crucial modulation on seafloor spreading processes that shape ~2/3 of our planet's surface. Standard thermal models treat the ridge axis as a steady-state boundary layer between the hydrosphere and asthenosphere, whose thermal structure primarily reflects the local spreading rate. This framework explains the deepening of axial melt lenses (AMLs)—a proxy for the basaltic solidus isotherm—from ~1 to ~3 km from fast- to intermediate-spreading ridges but fails to account for shallow crustal AMLs documented at slow-ultraslow spreading ridges. Here, we show that these can be explained by a numerical model that decouples the potentially transient ridge magma supply from spreading rate, captures the essential physics of hydrothermal convection, and considers multiple modes of melt emplacement. Our simulations show that melt flux is a better thermal predictor than spreading rate. While multiple combinations of melt/dike emplacement modes, permeability structure, and temporal fluctuations of melt supply can explain shallow crustal AMLs at slow-ultraslow ridges, they all require elevated melt fluxes compared to most ridge sections of comparable spreading rates. This highlights the importance of along-axis melt focusing at slow-ultraslow ridges and sheds light on the natural variability of their thermal regimes.
... The anharmonic thermal model is calculated by adding the temperature anomaly estimated from the V p anomaly to a one-dimensional oceanic thermal model, while anelasticity is incorporated by iteratively updating the temperature model (see Methods) 46,47 . We try to explain the lowvelocity zones as much as possible by thermal anomalies alone below layer 2A, but partial melt is required for temperatures above 1150°C 48 . Assuming a two-phase effective medium consisting of solid and molten basalt (crystal mush with interstitial partial melt) 5,49 , we estimate the melt fraction in regions with temperatures greater than 1150°C ( Fig. 3; Methods). ...
... This agrees with those estimated by seismic tomography from OBS and MCS data 5,10 , which is about two orders of magnitude greater than the volume emplaced during recent eruptions. The spatially variable melt content forms thermally and chemically zoned magma reservoirs in shallow crust 48,51 , which generated seismic reflections with varying amplitudes 11 and was tapped by a dike feeding the 2015 lava flows with a range of MgO content 9 . ...
The architecture of magma plumbing systems plays a fundamental role in volcano eruption and evolution. However, the precise configuration of crustal magma reservoirs and conduits responsible for supplying eruptions are difficult to explore across most active volcanic systems. Consequently, our understanding of their correlation with eruption dynamics is limited. Axial Seamount is an active submarine volcano located along the Juan de Fuca Ridge, with known eruptions in 1998, 2011, and 2015. Here we present high-resolution images of P-wave velocity, attenuation, and estimates of temperature and partial melt beneath the summit of Axial Seamount, derived from multi-parameter full waveform inversion of a 2D multi-channel seismic line. Multiple magma reservoirs, including a newly discovered western magma reservoir, are identified in the upper crust, with the maximum melt fraction of ~15–32% in the upper main magma reservoir (MMR) and lower fractions of 10% to 26% in other satellite reservoirs. In addition, a feeding conduit below the MMR with a melt fraction of ~4–11% and a low-velocity throat beneath the eastern caldera wall connecting the MMR roof with eruptive fissures are imaged. These findings delineate an asymmetric shallow plumbing system beneath Axial Seamount, providing insights into the magma pathways that fed recent eruptions.
... The layered structure of oceanic lithosphere is well studied by seismic observations of modern oceans and through geological investigations of ophiolites (e.g., Cann 1970;Detrick et al. 1987;Nicolas and Boudier 2015;Sinton and Detrick 1992), in which two major lithological or seismic boundaries have been well documented, i.e., the dike-gabbro transition between the upper and lower crust, and the Moho between the lower crust and mantle (Karson 2018;Koepke and Zhang 2021;Sinton and Detrick 1992). The dike-gabbro transition, occurring at the base of sheeted dike complexes and above lower crustal gabbros, has long been regarded as a site where magmatic and hydrothermal systems interact (e.g., Coogan et al. 2003;France et al. 2009France et al. , 2021Gillis 2008; Communicated by Dante Canil. ...
... The layered structure of oceanic lithosphere is well studied by seismic observations of modern oceans and through geological investigations of ophiolites (e.g., Cann 1970;Detrick et al. 1987;Nicolas and Boudier 2015;Sinton and Detrick 1992), in which two major lithological or seismic boundaries have been well documented, i.e., the dike-gabbro transition between the upper and lower crust, and the Moho between the lower crust and mantle (Karson 2018;Koepke and Zhang 2021;Sinton and Detrick 1992). The dike-gabbro transition, occurring at the base of sheeted dike complexes and above lower crustal gabbros, has long been regarded as a site where magmatic and hydrothermal systems interact (e.g., Coogan et al. 2003;France et al. 2009France et al. , 2021Gillis 2008; Communicated by Dante Canil. ...
The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km²) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms.
... (e.g.,Melson et al., 1976;Sinton and Detrick, 1992; Sanfilippo et al., 2018). Namely, while at the 673 East Pacific Rise primitive compositions (Mg# > 60) are limited to a few basaltic glasses, they 674 represent the majority of basalts recovered along the Gakkel Ridge (i.e., Gale et al., 2013 and 675 references therein). ...
At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.
Processes taking place within the magma plumbing system can exert an important control on the composition of mid-ocean ridge basalts (MORB). Plagioclase ultraphyric basalts (PUBs) found at magma-poor mid-ocean ridges exhibit diverse disequilibrium characteristics, which can provide vital insights for distinguishing the complex effects of melt transport from those of source heterogeneity on the compositions of MORBs. Here, we present new insights into magmatic processes using integrated petrologic and geochemical studies of the PUBs from two zones (~ 50° and ~ 64°E longitude) along the ultraslow-spreading southwest Indian ridge (SWIR). The studied PUBs have complex mineral morphologies, including skeletal and acicular crystals, glomerocrysts with open and closed structure, reverse and normally zoned crystals and external and internal resorption even in single samples. Both low- and high-Fo olivine and An plagioclase crystals are in disequilibrium with their matrix glasses. Some plagioclase phenocrysts have repeated oscillatory zoning (An77–86) going from their core to rim and an abrupt decrease in An content toward the rim. Disequilibrium Sr isotopic compositions are present at several scales: between cores and rims of plagioclase crystals, between different plagioclase crystals and between plagioclase and their host lavas. Inferred pressures of magma storage range from 0.3 to 11.3 kbar. The textural and compositional diversity of crystals together with the variability in melt compositions reflect the combined influences of source heterogeneity and magmatic processes (e.g. crystallization, assimilation and magma mixing processes) taking place within crystal mushes. Our data combined with previous studies suggest that the magmatic processes within the SWIR magma plumbing system involve formation, disaggregation and juxtaposition of crystal-rich mush zones.
Owing to their abundance and relative availability on Earth's seafloor, mid‐ocean ridge basalts (MORBs) have a well‐defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N‐MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N‐MORB composition. Utilizing revised parameterizations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallization events and melt‐crystal interaction. Modeling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N‐MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region.
Owing to their abundance and relative availability on Earth’s seafloor, mid-ocean ridge basalts (MORBs) have a well-defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N-MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N-MORB composition. Utilising revised parameterisations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallisation events and melt-crystal interaction. Modelling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N-MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region.
Apparently the ferrobasalts form in isolated magma chambers, which prevents them from mixing with primitive olivine tholeiite supplied to the East Pacific Rise magma chamber. (Preceding abstracts) -K.A.R.
We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0-0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. At the seafloor we find very low VP and VS/VP values around 2.2 km/s and ≤ 0.43. In the topmost 100-200 m of the crust, VP remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ~ 500 m below the seafloor. High attenuation values (QP < 100) are suggested in the topmost ~ 500 m of the crust. The layer 2-3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where VP ≃ 3 km/s and VS = 0. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. -from Authors
Abundant oceanic gabbros created in slow-spreading ridges have been collected by dredging, drilling or with submersibles (Atlantic ocean crust, Mid-Cayman Rise, South West Indian Ridge). A review of published studies as well as work in progress show that these gabbros may be extensively metamorphosed and more or less deformed. Structural and petrological investigations suggest that shearing starts in the lower crust at very high temperature, before the complete solidification of the magma chamber. Continuing shearing allows seawater penetration and formation of synkinematic amphibole as temperature decreases. In the absence of ductile deformation, metamorphic reactions result from interaction between gabbros and a seawater-derived fluid phase circulating through a crack network.
We propose a model to explain the metamorphic and deformational characteristics of oceanic gabbros. We suggest that early lithospheric stretching beneath the ridge allows seawater penetration in the lower crust when it is still very hot, through permeability created by shear zones and associated synkinematic cracks. Therefore, hydration of the lower crust starts at high temperature (750°C), in contrast with a simple cracking front model in which hydration starts at temperature below 500°C. The amount of stretching may be related to the spreading rate through the magma budget.
Gabbroic series from ophiolite complexes may show either this early stretching and high temperature metamorphism associated with ductile shear zones (Western Alps ophiolites) or a crack network related to the cracking front and moderate temperature metamorphism (Haylayn massif, Oman ophiolite).
Recent seismic studies along the northern East Pacific Rise have documented the existence of a thin, narrow crustal magma body that is significantly smaller than the magma chambers incorporated into many earlier ridge crest geological models. The predominately molten part of the chamber is only 1–2 km wide and less than a kilometer thick, although it can extend as a nearly continuous feature for distances of several kilometers to several tens of kilometers along the ridge crest. This thin, sill-like body of melt is surrounded by a much wider zone of anomalously low seismic velocities that is interpreted as ranging from a partially molten crystal mush to the solidified (but still hot) plutonic rocks of the lower oceanic crust. This magma-sill model of a mid-ocean ridge magma chamber has important implications for the petrological and geochemical variability of mid-ocean ridge basalts and the origin of the thick cumulate sections found in ophiolites.