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The earliest mantle fabrics formed during subduction zone infancy
Abstract
Harzburgites obtained from the oldest crust–mantle section in the
Philippine Sea plate (˜52 Ma) along the landward slope of the
southern Izu–Ogasawara Trench, preserve mantle fabrics formed
during the infancy of the subduction zone; that is during the initial
stages of Pacific plate subduction beneath the Philippine Sea plate. The
harzburgites have relatively fresh primary minerals despite of their
heavy serpentinizations, and show inequigranular interlobate textures,
and crystal preferred orientation patterns in olivine (001)[100] and Opx
(100)[001]. The harzburgites have the characteristics of residual
peridotites, whereas the dunites, obtained from the same location as the
harzburgites, provide evidence for the earliest stages of arc volcanism
during the inception of subduction. We propose that the (001)[100]
olivine patterns began forming in immature fore-arc mantle with an
increase in slab-derived hydrous fluids during the initial stages of
subduction in in situ oceanic island arc.
... Despite extensive work to elucidate the genesis of these diverse olivine fabrics [7][8][9][10][11][12][13][14][15][16][17][18], the mechanisms of their formation remain poorly constrained [12][13][14][15][16][17][18]. A simple model of olivine fabric has been established in the last decades, presenting the fabric types in a three-dimensional space encompassing temperature, stress, and content, which holds significant implications for inferring the temporal and spatial distribution of olivine fabrics and their impact on seismic anisotropy in subduction zones [12][13][14][15][16][17][18][19][20][21]. These olivine CPOs are commonly categorized into five fabric types, A, B, C, D, and E, which have been established based on the results of experimental studies and another olivine fabric type, previously known as the axial-[010] or [010]-fiber pattern named AG type, following on from the five fabric types of Karato [22]. ...
... Our understanding of the microstructures in olivine in the Mariana Trench is still very limited. Harigane et al. (2013) examined the deformation fabric of harzburgite in the southern landward slope of the Izu-Ogasawara Trench, and suggest that it preserves an early-stage mantle structure from the subduction zone [20]. Michibayashi et al. (2007) conducted preliminary studies on peridotites in the southern Mariana Trench, revealing significant fabric variations among samples, attributed to mantle melting and wedge complexities [27]. ...
... The coexistence of different types of dislocations within the same crystal further suggests that olivine crystals undergo dislocation slip at varying temperatures, with free dislocations forming at relatively low temperatures and dislocation arches and rings developing at higher temperatures within the same crystal (Figure 7). Harigane et al. (2013) studied the deformation fabric of gabbro in the southern land of the Izu-Bonin Trench [20]. They believe that these gabbro peridotites preserve the mantle structure formed in the early stage of the subduction zone and provide information of the initial stage of Pacific Plate subduction under the Philippine Sea Plate. ...
Olivine, the most abundant mineral in the upper mantle, exhibits elastic anisotropy. Understanding the seismic anisotropy and flow patterns in the upper mantle hinges on the crystallographic preferred orientation (CPO) of olivine. Similarly, hydrous minerals, which also display elastic anisotropy, play a crucial role in explaining seismic anisotropy in numerous subduction zones. High-temperature and -pressure simple shear experiments reveal that the CPO of amphibole can lead to significant seismic anisotropy. In this study, peridotite samples originating from the southern end of the Mariana Trench, commonly containing amphibole, were analyzed. The microdeformation fabric and seismic anisotropy were examined. The results indicate a weak fabric strength in olivine, yet identifiable deformation fabrics of A/D, D, and AG were observed. Various dislocation structures suggest that olivine experiences complex deformation across various temperatures. Not only can the original slip system transform, but the melt/fluid resulting from melting also has a substantial impact on the peridotite. Deformation precedes the melt/rock interaction, resulting in a strong melt/rock reaction under near-static conditions. Furthermore, the modal content of amphibole significantly alters the seismic anisotropy of peridotite. An increase in amphibole content (types I, III, and IV) enhances seismic anisotropy, particularly for type I amphibole. Notably, the presence of type I fabric amphibole promotes the Vs1 polarization direction parallel to the trench in subduction zones, a phenomenon observed in other subduction zones. Therefore, when considering mantle peridotite regions rich in amphibole, the impact of amphibole on seismic anisotropy must be accounted for.
... The serpentinized plagioclase-bearing wehrlite sample (KH07-02-D31-101) was collected from the landward slope of the Izu-Ogasawara trench during a dredging cruise (KH07-02 leg 4) of the R/V Hakuho-Maru operated by the Japan Agency for Marine-Earth Science and Technology (Harigane et al., 2013). In previous studies (Morishita et al., 2011;Harigane et al., 2013) it was reported that peridotite, troctolite, pyroxenite, gabbro, dolerite and basalt were collected from 5293 to 5792 metres below sea level during dredging and remotely operated vehicle operations. ...
... The serpentinized plagioclase-bearing wehrlite sample (KH07-02-D31-101) was collected from the landward slope of the Izu-Ogasawara trench during a dredging cruise (KH07-02 leg 4) of the R/V Hakuho-Maru operated by the Japan Agency for Marine-Earth Science and Technology (Harigane et al., 2013). In previous studies (Morishita et al., 2011;Harigane et al., 2013) it was reported that peridotite, troctolite, pyroxenite, gabbro, dolerite and basalt were collected from 5293 to 5792 metres below sea level during dredging and remotely operated vehicle operations. Some of the doleritic samples exhibited a fore-arc tholeiitic basalt signature (cf. ...
... Ishizuka et al., 2011). The variations in the rock samples suggest that this trench wall represents the structure of an immature fore-arc mantle at the time of initiation of the subduction zone (Morishita et al., 2011;Harigane et al., 2013). The plagioclase-bearing wehrlite sample was covered by layers of brown weathered material about 10 mm thick (Supplementary Data Fig. ...
Serpentinization of oceanic lithosphere commonly proceeds with the development of mesh texture. Examination of a serpentinized harzburgite and a plagioclase-bearing wehrlite revealed conspicuous zoning of Al in a serpentine mesh texture, with Al-rich cores and Al-poor rims, as well as Al-rich veins, indicating local transport of Al from plagioclase and pyroxene during serpentinization. To reveal the influences of Al on the reaction mechanisms and textural development during serpentinization, we conducted hydrothermal experiments in the olivine (Ol)-plagioclase (Pl)-H 2 O system and analyzed the variations in mineralogy and microtexture of olivine replacements as a function of distance from the Ol-Pl boundary. The Al-Si metasomatic zone, where Al-serpentine and a minor amount of Ca-saponite were formed, was developed in the Ol-hosted region close to the Ol-Pl boundary. Far from the Ol-Pl boundary, Al-free serpentine, brucite, and magnetite were formed, indicating the progress of an 'isochemical' reaction (separate from water). The aggregates of Al-serpentine after olivine in the meta-somatic zone showed a characteristic zoning of Al. Microtextural evidence indicates that the zoning was produced in response to the migration of an Al metasomatic front that involved an early-stage of serpentinization with an Al-free solution, and the subsequent pseudomorphic replacement of olivine and simultaneous development of overgrowths as the amount of Al increased in the solution. The Al-bearing aqueous solution caused the formation of olivine pseudomorphs and this contrasts with the lack of preservation of original olivine outlines in the isochemical zones. Comparisons of zoning in natural and experimentally produced mesh textures suggest that Al-poor rims in the mesh texture form at the start of the serpentinization process, followed by the coupled formation of Al-rich mesh cores and Al-rich veins. Our experimental results indicate that Al-zoning in the mesh texture represent the transition from a closed to an open system during serpentinization.
... The deformation characteristics and seismic properties of natural fore-arc peridotites, as well as their seismic implications for fore-arc regions, have been less well addressed (e.g. Mehl et al., 2003;Mizukami et al., 2004;Michibayashi et al., 2007Michibayashi et al., , 2009Tasaka et al., 2008;Soustelle et al., , 2013Harigane et al., 2013). As one of the most important consequences of plastic deformation, the crystal preferred orientations (CPOs) or fabrics of olivine have been used to infer the deformation states (e.g. ...
... Nevertheless, a simple model of olivine fabric occurrences in the 3D space of temperature, stress, and water content has been well established experimentally [see the summaries by Jung et al. (2006) and Karato et al. (2008)]. Benefiting from incorporating the influences of temperature and water content, this model is useful to infer the spatio-temporal distribution patterns of olivine fabrics and to investigate their effects on seismic anisotropies in subduction zones (Fig. 2a, b, d and e;Kneller et al., 2005Kneller et al., , 2007Ohuchi et al., 2012;Harigane et al., 2013;Nagaya et al., 2014). In contrast, Pré cigout & Almqvist (2014) recently proposed a different pattern of olivine fabric distribution in the mantle wedge ( Fig. 2c and f), in which the variation of olivine fabrics (A-to B-type transition) is controlled by the deformation mechanism of dislocation-accommodated grain boundary sliding (DisGBS), which is sensitive to temperature and independent of water content (Pré cigout & Hirth, 2014). ...
... Regardless of the pattern differences within these proposed models of olivine fabric distribution in the mantle wedge, one common aspect of these models is that they almost all hypothesize that the fore-arc mantle consists of only one type of olivine fabric ( Fig. 2; either B-, C-, or E-type). This hypothesis seems valid because of the progressive findings of B- (Song & Su, 1998;Mizukami et al., 2004;Tasaka et al., 2008;Jung, 2009;Jung et al., 2014;Nagaya et al., 2014;Kim & Jung, 2015;Park & Jung, 2015) and E-type (Jung, 2009;Mehl et al., 2003;Tommasi et al., 2006;Palasse et al., 2012;Harigane et al., 2013) olivine fabrics in fore-arc peridotites. However, this hypothesis may be only partly correct, because none of the proposed models could explain the frequent coexistence of these diverse olivine fabrics, and the universal occurrence of A-/D-type fabrics in natural peridotites from both fore-arc and back-arc settings (e.g. ...
The fore-arc mantle above a subducting slab is a unique site where complex partial melting, melt/fluid–rock interaction, and deformation of mantle rocks occur. To constrain these geological and geodynamic processes we analyzed the deformation microstructures, crystal preferred orientations (CPOs or fabrics), and water content in natural harzburgites that occur as exhumed massifs in the North Qilian suture zone, NW China. These harzburgites are very fresh, and have mineral assemblages of olivine (~81–87 vol. %), orthopyroxene (~11–17 vol. %), clinopyroxene (~1–2 vol. %), and spinel (~1 vol. %). Detailed analyses of mineral textures, CPO patterns, and rotation axis distributions suggest that the plastic deformation of olivine and pyroxene was accommodated by activating a series of slip systems of dislocation. The olivine (A-/D-type fabric) shows dominant (010)[100] and/or (001)[100] slip systems, as well as other minor [100]-glide, {0kl}[100], and [001]-glide slip systems. The orthopyroxene shows dominant (100)[001] and subordinate (010)[001] slip systems, with minor (100)[010] and (100)[0vw] slip systems. The water content is extremely low in the orthopyroxene (38–44ppm by weight), equilibrated olivine (4–7 ppm), and bulk-rock samples (9–14 ppm). Previously published refractory mineral and whole-rock compositions, as well as estimated low-pressure (~1–2 GPa), high-temperature (~1100–1300°C), low-stress (~1–4 MPa) and water-poor conditions of deformation, suggest that these harzburgites represent a remnant of fossil fore-arc lithospheric mantle that was probably both formed (experienced partial melting and high-temperature melt/fluid–rock reaction) and deformed in a young and warm fore-arc mantle setting (i.e. juvenile subduction zone). Based on these results, a refined schematic model of olivine fabric distributions in subduction zones is proposed. In this model, the A-/D-type olivine fabrics are inferred to be prevalent in fore-arc lithospheric mantle. The opposing polarizing directions of A-/D-type olivine fabrics with other underlying anisotropic sources in the mantle wedge (e.g. B-type olivine fabrics and oriented serpentinite layers) may weaken the trench-parallel fast S-wave anisotropy contributed by the deformed fore-arc mantle, and thus provide an alternative explanation for the short or nearly null delay times of local shear-wave splitting (sourced from intra-slab earthquakes) that have been detected in some fore-arc regions. In addition, topotactic antigorite fabrics after the A-/D-type olivine fabric might play a minor role in contributing to the low P- and S-wave velocities, high Vp/Vs ratios, and large seismic anisotropies that are typically observed in fore-arc mantle.
... (010)[100] is the more common slip system observed in oceanic and continental lithosphere and is recognised to be favoured at low-stress, high-temperature conditions (Ben Ismaïl and Mainprice 1998;Nicolas et al. 1971). Moreover,in SSZs,(010)[100] and (001)[100] slip systems have been observed to be the most common activated slip systems (Harigane et al. 2013;Hidas et al. 2007;Kim and Jung 2014;Michibayashi et al. 2009Michibayashi et al. , 2007Park and Jung 2014;Soustelle et al. 2013;Wang et al. 2013;Webber et al. 2008) after the (100)[001] B-type (Jung et al. 2006) predicted to be activated in the fore-arc mantle wedge (Kneller 2007) and observed in natural peridotite from subduction environments (e.g. Mizukami et al. 2004;Park and Jung 2014;Skemer et al. 2006;Tasaka et al. 2008). ...
... In a fore-arc position olivine B-type fabrics are predicted to operate ) but are almost nonexistent in the Marum ophiolite (small population in dunite B13, Fig. 10), while E-and A-type fabrics dominate the deformation in the host dunite and harzburgite (Figs. 7). Absence of olivine B-type has already been observed in other SSZ peridotites (Harigane et al. 2013;Mehl et al. 2003;Soustelle et al. 2013) and could be related to not favourable conditions to initiate [001] slip, or short activity (few Ma) of the Marum ophiolite. The coarse-grained texture and olivine strong crystal fabrics (strong J-index) could be formed at near-subsolidus temperature during asthenospheric flow consistent with the foliation and lineation in the dunite. ...
... This indicates that olivine pre-existing CPOs need more than 5 Ma to be modified in this context. This change in the orientation can be the result of complex mantle flow within the mantle wedge, producing different flows such as upwelling and/or lateral asthenospheric flow under low stress (Fig. 14b) (Harigane et al. 2013;Mehl et al. 2003). The variation from ol <100> axes parallel to the trench to variation up to 60° could be an indicator for the development of young fore-arc mantle in a supra-subduction zone. ...
New geochemical and microstructural data are presented for a suite of ultramafic rocks from the Marum
ophiolite in Papua New Guinea. Our results describe a piece of most depleted mantle made essentially of dunite and harzburgite showing compositions of supra-subduction zone peridotite. Strong olivine crystallographic preferred orientations (CPOs) in dunite and harzburgite inferred the activation of both (001)[100] and (010)[100] slip systems, which are activated at high-temperature and low-stress conditions. Clinopyroxene and orthopyroxene CPOs inferred the activation of (100)[001] and (010)[001] slip systems, which are common for pyroxenes deformed at high temperature. This plastic deformation is interpreted to have developed during the formation of the Marum ophiolite, prior to melt percolation. The orientation
of the foliation and olivine [100] slip directions sub-parallel to the subduction zone indicates that mantle
flow was parallel to the trench pointing a fast polarisation direction parallel to the arc. This provides new evidence that fast polarisation direction parallel to the arc could be caused by anisotropic peridotite and not by olivine [001] slip. After its formation, Marum ophiolite has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene (1–2 %) (P1) and formation of cm-scale ol-clinopyroxenite and ol-websterite veins cross-cutting the foliation (P2). This percolating melt shows silicarich magnesian affinities (boninite-like) related to suprasubduction zone in a young fore-arc environment. The peridotite has also been percolated by a melt with more tholeiite affinities precipitating plagioclase-rich wehrlite and thin gabbroic veins (P3); these are interpreted to form after the boninitic event. The small proportion of newly crystallised pyroxene in the dunite shows similar orientation of crystallographic axes to the host dunite (<100>ol parallel to <001>cpx–opx). In contrast, the pyroxenes in olclinopyroxenite, ol-websterite and the thin gabbroic veins in the wehrlite, record their own orientation with <001> axes at 45°–60° to olivine <100> axes. Our results indicate that for low melt proportion the crystallisation is governed by epitaxial growth, and when the proportion of melt is higher the newly formed minerals record synkinematic crystallisation. This switch of crystallographic axes orientation of newly formed minerals indicates a reorientation of the constraints during the boninitic and tholeiitic melts event probably due to a variation of lateral mantle flow within the fore-arc area. The variation of the crystallographic axes orientation could be an indicator for the development of a young fore-arc mantle in suprasubduction
zone.
... This is also confirmed by chemical data that show strong enrichments of fine-grained minerals in (proto)mylonites in fluid mobile elements (especially boron; Prigent et al., 2018). This observation is also in agreement with the predominant activation of olivine (001)[100] dislocation slip systems in (proto)mylonites to ultramylonite/UMB (Figure 7), which is generally activated in hydrous conditions (Cao et al., 2015;Harigane et al., 2013;Hidas et al., 2016;Jung et al., 2014;Katayama et al., 2004;Mehl et al., 2003;Michibayashi & Oohara, 2013;Précigout et al., 2017). The geochemistry of newly crystallized fine grains and whole rock isotopic compositions indicate that these aqueous fluids bear a subduction signature (Khedr et al., 2014;Prigent et al., 2018;Yoshikawa et al., 2015). ...
... Gray vertical overlay: shear zone location. DMM =Harigane et al., 2013;Hidas et al., 2016;Jung et al., 2014;Mehl et al., 2003;Michibayashi et al., 2009;Michibayashi & Oohara, 2013) or(2)in the presence of melt ...
This study reports on feedback mechanisms between fluid migration, ductile deformation, and strain localization processes in an incipiently forming mantle wedge: the basal banded unit of the Semail ophiolite. These peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction. During this stage, they were affected by coeval ductile deformation, forming (proto)mylonites at ~900–800 °C to ultramylonites at ~700 °C, and interaction with subduction fluids. From the petrological and microstructural study of these hydrated peridotites and their protolith (preserved lenses of porphyroclastic tectonites), we show that peridotite interaction with hydrous fluids triggered dissolution/precipitation processes. Dissolution of coarser grains and precipitation of new and smaller ones (mainly pyroxenes, spinel, and amphibole) resulted in a drastic grain size reduction, phase mixing and a switch from olivine dislocation to grain size sensitive creep in (proto)mylonites. Data also evidence a feedback process of fluid focusing in actively deforming shear zones. This rapid switch in olivine deformation mechanism, driven by subduction fluid-peridotite interaction, triggered an intense weakening of the peridotites at ~900–800 °C and a transition from a mechanically decoupled to coupled plate interface (as witnessed by the detachment and underplating of a high-temperature metamorphic sole to the base of the ophiolite). It also explains the intense strain localization (<1-km-thick shear zone) along this ductile portion of the plate interface. Considering that similar mechanisms take place in mature subduction zones, they may explain plate interface coupling at subarc depths in worldwide subduction zones.
... The few ridge peridotite data we have are from the Godzilla Megamullion (Harigane et al., 2011a) and the Mariana Trough (new data) (magenta points in Fig. 3). The trench peridotite data are from the Bonin (Ogasawara) Trench (Harigane et al., 2013), the South Mariana Trench (Michibayashi et al., 2007(Michibayashi et al., , 2009b; new data), the North Mariana Trench (new data), and the Tonga Trench (new data) (blue points in Fig. 3). The peridotite xenolith data are from Ichinomegata (Michibayashi et al., 2006a(Michibayashi et al., , 2006b, Avacha (Michibayashi et al., 2009a), Knippa (Satsukawa et al., 2010), Kilbourne Hole (Satsukawa et al., 2011), a petitspot (Harigane et al., 2011b), and Tasmania (Michibayashi et al., 2012) (green points in Fig. 3). ...
... 7C and 8B), although the occurrence of B type was predicted for both the Mariana and Tonga trenches by Jung and Karato (2001). Harigane et al. (2013) argued that E type is the earliest fabric to form during the initiation of subduction, but its occurrence is rare. Nonetheless, the variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate. ...
Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly depicted by pole figures for the [100], [010], and [001] axes, and they can be categorized into five well-known fabric types: A, B, C, D, and E. These fabric types can be related to olivine slip systems: A with (010)[100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (001)[100]. In addition, an AG type is commonly found in nature, but its origin is controversial, and could involve several contributing factors such as complex slip systems, non-coaxial strain types, or the effects of melt during plastic flow. In this paper we present all of our olivine fabric database published previously as well as new data mostly from ocean floor, mainly for the convergent margin of the western Pacific region, and we introduce a new index named Fabric-Index Angle (FIA), which is related to the P-wave property of a single olivine crystal. The FIA can be used as an alternative to classifying the CPOs into the six fabric types, and it allows a set of CPOs to be expressed as a single angle in a range between −90° and 180°. The six olivine fabric types have unique values of FIA: 63° for A type, −28° for B type, 158° for C type, 90° for D type, 106° for E type, and 0° for AG type. We divided our olivine database into five tectonic groups: ophiolites, ridge peridotites, trench peridotites, peridotite xenoliths, and peridotites enclosed in high-pressure metamorphic rocks. Our results show that although our database is not yet large enough (except for trench peridotites) to define the characteristics of the five tectonic groups, the natural olivine fabrics vary in their range of FIA: 0° to 150° for the ophiolites, 40° to 80° for the ridge peridotites, −40° to 100° for the trench peridotites, 0° to 100° for the peridotite xenoliths, and −40° to 10° for the peridotites enclosed in high-pressure metamorphic rocks. The trench peridotites show a statistically unimodal distribution of FIA consisting of the high peak equivalent of the A type, but with some FIAs close to the AG and D types. The variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate.
... In this study, all three Isua samples record B-type fabrics. However, in a supra-subduction environment (010)[100] A-type and (001)[100] E-type fabrics are the more commonly observed fabric types 25,27,[30][31][32] , and in some case B-type can even be absent 23,51,52 . The progression of fabric-type development in the supra-subduction environment is therefore likely to be complex and it has been suggested that the earliest stage of fabric development, associated with subduction initiation, may be the formation of E-type fabrics 51 . ...
... However, in a supra-subduction environment (010)[100] A-type and (001)[100] E-type fabrics are the more commonly observed fabric types 25,27,[30][31][32] , and in some case B-type can even be absent 23,51,52 . The progression of fabric-type development in the supra-subduction environment is therefore likely to be complex and it has been suggested that the earliest stage of fabric development, associated with subduction initiation, may be the formation of E-type fabrics 51 . If this is correct, it may indicate that the activation of B-type slip system in Isua dunite may represent deformation in a more mature, advanced subduction setting. ...
The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼ 3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.
... The IOT is an erosive trench 36,37 , where deformation of the hangingwall occurs. Previous studies of the distribution of mafic and ultramafic rocks dredged from the IOT and geochemical and geochronological data have revealed that (i) outcrops of peridotite, gabbro, and dolerite occur from the slope base to slope top (7000-3000 mbsl) at the seamount 38 , (ii) ultramafic rock is exposed on the slope at >5000 mbsl 1 (Fig. 1a), and (iii) the IOT comprises immature forearc mantle-crust related to subduction zone initiation [38][39][40] . ...
The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. Circulation of seawater in the mantle rock induces carbonate precipitation within the deep-sea forearc mantle, but the timescale and rates of the circulation are unclear. Here we investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400m water depth in the Izu–Ogasawara Trench. On the basis of microtextures, strontium–carbon–oxygen isotope geochemistry, and radiocarbon analysis, we found that the aragonite is sourced from seawater that accumulated for more than 42,000 years. Aragonite precipitation is triggered by episodic rupture events that expel the accumulated fluids at 10−2–10−1ms−1 and which continue for a few decades at most. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior.
https://doi.org/10.1038/s43247-021-00317-1 OPEN
1
... While Frese et al. (2003) discovered C-Type from hydrated samples below Cima di Gagnone, Jung (2009) revealed E-type as well as B-type close to Val Malenco. Considering the Izu-Ogasawara trench, Harigane et al. (2013) revealed indications for the presence of E-type and concluded that slab-induced hydration led to transition of previously occurred A-or D-type fabric. Mehl et al. (2003) documented E-type in rocks from the exhumed Talkeetna arc. ...
Shear-wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, as caused by the occurrence of olivine, to constrain the dominant directions of upper mantle deformation. While SK(K)S splitting measurements at individual seismic stations are often averaged before interpretation, it is useful to consider additional information, for example, based on the variation of splitting parameters with azimuth due to the non-vertical incidence of core-phases. These constraints in theory enable a differentiation between various types of olivine and may allow us to infer otherwise poorly known upper mantle parameters such as stress, temperature, and water content. In this study, we predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine fabrics and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain in the Pacific Northwest US. This helps to constrain the amount of water in the upper mantle in the back-arc of the Cascadia subduction zone, known for its consistent E-W oriented seismic anisotropy, and particularly large splitting delay times. Our investigation renders the C-type olivine mechanism improbable for this location; A- and E-type fabrics match the observations, although differentiating between them is difficult. However, the agreement of the amplitude of backazimuthal variation of the fast orientation, plus the potential to explain large splitting delay times, suggest the occurrence of E-type olivine, and thus the likely presence of a moderately hydrated upper mantle beneath Cascadia's back-arc.
... An example has been reported from the terrestrial-ward slope of the Challenger Deep, southern MT (Okumura et al., 2016b). The exposure of serpentinized peridotite below 5500 m at the terrestrial-ward slope of the IOT (Morishita et al., 2011;Harigane et al., 2013) indicates that discharge of CH 4 -rich geofluids occurring in the IOT cannot be ruled out, but such geofluid systems have never been discovered before. Seafloor serpentinization geofluids reported so far exhibit δ 13 C CH 4 values higher than −40 ‰ (Konn et al., 2015) and are not compatible with the hadal CH 4 anomaly observed in this study. ...
Full-depth profiles of hydrographic and geochemical
properties at the Izu–Ogasawara Trench were observed for the first time
using a CTD-CMS (conductivity–temperature–depth profiler with
carousel multiple sampling) system. Additionally, comparative samplings were
done at the northern Mariana Trench using the same methods. A well-mixed
hydrographic structure below 7000 m was observed within the Izu–Ogasawara
Trench. Seawater samples collected from this well-mixed hadal layer
exhibited constant concentrations of nitrate, phosphate, silicate, and
nitrous oxide as well as constant nitrogen and oxygen isotopic compositions
of nitrate and nitrous oxide. These results agree well with previous
observations of the Izu–Ogasawara hadal waters and deep-sea
water surrounding the Izu–Ogasawara Trench. In turn, methane concentrations and
isotopic compositions indicated spatial heterogeneity within the well-mixed
hadal water mass, strongly suggesting a local methane source within the
trench, in addition to the background methane originating from the general
deep-sea bottom water. Sedimentary compound releases, associated with
sediment re-suspensions, are considered to be the most likely mechanism for
generating this significant CH4 anomaly.
... An example has been reported from the terrestrial-ward slope of the Challenger Deep, southern MT (Okumura et al., 2016b). The exposure of serpentinized peridotite below 5,500 m at the terrestrial-ward slope of the IOT (Morishita et al., 2011;Harigane et al., 2013) indicates that discharge of CH 4 -rich geofluids occurring in the IOT cannot be ruled out, but such geofluid systems have never been 5 discovered before. Seafloor serpentinization geofluids reported so far exhibit δ 13 C CH4 values higher than -40‰ (Konn et al., 2015) and are not compatible with the hadal CH 4 anomaly observed in this study. ...
Full-depth profiles of hydrographic and geochemical properties at the Izu-Ogasawara Trench were observed for the first time using a CTD-CMS (Conductivity Temperature Depth profiler with Carousel Multiple Sampling) system, supplemented by some comparative samplings at the northern Mariana Trench using the same methods. The CTD sensor, calibrated from measurements of seawater samples, demonstrated a well-mixed hydrographic structure below 7000 m within the Izu-Ogasawara Trench. Seawater samples collected from this well-mixed hadal layer exhibited constant concentrations of nitrate, phosphate, silicate, and nitrous oxide as well as constant nitrogen and oxygen isotopic compositions of nitrate and nitrous oxide. These results agree well with previous observations of the Izu-Ogasawara hadal waters and deep-sea water surrounding the Izu-Ogasawara Trench. In turn, methane concentrations and isotopic compositions indicated spatial heterogeneity within the well-mixed hadal water mass, strongly suggesting a local methane source within the trench, in addition to the background methane originating from the general deep-sea bottom water. Sedimentary compound releases, associated with sediment re-suspensions, are considered to be the most likely mechanism for generating this significant CH4 anomaly.
... We also found type-E olivine LPO in the wehrlites in our study. The proposed mechanisms for producing type-E olivine LPO are a pre-existing mechanical anisotropy on the mantle lithosphere (Michibayashi and Mainprice 2004) and the deformation of olivine in the presence of a moderate amount of water at low stress (Mehl et al. 2003;Katayama et al. 2004;Jung et al. 2006;Harigane et al. 2013). Michibayashi and Mainprice (2004) showed that type-A LPO (the crystallographic [100] axes of olivine are aligned subparallel to the lineation, and the [010] axes are aligned subnormal to the foliation) of olivine, which was formed by E-W mantle flow, was transformed to type-E olivine LPO due to the NW-SE strike-slip shear. ...
... Hitherto, many studies have addressed the olivine CPOs in the forearc region (e.g., Cao et al., 2015;Cordellier et al., 1981;Harigane et al., 2013;Ji et al., 1994;Jung, 2009;Jung et al., 2014;Kaczmarek et al., 2015;Kim and Jung, 2015;Mehl et al., 2003;Michibayashi et al., 2007;Mizukami et al., 2004;Palasse et al., 2012;Park and Jung, 2015;Soustelle et al., 2013;Soustelle et al., 2010;Tasaka et al., 2008;Tommasi et al., 2006;Wang et al., 2016). Consistent with the natural observations, Cao et al. (2015) recently conceived that the forearc lithospheric mantle is dominated by A-/D-type CPOs, whereas B-, C-and E-type CPOs mainly occur in the deeper asthenospheric mantle. ...
Though extensively studied, the roles of olivine crystal preferred orientations (CPOs or fabrics) in affecting the seismic anisotropies in the Earth's upper mantle are rather complicated and still not fully known. In this study, we attempted to address this issue by analyzing the seismic anisotropies [e.g., P-wave anisotropy (AVp), S-wave polarization anisotropy (AVs), radial anisotropy (ξ), and Rayleigh wave anisotropy (G)] of the Songshugou peridotites (dunite dominated) in the Qinling orogen in central China, based on our previously reported olivine CPOs. The seismic anisotropy patterns of olivine aggregates in our studied samples are well consistent with the prediction for their olivine CPO types; and the magnitude of seismic anisotropies shows a striking positive correlation with equilibrium pressure and temperature (P–T) conditions. Significant reductions of seismic anisotropies (AVp, max. AVs, and G) are observed in porphyroclastic dunite compared to coarse- and fine-grained dunites, as the results of olivine CPO transition (from A-/D-type in coarse-grained dunite, through AG-type-like in porphyroclastic dunite, to B-type-like in fine-grained dunite) and strength variation (weakening: A-/D-type → AG-type-like; strengthening: AG-type-like → B-type-like) during dynamic recrystallization. The transition of olivine CPOs from A-/D-type to B-/AG-type-like in the forearc mantle may weaken the seismic anisotropies and deviate the fast velocity direction and the fast S-wave polarization direction from trench-perpendicular to trench-oblique direction with the cooling and aging of forearc mantle. Depending on the size and distribution of the peridotite body such as the Songshugou peridotites, B- and AG-type-like olivine CPOs can be an additional (despite minor) local contributor to the orogen-parallel fast velocity direction and fast shear-wave polarization direction in the orogenic crust such as in the Songshugou area in Qinling orogen.
... C-, D-, and E-type fabrics are observed in many peridotites, but relatively poorly understood [e.g., Ben Ismaïl and Mainprice, 1998;Harigane et al., 2013;Hidas et al., 2007;Kaczmarek and Reddy, 2013;Kaczmarek et al., 2015;Karato et al., 2008;Kim and Jung, 2015;Mainprice, 2007;Mehl et al., 2003;Michibayashi and Mainprice, 2004;Michibayashi and Oohara, 2013;Michibayashi et al., 2007Michibayashi et al., , 2009Palasse et al., 2012;Park and Jung, 2015;Sawaguchi, 2004;Skemer et al., 2010;Soustelle et al., 2013;Tommasi et al., 2000;Wang et al., 2013;Webber et al., 2008]. C-type fabric is formed by deformation using (100)[001] slip systems and tends to result in Pattern III anisotropy: V p (Z) > V p (X) > V p (Y). E-type fabric formed by (001)[100] slip systems would yield Pattern IV anisotropy: V p (X) > V p (Z) > V p (Y). D-type fabric is characterized by [100] axes subparallel to the X direction, and [010] and [001] axes forming a girdle normal to the X direction. ...
In order to constrain the effects of olivine fabric, melt-rock reaction, and hydration on the seismic properties and anisotropy of mantle rocks, we investigated serpentinized peridotites from the Luobusha ophiolite in the Indus-Tsangpo suture of the Tibetan Plateau. A-type and almost random olivine crystal-preferred orientations (CPO) occur in harzburgite and dunite samples, respectively. The dunite resulted from interactions of harzburgite with boninitic melt at ~800–970°C, yielding pyroxene dissolution and olivine precipitation. The olivine neoblasts formed from the melt-rock reaction show no evidence of dislocation creep and developed almost random CPO. Hence, the melt-rock reaction reduced seismic anisotropy. Our results together with those from the literature indicate that A-, B-, C-, D-, and E-type CPOs of olivine generally induce Vp anisotropy patterns with Vp(X) > Vp(Y) > Vp(Z), Vp(Y) > Vp(X) > Vp(Z), Vp(Z) > Vp(X) > Vp(Y), Vp(X) > Vp(Y) ≈ Vp(Z), and Vp(X) > Vp(Z) > Vp(Y), respectively. The effect of serpentinization was calibrated by the comparison of seismic velocities and anisotropy measured up to 600 MPa with the values calculated from the CPO data. Although the low-temperature (LT, <300°C) serpentinization (lizardite and chrysotile) decreases Vp by ~6–10% and Vs by ~12%, it does not change the anisotropy pattern because the mesh-texture characterized by serpentine veins perpendicular to the principal structural directions (X, Y, and Z) reduces the velocities in these orthogonal directions to almost equal extent. Thus, the magnitude of seismic anisotropy alone cannot be used as an indicator of the degree of LT serpentinization in the mantle rocks. Furthermore, Birch's law is found to hold when peridotites undergo serpentinization.
... We also found type-E olivine LPO in the wehrlites in our study. The proposed mechanisms for producing type-E olivine LPO are a pre-existing mechanical anisotropy on the mantle lithosphere (Michibayashi and Mainprice 2004) and the deformation of olivine in the presence of a moderate amount of water at low stress (Mehl et al. 2003;Katayama et al. 2004;Jung et al. 2006;Harigane et al. 2013). Michibayashi and Mainprice (2004) showed that type-A LPO (the crystallographic [100] axes of olivine are aligned subparallel to the lineation, and the [010] axes are aligned subnormal to the foliation) of olivine, which was formed by E-W mantle flow, was transformed to type-E olivine LPO due to the NW-SE strike-slip shear. ...
Deformation microstructures, including lattice-preferred orientations (LPOs) of olivine, enstatite, and diopside, in mantle xenoliths at Shanwang, eastern China, were studied to understand the deformation mechanism and seismic anisotropy of the upper mantle. The Shanwang is located across the Tan-Lu fault zone, which was formed due to the collision between the Sino-Korean and South China cratons. All samples are spinel lherzolites and wehrlites, and LPOs of minerals were determined using scanning electron microscope/electron backscattered diffraction. We found two types of olivine LPO: type-B in spinel lherzolites and type-E in wehrlites. Enstatite showed two types of LPO (types BC and AC), and diopside showed four different types of LPO. Observations of strong LPOs and numerous dislocations in olivine suggest that samples showing both type-B and -E LPOs were deformed in dislocation creep. The seismic anisotropy of the P-wave was in the range of 2.2-11.6% for olivine, 1.2-2.3% for enstatite, and 2.1-6.4% for diopside. The maximum seismic anisotropy of the shear wave was in the range 1.93-7.53% for olivine, 1.53-2.46% for enstatite, and 1.81-6.57% for diopside. Furthermore, the thickness of the anisotropic layer was calculated for four geodynamic models to understand the origin of seismic anisotropy under the study area by using delay time from shear wave splitting, and S-wave velocity and anisotropy from mineral LPOs. We suggest that the seismic anisotropy under the study area can be most likely explained by two deformation modes that might have occurred at different times: one of deformed lherzolites with a type-B olivine LPO by lateral shear during/after the period of the Mesozoic continental collision between the Sino-Korean and South China cratons; and the other deformed the wehrlites with a type-E olivine LPO by horizontal extension during the period of change in absolute plate motion in relation to the westward-subducting Pacific plate.
Our understanding of the processes at work in the lower crust/upper mantle transition zone during subduction initiation and early arc development has suffered from a general lack of in situ samples. Here, we present the results of petrographic and geochemical analysis of 34 samples (9 harzburgites, 13 dunites, 2 orthopyroxenites, 3 olivine-gabbros, and 7 wehrlites) collected from the inner trench wall of the Bonin Ridge, Izu–Bonin forearc. The sample suite records three main melt–rock reaction events involving melts with forearc basalt (FAB)-like, boninitic, and transitional compositions. The wehrlitic and gabbroic rocks trend towards more transitional to FAB compositions and the rest towards more boninitic compositions. The crosscutting occurrence of all three events in a single sample (wehrlite D31–106) establishes a relative timing of the events like that reported for the volcanic edifice of the Bonin Ridge, which transitioned from forearc basalt volcanism at subduction initiation (c.a., 51–52 Ma) to boninitic volcanism (c.a., 50–51 Ma) as the subduction system matured. We therefore suggest that the lower crust/upper mantle transition of the Bonin Ridge preserves a record of the transition from FAB melts created by decompression melting at subduction initiation to arc-type flux melting and boninitic volcanism thereafter. Orthopyroxenites and two anomalously fresh harzburgites from the sample suite are suggested to represent the later boninitic melts and possibly the result of hybridization between such melts and residual peridotites, respectively. Diffuse melt–rock reaction between the later boninites and/or subduction-related fluids and the earlier-formed FAB-related crust is recorded by enrichments in fluid mobile elements and depletions in first row transition metals in clinopyroxenes from a metasomatic vein in wehrlite sample D31–106. The chemistry of the wehrlitic and gabbroic clinopyroxenes suggests that they crystallized from hydrous, highly depleted melts which lack a slab fluid signature. We thus suggest that highly depleted melt fractions might be created early on during subduction initiation by the introduction of seawater into the proto-mantle wedge. The overall FAB-like nature of the crustal wehrlites and gabbros would suggest that most of the lower arc crust was created by forearc extension during/following subduction initiation and that later, mature arc volcanism may have contributed little or no material to the lower crust/upper mantle record in the outer forearc.
Ultramafic rocks, i.e., peridotites and pyroxenites, occur in a variety of tectonic settings on Earth. Ultramafic rocks can form as accumulation of mafic minerals from basaltic to komatiitic melts and be a major component of the Earth's mantle. The origin and history of ultramafic rocks are expected to provide information on the processes of partial melting and melt migration/extraction in the mantle and on the tectonic evolution of geologic units containing ultramafic rocks. I study ultramafic rocks in metamorphic belts, ocean floor, and mantle sections of ophiolites. My career began with a study of the Horoman Peridotite Complex in the Hidaka metamorphic belt in Japan. The ultramafic rocks and associated mafic rocks in the Horoman body record a very complex evolutionary history from the mantle conditions to crustal conditions. It is difficult to constrain the tectonic setting affecting events in the Horoman Peridotite Complex. On the other hand, ultramafic rocks in the mantle section of ophiolites and abyssal peridotites directly recovered from ocean floor to study melting processes and melt-rock interactions in the mantle can be used to constrain their tectonic setting, or at least as analogs to these tectonic settings. Studies on the Oman ophiolite by Japanese groups and literature studies of other ophiolites suggest that many ophiolites are later modified by subduction-related magmatism. Several ophiolites are being studied to elucidate the maturing process by subduction-related magmatism. Simple partial melting and melt extraction is expected in the adiabatically upwelling mantle beneath the mid-ocean ridge. In fact, abyssal peridotites directly recovered from mid-ocean ridges provided a unique opportunity to elucidate these processes. Comparison of abyssal peridotites recovered from the mid-ocean ridges and arc regions (fore arc and back arc) is key to understand the differences in magmatic processes in the two regions. Ocean science with research vessels has a well-defined working hypothesis that can only be addressed by direct sampling from the seafloor. To understand a crucial issue in Earth science as to why plate tectonics occurs on Earth, it is essential to elucidate the life of the oceanic lithosphere from its birth to its subduction into the mantle. Direct sampling of oceanic lithosphere by drilling is the key to solving this issue. I would like to emphasize that members of the Japan Association of Mineralogical science can play an essential role in leading analyses of rock samples directly recovered from seafloor. Rock samples recovered from seafloor by drilling and any methodology, as well as samples from anywhere on Earth, should be published in as papers, and these data would help integrate knowledge about the history of the Earth and planet and its future.
Melt-rock interactions are important for understanding the long-term evolution of the Earth and for addressing various mantle compositions. However, details of the mechanisms and evolution of interactions in supra-subduction zones remain poorly understood. This study presents petrographic and geochemical analyses of the harzburgites from the Moa-Baracoa Ophiolitic Massif, eastern Cuba, to constrain the melt-rock interaction within the lithospheric mantle. The harzburgites preserve pristine microstructures by recording three-stage petrogenetic processes: incongruent melting of orthopyroxene (Opx) in stage I; crystallization of interstitial clinopyroxene (Cpx), spinel (Sp), and base-metal sulfides (BMS) at the expense of Opx in stage II; and re-equilibration characterized by Sp-Cpx symplectite in stage III. The harzburgite compositions are highly refractory with Al2O3 (0.21–0.81 wt%) and TiO2 (~0.04 wt%) and show “U”-shaped rare earth element (REE) patterns and significant LREE and large ion lithophile element (LILE) enrichments. The olivines have an E-type fabric and a narrow range of compositions (Fo = 90.6–91.6). Interstitial Cpx has moderate Mg# (92.1–94.9) and Al2O3 (1.06–2.88 wt%) and extremely low TiO2 contents (<0.03 wt%). Opx and interstitial Cpx have low ∑REE contents (0.09–0.14 ppm and 0.24–0.49 ppm, respectively) and are depleted in LREE and variably enriched in LILE. Spinels possess similar Cr# values (56.7–64.8) but variable Mg# values (37.2–58.4). BMS is primarily dominated by pentlandite, which is partly or completely replaced by magnetite and/or heazlewoodite. Modeling of whole-rock HREE variations suggests that the harzburgites experienced >25% partial melting. Widespread interstitial Cpx and BMS, elevated whole-rock Cu and Pt, and variable LILE enrichment in Opx and Cpx indicate the interaction of the refractory harzburgites with migrating sulfur-saturated, low-silica melts. Melts equilibrated with interstitial Cpx appear to have an affinity for FAB. Mineralogical, chemical, and olivine fabric evidence suggests that the Moa-Baracoa harzburgites originated in a nascent forearc mantle. The harzburgites experienced partial melting facilitated by migrating low-silica melts in stage I. Subsequent interactions with FAB melts at relatively high temperatures precipitated Sp, Cpx, and BMS in stage II. Finally, the re-equilibration of high-temperature/pressure pyroxenes produced Sp-Cpx symplectite as the harzburgites were rapidly emplaced into the lithospheric mantle during subduction initiation.
The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7Ma. The overlying basalts have 40Ar/39Ar ages of 48–52Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/39Ar age of 51.1Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52Ma. The change to flux melting and boninitic volcanism took 2–4m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50Ma suggests that the two events are intimately linked.
Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These “fore-arc basalts” (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.
A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia. The model accounts for several key observations about the earliest stages in the evolution of the IBM system: (1) subduction nucleated along an active transform boundary, which separated younger, less-dense lithosphere in the west from older, more-dense lithosphere to the east; (2) initial arc magmatic activity occupied a much broader zone than existed later; (3) initial magmatism extended up to the modern trench, over a region now characterized by subnormal heat flow; (4) early are magmatism was characterized by depleted (tholeiitic) and ultra-depleted (boninitic) magmas, indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth; (5) early arc magmatism was strongly extensional, with crust forming in a manner similar to slow-spreading ridges; and (6) crust production rates were 120 to 180 km³/km-Ma, several times greater than for mature arc systems. These observations require that the earliest stages of subduction involve rapid retreat of the trench; we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithospheric subduction. Application of the subduction-zone infancy model to the Late Jurassic ophiolites of California provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger Sierran magmatic arc and the Franciscan subduction complex, provides a mechanism for the formation and subsidence of the Great Valley forearc basin, and explains the limited duration of high-T, high-P metamorphism experienced by Franciscan mélanges.
We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001) liz, corresponding to the pseudofibres observed optically. The plane (001) liz is locally parallel to the narrow boundary ol-liz; thus column orientations register the interface of serpentinization. The ol-liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010) ol. In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010]. atg show domains of very regular modulation with a 43·5 Å wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP-HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100] atg//[010]. ol and <100>. atg//<100>. ol; the planes in contact are (001) atg//(100) ol and (001) atg//(010)\ ol, respectively. The [010] atg//[001] ol and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine-serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.
A new type of olivine fabric was found by high-strain simple-shear deformation experiments in the presence of trace amounts of water at ˜0.5 2.2 GPa and ˜1470 1570 K. In this fabric, called E-type fabric, the olivine [100] axis is subparallel to the shear direction, and the (001) plane is parallel to the shear plane; this geometry suggests that the [100](001) slip system makes the dominant contribution to total strain. This fabric is dominant at a modest water content, 200 < COH < 1000 H/106Si at low stresses and high temperatures. Some mylonites from peridotite massifs show this type of olivine fabric, which suggests the presence of water during the shear localization. The seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric (A type), but the magnitudes of anisotropy are different between the two types: for horizontal flow, the amplitude of VSH/VSV anisotropy is weaker, but the amplitude of shear-wave splitting is stronger for the E-type fabric than for the A-type dry fabric. Seismic anisotropy in the oceanic upper mantle may be due to the olivine E-type fabric.
Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an ~300-400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large ~1km+ thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust-mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt-rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep. The average Kane transform peridotite is a lherzolite with 5·2% Cpx, whereas that from the main core complex is a harzburgite with only 3·5% Cpx. As the area corresponds to a regional bathymetric low, and the crust is apparently thin, it is likely that most residual mantle along the MAR is significantly more depleted. Thus, harzburgitic and lherzolitic ophiolite subtypes cannot be simply interpreted as slowand fast-spreading ridges respectively. The mantle peridotites are consistent with a transform edge effect caused by juxtaposition of old cold lithosphere against upwelling mantle at the ridge-transform intersection. This effect is far more local, confined to within 10 km of the transform slip zone, and far smaller than previously thought, corresponding to ~8% as opposed to 12·5% melting of a pyrolitic mantle away from the transform. Excluding the transform, the overall degree of melting over 3 Myr indicated by the peridotites is uniform, ranging from ~11·3 to 13·8%. Large variations in composition for a single dredge or ROV dive, however, reflect local melt transport through the shallow mantle. This produced variable extents of melt-rock reaction, dunite formation, and melt impregnation. At least three styles of late mantle metasomatism are present. Small amounts of plagioclase with elevated sodium and titanium and alumina-depletion in pyroxene relative to residual spinel peridotites represent impregnation by a MORB-like melt. Highly variable alumina depletion in pyroxene rims in spinel peridotite probably represents cryptic metasomatism by small volumes of late transient silica-rich melts meandering through the shallow mantle. Direct evidence for such melts is seen in orthopyroxenite veins. Finally, a late hydrous fluid may be required to explain anomalous pyroxene sodium enrichment in spinel peridotites. The discontinuous thin lower crust exposed at Kane Megamullion contrasts with the >700 km2 1·5 km+ thick Atlantis Bank gabbro massif at the SW Indian Ridge (SWIR), clearly showing more robust magmatism at the latter. However, the SWIR spreading rate is 54% of the MAR rate, the offset of the Atlantis II Fracture Zone is 46% greater and Na8 of the spatially associated basalts 16% greaterçall of which predict precisely the opposite. At the same time, the average compositions of Kane and Atlantis II transform peridotites are nearly identical. This is best explained by a more fertile parent mantle beneath the SWIR and demonstrates that crustal thickness predicted by simply inverting MORB compositions can be significantly in error.
Magmatic processes during the earliest stage of subduction initiation are still not well understood. We examined peridotites recovered from an exhumed crust-mantle section exposed along the landward slopes of the northern Izu-Bonin Trench using the Japan Agency for Marine-Earth Science and Technology's remotely operated vehicle KAIKO7000II. Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (>0.8) dunite and (2) medium-Cr# (0.4-0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites: a boninitic melt for the high-Cr# dunite and a mid-oceanic ridge basalt (MORB)-like melt for the medium-Cr# dunite. The TiO2 content of the latter melt is lower than typical MORB compositions. We suggest that the medium-Cr# dunite was a melt conduit for a basalt recently reported from the Mariana forearc that was erupted at the inception of subduction. The wide range of variation in the Cr#s of spinels in dunites from the Izu-Bonin-Mariana forearc probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids and/or melts during subduction initiation.
Seismic anisotropy is caused mainly by the lattice-preferred orientation of anisotropic minerals. Major breakthroughs have occurred in the study of lattice-preferred orientation in olivine during the past ∼10 years through large-strain, shear deformation experiments at high pressures. The role of water as well as stress, temperature, pressure, and partial melting has been addressed. The influence of water is large, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active regions such as subduction zones, asthenosphere, and plumes. The main effect of partial melting on deformation fabrics is through the redistribution of water, not through a change in deformation geometry. A combination of new experimental results with seismological observations provides new insights into the distribution of water associated with plume-asthenosphere interactions, formation of the oceanic lithosphere, and subduction. However, large uncertainties remain regardi...
The anisotropic single crystal seismic properties are reviewed in the light of recent experimental and theoretical determinations. Although considerable progress has been made on the determination of single crystal properties, data are still lacking, particularly for the temperature derivatives of transition zone and lower mantle phases. The common types of LPO of olivine, opx and cpx are presented together with their associated seismic properties. It is emphasized that simple seismic symmetry pattern of upper mantle rocks are a direct result of the interaction of olivine, opx and cpx. Using the standard structural frame (X lineation, Z pole to foliation) for typical LPOs of olivine, opx and cpx have the maximum Vp parallel to X, in the XZ plane and parallel to Y respectively. Destructive interference occurs between these minerals and hence P-wave anisotropy should be sensitive to the aggregate composition. For shear wave splitting (dVs) typical olivine and opx LPOs result in similar patterns with the maximum dVs in the YZ plane and the fast split shear wave (Vs1) polarized parallel to the foliation. A typical cpx LPO on the other hand produces destructive interference as the max dVs is close to X. By comparison with experiments and numerical simulations, it is estimated that upper mantle samples have an olivine LPO strength which recorded shear strain gamma of between 0.25 and 2.0. Pyrolite and piclogite models are compared with global transverse isotropic models. The slowly reducing P-wave anisotropy in the first 200 km can be explained by a model with constant composition and LPO strength. The sharp decrease in the observed anisotropy in the global models cannot be explained by the transformation of opx to cpx at 300 km, it is proposed that this decrease is due to a reduction in LPO strength from 200 to 350 km at the base of the lithosphere.
1] Residual mantle exposures in the accreted Talkeetna arc, Alaska, provide the first rock analog for the arc-parallel flow that is inferred from seismic anisotropy at several modern arcs. The peridotites exposed at the base of the Jurassic Talkeetna arc have a Moho-parallel foliation and indicate dislocation creep of olivine at temperatures of $1000° to >1100°C. Slip occurred chiefly on the (001)[100] slip system, which has only rarely been observed to be the dominant slip system in olivine. Stretching lineations and olivine [100] slip directions are subparallel to the long axis of the Talkeetna arc for over 200 km, indicating that mantle flow was parallel to the arc axis. The alignment of the olivine [100] axes yields a calculated S wave anisotropy with the fast polarization direction parallel to the arc. Thus (1) the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog; (2) arc-parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones, fluid-filled pockets, or glide on the (010)[001] H 2 O-induced slip system; (3) seismic anisotropy beneath modern arcs may be due to slip on (001)[100] with a horizontal foliation rather than slip on (010)[100] with a vertical foliation; and (4) the observed dominance of the (001)[100] slip system may be due to high H 2 O concentrations, suggesting that strain in the oceanic upper mantle may be accommodated dominantly by (001)[100] olivine slip., Arc-parallel flow within the mantle wedge: Evidence from the accreted Talkeetna arc, south central Alaska, J. Geophys. Res., 108(B8), 2375, doi:10.1029/2002JB002233, 2003.
Bathymetric data at a 200-m contour interval for the entire Mariana forearc, from south of 13°N to 25°N, permits the first comprehensive overview of this feature. The Mariana forearc represents a sediment-starved end-member. The forearc in its southern and central sections is divisible into a structurally complex eastern province and a less-deformed western province. Despite the absence of an accretionary complex the Mariana forearc has a well-defined outer-arc high; this probably results from a greater concentration of low-density serpentinized mantle lithosphere beneath the outer forearc relative to the inner forearc. This serpentinization gradient is coupled with differing deformational styles of thinner and more brittle lithosphere beneath the outer forearc compared to thicker and more ductile lithosphere beneath the inner forearc. The bathymetric data also support models calling for extension along-strike of the forearc, reflecting an increase in arc length accompanying the crescent-shaped opening of the Mariana Trough back-arc basin. Both northeast and northwest ridges and grabens can be identified, with the latter restricted to the southern part of the forearc and the former widely distributed in the central and northern forearc. Northeast-oriented extensional structures are supplanted northward by long, linear northwest-trending structures that are interpreted as left-lateral strike–slip faults. These variations in deformation along-strike of the forearc manifest a transition from nearly orthogonal convergence in the south to highly oblique convergence in the north.
The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr#0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.Based on an examination of co-existing rock and spinel compositions in lavas, it appears that spinel is a sensitive indicator of melt composition and pressure of crystallization. The close similarity of spinel composition fields in genetically related basalts, dunites and peridotites at localities in the oceans and in ophiolite complexes indicates that its composition reflects the degree of melting in the mantle source region. Accordingly, we infer from the restricted range of spinel compositions in abyssal basalts that the degree of mantle melting beneath mid-ocean ridges is generally limited to that found in Type I alpine-type peridotites. It is apparent, therefore, that the phase boundary OL-EN-DI-SP +meltOL-EN-SP+melt has limited the degree of melting of the mantle beneath mid-ocean ridges. This was clearly not the case for many alpine-type peridotites, implying very different melting conditions in the mantle, probably involving the presence of water.
The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo–Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new subduction zones along the western edge of the Pacific plate during the Eocene. Development of self-sustaining subduction in the case of SNSZ is signaled by the beginning of down-dip slab motion, causing chilling of the forearc mantle and retreat of the magmatic arc to a position that is 100–200 km from the trench. INSZ may affect only part of a plate margin, but SNSZ affects the entire margin in the new direction of convergence. INSZ and SNSZ can be distinguished by the record left on the upper plates: INSZ begins with strong compression and uplift, whereas SNSZ begins with rifting and seafloor spreading. Understanding conditions leading to SNSZ and how hinged subsidence of lithosphere changes to true subduction promise to be exciting and fruitful areas of future research.
Seismic anisotropy in subduction zones results from a combination of various processes. Although it depends primarily on the orientation of olivine in response to flow, the presence of water and melt in the wedge may modify the deformation of olivine. The melt distribution also influences anisotropy. Direct observations of the deformation and melt-rock interactions in a strongly depleted spinel-harzburgite massif from the Cache Creek terrane in the Canadian Cordillera allow evaluating the relative contribution of each process. Structural mapping shows that this massif has recorded high-temperature, low-stress deformation, high degrees of partial melting, and synkinematic melt-rock interaction at shallow depths (< 70 km) in the mantle, probably above an oblique subduction. Deformation, marked by shallow-dipping lineations and steep foliations, controlled melt distribution: reactive dunites and pyroxenite dykes are dominantly parallel to the foliation. Analysis of olivine crystal preferred orientations (CPO) indicates deformation by dislocation creep with dominant [100] glide. Glide planes are however different in harzburgites and dunites, suggesting that higher melt contents may favor glide on (001) relative to (010). Seismic properties, calculated by considering explicitly the large-scale structure of the massif, the olivine and pyroxene CPO, and possible melt distributions, show that the strain-induced olivine CPO results in up to 5% P- and S-wave anisotropy with fast seismic directions parallel to the lineation. Synkinematic melt transport by diffuse porous flow leading to melt pockets or dykes aligned in the foliation may significantly enhance this anisotropy, in particular for S-waves. In contrast, focused melt flow is not recorded by seismic anisotropy, unless associated with very high instantaneous melt fractions. Orientation of pyroxenite dykes suggests that the present orientation of the structures is representative of the pre-obduction situation, implying trench-parallel fast polarizations and high delay times as observed above the Kurils, Ryukyu, Taiwan, and Tonga subductions.
Subduction zones are where sediments, oceanic crust, and mantle lithosphere return to and reequilibrate with Earth's mantle. Subduction zones are interior expressions of Earth's 55,000 km of convergent plate margins and are the geodynamic system that builds island arcs. Excess density of the mantle lithosphere in subduction zones provides most of the power needed to move the plates while inducing convection in the overriding mantle wedge. Asthenospheric mantle sucked toward the trench by the sinking slab interacts with water and incompatible elements rising from the sinking plate, and this interaction causes the mantle to melt. These melts rise vertically through downwelling mantle to erupt at arc volcanoes. Subduction zones are thus interior Earth systems of unparalleled scale and complexity. Subduction zone igneous activity formed most ore deposits and continental crust, and earthquakes caused by the downgoing plate present a growing hazard to society. This overview summarizes our present understandi
[1] Two N-S fault zones in the southern Mariana fore arc record at least 20 km of left-lateral displacement. We examined the eastward facing slope of one of the fault zones (the West Santa Rosa Bank fault) from 6469 to 5957 m water depth using the submersible Shinkai 6500 (YK06-12 Dive 973) as part of a cruise by the R/V Yokosuka in 2006. The dive recovered residual but still partly fertile lherzolite, residual lherzolite, and dunite; the samples show mylonitic, porphyroclastic, and coarse, moderately deformed secondary textures. Crystal-preferred orientations of olivine within the peridotites show a typical [100](010) pattern, with the fabric intensity decreasing from rocks with coarse secondary texture to mylonites. The sampled peridotites therefore represent a ductile shear zone within the lithospheric mantle of the overriding plate. Peridotites were probably exposed in association with a tear in the subducting slab, previously inferred from bathymetry and seismicity. Furthermore, although the dive site is located in the fore arc close to the Mariana Trench, spinel compositions within the sampled peridotites are comparable to those from the Mariana Trough back arc, suggesting that back-arc basin mantle is exposed along the West Santa Rosa Bank fault.
Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth's interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals. Although seismic anisotropy in the upper mantle is generally attributed to the crystal-preferred orientation of olivine, the strong trench-parallel anisotropy (delay time of one to two seconds) observed in several subduction systems is difficult to explain in terms of olivine anisotropy, even if the entire mantle wedge were to act as an anisotropic source. Here we show that the crystal-preferred orientation of serpentine, the main hydrous mineral in the upper mantle, can produce the strong trench-parallel seismic anisotropy observed in subduction systems. High-pressure deformation experiments reveal that the serpentine c-axis tends to rotate to an orientation normal to the shear plane during deformation; consequently, seismic velocity propagating normal to the shear plane (plate interface) is much slower than that in other directions. The seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine, and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench-parallel seismic anisotropy in the case of a steeply subducting slab. This hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge, as inferred from anomalously low seismic-wave velocities.
Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.
Structural and fabric analysis of the well-exposed Hilti mantle section, Oman ophiolite, suggests that shear zone development, which may have resulted from oceanic plate fragmentation, was influenced by pre-existing mantle fabric present at the paleo-ridge. Detailed structural mapping in the mantle section revealed a gently undulating structure with an east–west flow direction. A NW–SE strike-slip shear zone cuts across this horizontal structure. The crystal preferred orientation (CPO) of olivine within the foliation is dominated by (010) axial patterns rather than more commonly observed (010)[100] patterns, suggesting that the horizontal flow close to the Moho involved non-coaxial flow. Olivine CPO within the shear zone formed at low temperature is characterized by (001)[100] patterns and a sinistral sense of shear. The olivine CPO becomes weaker with progressive mylonitization and accompanying grain size reduction, and ultimately develops into an ultra-mylonite with a random CPO pattern. The olivine [010]-axis is consistently sub-vertical, even where the horizontal foliation has been rotated to a sub-vertical orientation within the shear zone. These observations suggest that the primary mechanical anisotropy (mantle fabric) has been readily transformed into a secondary structure (shear zone) with minimum modification. This occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling. We propose that primary olivine CPO fabrics may play a significant role in the subsequent structural development of the mantle. Thus, the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center.
Recent studies of forearcs in the circum-Pacific regions have revealed that the widespread serpentinization of mantle wedge peridotite occurs along the subducting slab at depths of 15-30 km due to water supplied from the slab. A huge zone of diapiric serpentinite seamounts along the trench axis in the Izu-Bonin and Mariana forearcs suggests that voluminous and gravitationally unstable low-density serpentinites generated just above the subducting slab have risen to the seafloor to form the seamounts. During ODP Leg 125, metamorphic rock clasts recovered from Holes 778A and 779B at Conical seamount, one of the serpentinite seamounts, have provided essential information on the interaction between forearc material and water. A geochemical study of the 778 A metabasalts indicates that the rocks have a chemical affinity with mid-ocean ridge basalts, some of which have zigzag REE patterns due to intense interaction with seawater. There are two possible origins that are worth considering. One is the trapped oceanic crust in the area between the trench and the volcanic front when subduction of the Pacific plate started, and the other is the accreted oceanic crust supplied directly from descending oceanic slab during subduction. The Hole 778A metabasalts commonly contain quartz veins, which have been produced prior to or during blueschist facies metamorphism, because high-pressure minerals, lawsonite, pumpellyite, and aragonite, were often crystallized in the vein. When the trapped or accreted oceanic crust had been squeezed deep down by the subducting slab, it encountered the pelagic sediments on top of the subducting slab. The SiO2-rich fluids having permeated the Hole 778A rocks were probably derived from these pelagic sediments. A phengite-rich clast, the only clast recovered from Hole 779B, is ultrabasic in composition, but is rich with incompatible elements, such as Zr, Ti and Th, and is relatively poor in compatible elements, such as Cr , Ni, and Co. Rocks with similar geochemical characteristics are found in the metasomatic reaction zone developed at the boundary between serpentinite and pelitic schist in the high-pressure Sanbagawa metamorphic belt, Japan. The clast may have been formed at the boundary between mantle wedge peridotite and subducting slab, where the hydrothermal metasomatic reactions have pervasively occurred between mantle wedge and pelagic sediments. 近年,環太平洋前弧域では,沈み込むスラブから供給される水により,深さ15-30 kmにおいてマントルウェッジ・カンラン岩の大規模な蛇紋岩化か発生していることが明らかにされてきた.伊豆-小笠原,マリアナ前弧域において海溝軸に沿って発達する巨大なダイアピル蛇紋岩海山群は,沈み込むスラブ直上に発達した重力的に不安定な低密度の蛇紋岩が上昇し,海底に蛇紋岩海山を形成したことを示している.国際掘削計画第125節において,蛇紋岩海山の一つであるコニカル海山の掘削孔778Aと779Bから回収された変成岩片は,前弧物質と水との相互作用に関する重大な情報を与えてくれる.778Aの変成玄武岩類は,その地球化学的特徴が中央海嶺玄武岩に類似しており,その中には海水の影響を強く受けた希土類元素存在度パターンを示すものがある.これらは,太平洋プレートの沈み込み開始時に,マリアナ前弧域に封じ込められた海洋地殻あるいは直接付加した太平洋プレートの断片である可能性が強い.封じ込められたあるいは付加した海洋地殻は,スラブの沈み込みにより高圧変成作用が発生している深所まで引きずり込まれたと考えられる.778Aの変成玄武岩には通常石英脈が認められる.この石英脈には,ローソナイト,パンペリ一石,アラレ石などの高圧鉱物が再結晶していることから,脈形成の時期は変成作用時,あるいはそれ以前であることがわかる.スラブの沈み込みにより削剥・破砕され断片化した海洋地殻は,沈み込むスラブ直上の泥質堆積物に接触する.778Aの岩石類に浸透するシリカに富んだ流体は,おそらく,この泥質堆積物に由来するであろう.779Bから回収された唯一の変成岩である雲母に富んだ岩片は,超塩基性の組成をもつが,ZrやTiなどの不適合元素に富みCrやNiなどの適合元素に乏しい.同様の岩石が,三波川変成帯の蛇紋岩と泥質片岩の境界部に発達する反応帯に認められことから,この岩片は,交代作用が普遍的に起こりうるマントルウェッジと沈み込むスラブとの境界部で形成された可能性が高い.
Ocean drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (>4%), chrome-rich spinels (cr-number = 0.40-0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine-spinel equilibration temperatures that are low compared with abyssal peridotites, pssibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite-magnetite-quartz) - 1. 1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunmites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculat4ed oxygen fugacities of FMQ-0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harsburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction -zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20-25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lthosphere may have undergone extensive interaction with subduction-derived magmas.
In this manuscript we review experimental constraints for the viscosity
of the upper mantle. We first analyze experimental data to provide a
critical review of flow law parameters for olivine aggregates and single
crystals deformed in the diffusion creep and dislocation creep regimes
under both wet and dry conditions. Using reasonable values for the
physical state of the upper mantle, the viscosities predicted by
extrapolation of the experimental flow laws compare well with
independent estimates for the viscosity of the oceanic mantle, which is
approximately 1019 Pa s at a depth of ˜100 km. The
viscosity of the mantle wedge of subduction zones could be even lower if
the flux of water through it can result in olivine water contents
greater than those estimated for the oceanic asthenosphere and promote
the onset of melting. Calculations of the partitioning of water between
hydrous melt and mantle peridotite suggest that the water content of the
residue of arc melting is similar to that estimated for the
asthenosphere. Thus, transport of water from the slab into the mantle
wedge can continually replenish the water content of the upper mantle
and facilitate the existence of a low viscosity asthenosphere.
Eleven harzburgites and one dunite from Ocean Drilling Program Leg 209 Hole 1274A preserve high-temperature mantle textures. Electron backscatter diffraction (EBSD) analysis shows moderately developed crystal lattice preferred orientations (LPOs) in olivine and orthopyroxene (M-indices≈0.1) indicative of crystal-plastic deformation at ~1250°C. These rocks preserve a protogranular texture with a weak olivine foliation, a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene LPO, and minor interstitial clinopyroxene and spinel. Olivine grain size distributions, along with melt-related microstructures in orthopyroxene, clinopyroxene and spinel suggest that high-temperature deformation textures have been overprinted by pervasive post-deformation melt-rock interaction. Paleomagnetic data constrain the olivine [100] axes to be subhorizontal and oriented at low angle (≤28.6°±10.6°) to the ridge axis at the onset of serpentinization. This orientation is consistent with either complex 3-D mantle upwelling or 2-D mantle upwelling coupled with complex 3-D tectonic emplacement to the seafloor.
Petrological studies have suggested that oceanic crust is formed during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in the fore-arc region. We conducted an active-source seismic survey in the fore-arc region of the Izu-Bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction. We used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction-stack migration of picked reflection travel times to derive reflectivity images. These data show a remarkably thin crust (
The first part of this paper is a review on the main mineralogical, petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth's surface. These criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history. The unifying concept is that of adiabatic ascent rate, related to rifting and spreading rates. Rates lower than 0.5 cm/yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km. This inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue. Higher rates, probably not exceeding 1 cm/yr, correspond to oceanic rifts with the main melt extraction completed around 15 km, generating a thin oceanic crust and plagioclase lherzolites as residue. Finally, rates greater than 1–2 cm/yr correspond to oceanic ridges with melt being extracted at Moho depth, thus generating a 6-km-thick crust and leaving depleted harzburgites as residue. Thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin. This conclusion must however be tempered by the fact that the spreading rate, envisioned here as the principal controlling parameter, can change with space and time in a given environment (mantle diapirism, rifting, seafloor spreading, crust generation).
A large number of serpentinite seamounts in the Izu-Ogasawara-Mariana forearc reflect serpentinite diapirism under a tensional stress field. The Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied by serpentinite mélange including deep, high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, contains lesser amounts of serpentinite and is not accompanied by serpentinite mélange. The former were probably formed under a tensional stress regime, and the latter under a compressional one. The restricted occurrence of serpentinite to high-grade pelitic schists in the Sanbagawa belt suggests that the serpentinites were transported from the mantle wedge into pelagic sediments on the top of the subducting plate at the initiation of accretion. Tremolite rocks and phengite-chlorite schists near serpentinite in the Sanbagawa belt and from Chamorro Seamount, Mariana forearc, may suggest widespread metasomatism along the boundary between the mantle-wedge serpentinite and pelagic sediments.
The seismic anisotropy of the deep Earth is reviewed as a profile from the upper mantle to the solid inner core at the centre of the Earth. The upper mantle is by far the most anisotropic region of the Earth, followed the D" layer above the core mantle boundary. In contrast it is shown that many minerals that are present in the upper mantle, transition zone, lower mantle, D" layer and solid inner core are elastically anisotropic to different degrees. The anisotropy of hydrous phases present in subduction zones is briefly introduced. The basic concepts of single crystal and polycrystalline elasticity and the extrapolation of elastic properties to high temperature and pressure are presented. The specific features of elastic wave propagation in an anisotropic medium of any arbitrary symmetry are illustrated using mantle and inner core phases. The roles of crystal preferred orientation, water and melt in producing seismic anisotropy in the upper mantle are discussed.
A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74 ± 0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0 ± 0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84 ± 0.14 to 43.88 ± 0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench.
Shear-wave polarization anisotropy in the central part of northeastern (NE) Japan was investigated. We analyzed S phases of 293 intermediate-depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters. The obtained results show the systematic variation in splitting parameters across the arc. The leading shear-wave polarization directions (fast direction) obtained at stations in the western side of the study area are oriented nearly E–W, which is sub-parallel to the direction of relative plate motion between the Pacific plate and the North American/Okhotsk plate. In contrast, fast directions obtained at stations in the eastern side of the study area are oriented approximately N–S. Stations in the western side have a larger average delay time compared to those in the eastern side. These variations suggest that the nature of anisotropy is quite different between the eastern and the western sides. Both experimental studies on the deformation of olivine and numerical simulations of plate-driven flow predict that the lattice-preferred orientation of olivine causes the fast direction sub-parallel to the flow direction for the conditions in the mantle wedge of NE Japan. We thus infer that the lattice-preferred orientation of olivine generated by flow-induced strain is the most likely candidate for anisotropy that produces splitting with E–W fast direction. Three possible causes of anisotropy in the eastern side are considered: deformation of water-rich olivine in the mantle wedge, trench-parallel flow in the mantle wedge due to along-strike dip variations in the slab and anisotropy in the crust and the slab. We could not, however, determine which causes are dominant. Further systematic study is required to reveal the cause of anisotropy in the eastern side.
We conducted a microstructural study of samples from a natural serpentinite shear zone in the Ohmachi Seamount, Izu-Bonin frontal arc. The serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains (interpenetrating) to aligned (schistose) forms. Because the two types of grains have similar compositions, these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone. The crystal-preferred orientation (CPO) of antigorite with interpenetrating texture is almost randomly oriented, whereas in the case of schistose texture the CPO shows a typical [010](001) pattern. We also found that with increasing intensity of schistosity, the polarization plane of Vs1 for antigorite grains becomes aligned parallel to the flow plane, consistent with a plane oriented normal to the maximum concentration of slow antigorite c-axes. This configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains. These findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains (γ > ~ 2), the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge, depending on the dip angle of the subducting slab.
Subhorizontal mantle structures subparallel to the Moho are rotated into NW–SE subvertical orientations across a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite. Dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture. Mean olivine grain size stabilized at ∼0.7 mm within the shear zone center; this may reflect the steady-state grain size of dynamically recrystallized olivine, as determined by the deviatoric stress, which in this case was as low as 10 MPa. Crystal-preferred orientation (CPO) patterns of olivine are consistently [100]-fiber or partial fiber texture, indicating that olivine slip systems did not change during shearing. Dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center, but this weakening is counterbalanced by CPO strengthening due to dislocation glide. This process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone, in contrast to a gradual decrease in olivine fabric intensity and mean grain size.The measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations. Despite the development of the shear zone, dispersion of both P- and S-waves in the shear zone may be of little effect with respect to the overall seismic anisotropy. This is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high-T structure region. It suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure, as localized structures may be obscured in the region of the observation.
A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge. However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous. Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic velocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity, an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength. Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin–Mariana, and central Japan. Serpentinization may explain why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the low density, weak serpentinized forearc mantle. Large volumes of H2O may be released from serpentinized forearc mantle by heating during ridge subduction or continent collision.
We present a unique database of 110 olivine petrofabrics and their calculated seismic properties. The samples come from a variety of the upper mantle geodynamic environments (ophiolites, subduction zones, and kimberlites) with a wide range of micro-structures. A phenomenological relationship is established between P- and S-wave seismic anisotropy and the degree of crystal alignment (fabric strength). Seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20% for P-waves and 10 to 15% for S-waves. Despite a large variation in the symmetry of fabric patterns, the average seismic properties of the different fabric, micro-structural and geodynamic settings have similar anisotropies in both magnitude and symmetry. Hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known, but not geodynamic environment or details of the petrofabric pattern. A simple pattern of seismic anisotropy characterises the average sample of the database, which has the following features: the polarisation plane of the fastest S-wave is parallel or sub-parallel to the foliation plane; the maximum shear wave splitting is parallel to the Y structural direction (in foliation plane and normal to the lineation); the maximum of the P-wave velocity is parallel to the high concentration of [100] axes, which is sub-parallel to the lineation. The [100] orientation distribution has the greatest influence on the P-wave seismic anisotropy. The [100] and [001] orientation distributions have the greatest influence on the symmetry of S-wave anisotropy, although the magnitude of anisotropy is influenced by the distribution of all three principal axes. Although the database only contains olivine petrofabrics, this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle.
Material recycling in subduction zones, including the generation and migration of aqueous fluids and melts, is key to understanding the origin of volcanism in subduction zones and is also important for understanding the global circulation of materials. Recent knowledge concerning the phase relationships of hydrous peridotitic and basaltic systems allows us to model the fluid generation and migration in subduction zones. Here I present a numerical model, in which the aqueous fluid migrates by permeable flow and interacts chemically with the convecting solid, including melting. The calculation results suggest that nearly all the H2O expelled from the subducting slab will be hosted by serpentine and chlorite just above the slab, and is brought down by up to 150 km, depending on the temperature along the slab. Breakdown of serpentine and chlorite at these depths results in the formation of a fluid column through which H2O is transported upwards. The fluid reaches a depth corresponding to a cusp of the H2O-undersaturated solidus of peridotite (minimum at 2.5 GPa) and initiates extensive melting whose depth and the lateral extent toward the trench side is nearly fixed, irrespective of the age of the slab. This is because the flux of H2O and the depression of the practical solidus temperature in the mantle wedge are similar for models with different slab ages. Exceptionally, for very young slabs (e.g. ≪10 Myr when the subduction velocity is ∼6 cm/y), different melting regimes occur, such as melting in the forearc region and slab melting. If the aqueous fluid released from the slab migrates upwards in disequilibrium (e.g. through fractures), significant melting occurs in the forearc region, since the serpentinite layer, an effective H2O-absorber, is not formed. However, this is not the case in most subduction zones. Further studies with various subduction parameters, and melt segregation processes which are not included in this study, are required to compare the model results with the observed distribution and chemistry of arc magmas.
Observations of seismic anisotropy yield some of the most direct constraints available on both past and present-day deformation in the Earth's mantle. Insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales. We highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress, particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it. To first order, the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins, but the fit is poorer in regions of greater tectonic complexity, such as beneath continental interiors and within subduction systems. In many regions of the upper mantle, models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations, which suggests that our understanding of complex anisotropic regions remains incomplete. Observations of anisotropy in the D" layer hold promise for improving our understanding of dynamic processes in the deep Earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow, geochemical heterogeneity, or phase transitions. Major outstanding problems related to understanding mantle anisotropy remain, particularly regarding the deformation and evolution of continents, the nature of the asthenosphere, subduction zone geodynamics, and the thermo-chemical state of the lowermost mantle. However, we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas.
The hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths. In the absence of data, Newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions [Hilairet, N., et al., 2007. High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. Science, 318(5858): 1910–1913]. Using a simple 1D model of a serpentinized channel and – as opposed to previous models – in this power law rheology, we examine the influence of channel thickness, temperature and subduction angle on serpentine flow driven by density contrast (serpentinization degree) with the surroundings. At temperatures of 200–500 °C relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2–3 km. For channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 1018 Pa s. The velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed HP-rocks. Upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature. Competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation. At shallower levels the thickness allowed for a channel may be up to ~ 8–10 km if the rheology has a higher dependence on stress. We therefore propose that the exhumation of HP oceanic units in serpentinite channels is organized in two levels, the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow. The thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies. The deepest serpentinite channel may thus be difficult to detect by seismic methods, but it will have a strong influence on the mechanical coupling between the slab and mantle wedge.
Within the undisrupted, 6 km thick mantle section of the Table Mountain massif (Bay of Islands ophiolite, Newfoundland), several distinct peridotite microstructures are recognized. In the uppermost mantle section, very high temperature (T ∼ 1200°), high strain microstructures with evidence for syntectonic melt infiltration are preserved. Remarkable among these is an orthopyroxene-impregnated dunite. At an intermediate level of the mantle section, peridotites have been affected by a weak, lower temperature (T = 1000–1100°C) overprint. They are underlain by a thin sliver of low strain, high temperature peridotites. All these peridotites are interpreted to have formed during spreading-related strain. Lower temperature (T < 1000°C), high stress microstructures with high accumulated strain are locally preserved at intermediate levels of the mantle section and are regionally present at its base. They culminate in (metasomatic) ultramafic ultramylonites and are related to detachment of the ophiolite.
In oceanic subduction zones, dehydration of slab’s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength. This should enhance the upper plate base reheating and markedly alter its thermal structure. To quantify this phenomenon, we model slab subduction within a viscous mantle, dehydration–hydration processes, and the rock strength dependence on water content. We use accurate phase diagrams for a H2O-satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock. Transition phases are supposed to be not metastable. Water is released from the oceanic crust and from the altered peridotite portion of the slab. Dehydration of the subducting lithosphere occurs first at 60–75 km depth, when the crust is eclogitized, and second, deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable. For high convergence rates, because of cold P–T–t paths in the slab, serpentine can be transformed into the hydrated phase A and water is recycled by the slab until great depth. However, in all investigated cases, the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217±55 km away from the trench and, as it goes up, to form hydrated minerals in the overlying lithosphere over a significant volume. The convergence rate slightly shifts dehydration fronts location, and consequently widens or reduces the hydrated mantle wedge. Note that for the dynamic corner flow modelled here, with a non-Newtonian rheology, the slab surface is significantly warmer than for an isoviscous analytical corner flow model, yielding plausible crust melting. We assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water. The rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge. This results in cooler temperatures along the slab top, but in an enhanced corner flow effect on the upper plate thermal structure. For large hydration strength reductions, a strong thermal erosion of the overlapping lithosphere develops in less than 15 Myr due to convective destabilization. This weakens drastically the upper plate at a distance from 110 to 220 km away from the trench. The geometry of the eroded region could correspond to the low-velocity zone observed below the arc region.
We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594-7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region.
Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an 'inverted' continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.
Petrological studies of peridotites from diapiric serpentinite seamounts in the Izu-Ogasawara-Mariana fore-arc, Leg 125
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