Forearc Mantle Research Articles

Petrology of a Neoproterozoic mantle peridotite–chromitite association from Abu Dahr area, Eastern Desert, Egypt: Infiltration of a boninitic melt in highly depleted harzburgite

Peridotites of Abu Dahr represent the main litho–unit of a Neoproterozoic dismembered ophiolite sequence and are among the best–preserved and well–exposed mantle rocks in South Eastern Desert of Egypt. Here, we present new geochemical and mineral chemical data for peridotites and associated pyroxenites and for chromitites and their platinum–group minerals to constrain their petrogenesis and geotectonic setting. The Abu Dahr ophiolite mantle section consists mainly of harzburgites, cut by pyroxenite dykes and containing dunite–chromitite lenses. The harzburgites are composed of olivine, orthopyroxene, spinel and minor clinopyroxene (˂1.0 vol %) and amphibole. Olivine from harzburgites is highly magnesian (Fo 91–93) and Cr–spinel shows a wide–range of Cr2O3 and Al2O3 contents. The enstatite component of orthopyroxene decreases from harzburgite (En = 90–91) to orthopyroxenite (En = 84–87). Amphiboles are represented by magnesiohornblende and tschermakite. The chromitites are massive to disseminated and composed of magnesiochromite with high Cr# (83–93) and Mg# (66–79), and low TiO2 (<0.1 wt%) content. Solid inclusions in chromite include olivine, orthopyroxene and hornblende. Laurite (RuS2) is the most common PGM detected in the investigated chromitite samples and forms micrometer–size inclusions in fresh chromite. Various Ni–sulfides are found both in fresh chromite and along serpentine veinlets. Harzburgites have a refractory composition with a very low Al2O3 (0.4–0.8 wt%) and CaO (0.2–1.6 wt%) contents and high bulk–rock Mg# (89–92). Geochemical data suggest that the Abu Dahr peridotites are highly depleted SSZ peridotites formed in a forearc mantle wedge setting by high degrees of hydrous partial melting and emplaced as a result of the collision of the intra–oceanic arc with the Beitan gneisses. The podiform chromitites and orthopyroxenites were formed due to impregnation of mantle wedge harzburgites by boninitic melt. The highly depleted nature of the harzburgite is responsible for the small reserves of chromite ore at Abu Dahr and in the South Eastern Desert in general.

Investigation of Fluid-driven Carbonation of a Hydrated, Forearc Mantle Wedge using Serpentinite Cores in High-pressure Experiments

AbstractHigh-pressure experiments were performed to investigate the effectiveness, rate and mechanism of carbonation of serpentinites by a carbon-saturated COH fluid at 1·5–2·5 GPa and 375–700 °C. This allows a better understanding of the fate and redistribution of slab-derived carbonic fluids when they react with the partially hydrated mantle within and above the subducting slab under pressure and temperature conditions corresponding to the forearc mantle. Interactions between carbon-saturated CO2–H2O–CH4 fluids and serpentinite were investigated using natural serpentinite cylinders with natural grain sizes and shapes in piston-cylinder experiments. The volatile composition of post-run fluids was quantified by gas chromatography. Solid phases were examined by Raman spectroscopy, electron microscopy and laser ablation inductively coupled plasma mass spectrometry. Textures, porosity and phase abundances of recovered rock cores were visualized and quantified by three-dimensional, high-resolution computed tomography. We find that carbonation of serpentinites is efficient at sequestering CO2 from the interacting fluid into newly formed magnesite. Time-series experiments demonstrate that carbonation is completed within ∼96 h at 2 GPa and 600 °C. With decreasing CO2,aq antigorite is replaced first by magnesite + quartz followed by magnesite + talc + chlorite in distinct, metasomatic fronts. Above antigorite stability magnesite + enstatite + talc + chlorite occur additionally. The formation of fluid-permeable reaction zones enhances the reaction rate and efficiency of carbonation. Carbonation probably occurs via an interface-coupled replacement process, whereby interconnected porosity is present within reaction zones after the experiment. Consequently, carbonation of serpentinites is self-promoting and efficient even if fluid flow is channelized into veins. We conclude that significant amounts of carbonates may accumulate, over time, in the hydrated forearc mantle.

Effects of reactive dissolution of orthopyroxene in producing incompatible element depleted melts and refractory mantle residues during early fore-arc spreading: constraints from ophiolites in eastern Mediterranean

Compositional variations of peridotites from the Cretaceous ophiolites in southern Turkey and Northern Cyprus are presented to document the nature of partial melting and possible effects of reactive dissolution of primary mantle phases during fore-arc spreading. The peridotites overall exhibit a range of 187Os/188Os ratios from 0.1171 to 0.1266 and appear to represent a mantle region that preserves a record of ancient melt depletion. The samples are depleted in 187Os/188Os compared to the ambient oceanic upper mantle (187Os/188Os ~0.127), suggesting that they are representatives of a shallow fore-arc mantle where transport of radiogenic 187Os during slab dehydration was limited. Chemical variations of primary mantle minerals indicate that the peridotites are the residues of moderate to high degrees (>16%) of partial melting and have experienced significant modal and chemical compositional modification through interaction with oxidizing hydrous basaltic melts. Interacting melts, which appear to be similar in composition to primitive arc tholeiite, are likely to have originated from sub-lithospheric lower part of the mantle wedge during early stages of fore-arc spreading and migrated upward to react with variably depleted harzburgites to induce further melting in the overlying lithospheric mantle through open-system reactive flow. This second stage melting resulted in (1) common occurrence of reactive harzburgites and dunites by incongruent melting of orthopyroxene and crystallization of olivine through interaction with olivine saturated melt; and (2) local development of refertilized peridotites by shallower melt impregnation that involves interaction with olivine + clinopyroxene saturated melt. The dissolution of orthopyroxene caused the reacting melt to be enriched in silica and diluted in incompatible elements which led to the production of the final melts similar in composition to fore arc basalt and boninite. Involvement of compositionally variable mantle and melt components with different rates of melt influx therefore appears to explain the generation of fore-arc crust with a range of diverse rock suites including temporally and spatially associated arc tholeiites and boninites with significant depletion in incompatible elements.

Neoproterozoic and Cretaceous mantle oxidation states: Controls and heterogeneity through time

To estimate the oxygen fugacity (fO2) of the Neoproterozoic and Cretaceous suprasubduction zone mantle, and to evaluate the possible secular changes in the upper mantle oxidation state, the compositions of spinel, olivine and orthopyroxene of Neoproterozoic (Egypt and Saudi Arabia) and late Cretaceous (Iran) mantle rocks were determined. For accurate estimation of fO2, spinel ferric iron was calculated after correcting the electron microprobe data using a set of spinel standards for which the ferric iron content was measured by Mӧssbauer spectroscopy. The Neoproterozoic samples record strongly heterogenous fO2 values ranging from moderately oxidized (FMQ +0.54) to ultra-reduced (FMQ -4.73) for harzburgites, from highly oxidized (FMQ +1.49) to moderately reduced (FMQ -0.60) for dunites as well as one highly reduced (FMQ -1.61) value for chromitite. Such heterogeneity is not apparent in the late Cretaceous harzburgites that record fO2 values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The fO2 of the Neoproterozoic forearc mantle is most easily explained by melt-mantle interaction and deep-mantle recycling, while that of the late Cretaceous forearc mantle can be attributed to variable degrees of melt-mantle interaction. The estimated fO2 values of Neoproterozoic/Cretaceous mantle unaffected by melt-rock interaction and deep-mantle recycling, and published values of Precambrian and Modern mantle suggest a consistent upper mantle oxidation state from Proterozoic to present day.

Mariana serpentinite mud volcanism exhumes subducted seamount materials: implications for the origin of life.

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life.This article is part of a discussion meeting issue ‘Serpentine in the Earth system’.

Petrology and geochemistry of ophiolitic ultramafic rocks and chromitites across the Eastern Desert of Egypt: Insights into the composition and nature of a Neoproterozoic mantle and implication for the evolution of SSZ system

Large outcrops of ultramafic rocks are common in the Central Eastern Desert (CED) of Egypt and represent residual mantle sections and cumulates of dismembered Neoproterozoic oceanic lithosphere. This study integrates field observations, petrography, geochemistry and mineral chemistry to investigate ultramafic rocks of the dismembered ophiolitic rocks at four areas distributed from east to west across the CED, with the aim of gaining a better understanding the tectonic settings and mantle conditions under which the ultramafic rocks formed. The ultramafic masses studied are composed of serpentinized peridotites, serpentinites and pyroxenites, and occasionally host podiform chromitites. Preserved textures, relict primary minerals and whole-rock compositions indicate that the protoliths of these ultramafic rocks were dominated by harzburgites. The Al2O3- and CaO-depleted nature of residual harzburgites, the high Fo and NiO contents of primary olivines and the high Cr# and low TiO2 contents of fresh Cr-spinels suggest that these ultramafic rocks are remnants of depleted to highly depleted forearc mantle of suprasubduction zone (SSZ) system. Temperatures from olivine-spinel geothermometry (668–778 °C) and Al-in-orthopyroxene geothermometry (984 °C) estimated for residual harzburgites are comparable to those reported for modern forearc mantle peridotites. The range of oxygen fugacity [ΔlogƒO2 (FMQ)] values (−3.01 to +0.32) calculated for the residual harzburgites reveals heterogeneity in the Neoproterozoic mantle oxidation state and suggests that SSZ mantle was not consistently more oxidized than MOR mantle. The podiform chromitites of different ultramafic masses were formed following the interaction of residual mantle peridotites with forearc basaltic and boninitic melts, implying the evolution of Neoproterozoic SSZ system from subduction initiation to more mature island arc stage. Integration of the analyses carried out in this study shows a Neoproterozoic SSZ system where slab-derived melts evolve in composition and oxidation state from early reduced forearc basalt in the east to more oxidised boninitic melts in the west.

Saline aqueous fluid circulation in mantle wedge inferred from olivine wetting properties

Recently, high electrical conductors have been detected beneath some fore-arcs and are believed to store voluminous slab-derived fluids. This implies that the for-arc mantle wedge is permeable for aqueous fluids. Here, we precisely determine the dihedral (wetting) angle in an olivine–NaCl–H2O system at fore-arc mantle conditions to assess the effect of salinity of subduction-zone fluids on the fluid connectivity. We find that NaCl significantly decreases the dihedral angle to below 60° in all investigated conditions at concentrations above 5 wt% and, importantly, even at 1 wt% at 2 GPa. Our results show that slab-released fluid forms an interconnected network at relatively shallow depths of ~80 km and can partly reach the fore-arc crust without causing wet-melting and serpentinization of the mantle. Fluid transport through this permeable window of mantle wedge accounts for the location of the high electrical conductivity anomalies detected in fore-arc regions.

Multistage CO2 sequestration in the subduction zone: Insights from exhumed carbonated serpentinites, SW Tianshan UHP belt, China

Climate is regulated by the carbonate–silicate cycle in which slab outgassing of C into deep fluids and volcanic degassing of CO2 into the atmosphere are an important part. However, the mechanisms of C mobility in subduction zones remain largely unresolved. Previous research has focused mainly on investigating the upward transfer of slab-derived carbonic fluids for the forearc mantle metasomatism and partial melting. Furthermore, percolation of CO2-bearing fluids parallel to the downgoing plates can potentially drive carbonation of subducted rocks, which influences the global estimates of C fluxes at convergent margins. Nevertheless, the geological conditions and processes leading to the carbonation of subduction-zone lithologies by fluid–rock interactions are still poorly understood. Here, we present new field, petrological, and isotopic results of carbonated serpentinites—high-pressure (HP) ophidolomites and low-pressure (LP) ophimagnesites and listvenites—from the Chinese southwestern Tianshan HP–UHP metamorphic belt. These rocks recorded the carbonation of subduction-zone serpentinites at HP and LP conditions during exhumation, reflecting the multistage transfer and infiltration of carbonic fluids along the plate boundary. The HP ophidolomites are characterized by the growth of carbonates (dolomite, aragonite, and Mg-calcite) at the expense of silicates in the host serpentinites. Integrated Sr–C–O isotopic data and thermodynamic modelling suggest that carbonic fluids (containing a CO2,aq concentration of up to 1.9 molal) emanating from carbonate-bearing metamafic rocks (e.g., eclogites) likely contributed to HP carbonation of serpentinites at about 15–25 kbar and 550–600 °C. The close contact of ophidolomites and carbonated metasedimentary rocks in the field as well as their similarities in Sr isotope compositions suggests that the latter could also have acted as the possible C source. Alternatively, both lithologies may have formed by coupled HP carbonation along the lithological interface between serpentinites and metasedimentary rocks. Subsequent fluid–rock interactions at relatively shallow crustal levels resulted in a second stage of serpentinite carbonation to form LP ophimagnesites and listvenites, during which the metasomatic CO2-bearing fluids may have originated from metasedimentary rocks. The multistage CO2 sequestration in subduction-zone serpentinites implies that hydrated ultramafic rocks in subducted slabs are highly effective reactants to capture and store slab-released C over a wide range of P–T conditions, with the potential to substantially control the C distribution between shallow and deep reservoirs and thus modulate C fluxes in subduction zones.

Correlation of Elastic Moduli and Serpentine Content in Ultramafic Rocks

Understanding the physical properties of ultramafic rocks is important for evaluating a wide variety of petrologic models of the oceanic lithosphere, particularly upper mantle and lower crust. Hydration of oceanic peridotites results in increasing serpentine content, which affects lithospheric physical properties and the global bio/geochemical cycles of various elements. In understanding tectonic, magmatic, and metamorphic history of the oceanic crust, interpreting seismic velocities, rock composition, and elastic moduli are of fundamental importance. In this study, we show that as serpentine content increases, density decreases linearly with a slope of 7.85. Porosity of the samples does not show any systematic correlation with serpentine content, as it is more strongly affected by local weathering and erosional processes. We also correlate increase in serpentine content with a linear decline in shear, bulk, and Young’s moduli with slopes of 0.48, 0.77, and 0.45, respectively. Our results show that increase in serpentine content of mantle wedge and forearc mantle contributes to their brittle behavior and result in break-offs, obduction, and overthrusting. Therefore, serpentine content strongly affects tectonic processes at subduction zones, particularly serpentinization may be responsible for formation of weak fault zones. Also, serpentinization of fresh oceanic peridotite in slow and ultra-slow spreading ridges may be responsible for observed discontinuities in thin crust.

Variations in mantle lithosphere buoyancy reveal seismogenic behavior in the Sunda-Andaman subduction zone

Summary The distribution of historic earthquakes in the Sumatra subduction zone reveals, in the fore-arc region, the intense seismic activity and frequent occurrences of Mw &gt; 8 earthquakes throughout the whole area. In contrast, the neighboring region has less dense seismicity, and no large earthquake greater than Mw8 has been observed in the Java subduction zone. Such different seismic behaviors may be due to distinct degrees of the stress accumulation and release. In this study, the strength of plate coupling inferred from mantle lithosphere buoyancy (Hm) estimation is used to explain the seismogenic behavior in the Sunda-Andaman subduction zone. Strong and weak plate coupling status are obtained in the Sumatra and Java subduction zones, respectively. These results can explain the significant differences in seismogenic behaviors in the Sunda-Andaman subduction zone. In assessing the global implications of this finding, we observe that uplifted serpentinized fore-arc mantle peridotite is the critical phenomenon in weak plate coupling cases and leads to a limit on the width of the coupling zone. Strong plate coupling can cause a relatively low gravity anomaly as well as a negative trench-parallel gravity anomaly (TPGA) in the fore-arc regions and correlates well with the occurrence of large earthquakes, whereas weak plate coupling can cause a positive TPGA and constrain the potential occurrence of large earthquakes.

Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region

Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24-40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich (similar to 42 wt%), but have low contents in Al2O3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%-25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount (mostly >25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements (Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.

Evolution and maturation of the nascent Mariana arc

Long-standing debate remains regarding the processes by which nascent island arcs evolve into thicker-crust, mature volcanic edifices, long before being accreted into continental crust. How mature island arcs maintain their location for millions of years is still unclear. Arc location is currently believed to be controlled by slab dehydration or by the thermal state of the sub-arc mantle. However, most studies have focused on modern island arcs that have developed mature and stable volcanic edifices. Using published geochemical datasets, here we show that the southern Mariana arc is an incipient island arc that evolves into stable, thicker-crust stratovolcanoes to the north. Because it shares similar geochemical features with the early arc that formed during subduction infancy, it can place new important constraints onto the processes that occurred during island arc infancy. Examination of the Mariana arc along a S-N transect suggests that the growth of the serpentinized fore-arc mantle has the potential to modulate the depth of arc magma generation, which allows, in return, stabilization of the volcanic edifices during arc maturation. Arc stabilization further enables the deeply-sourced, slab fluids to infiltrate the sub-arc mantle wedge, which results in magmas with a typical arc fingerprint to the north. Melt focusing along magmatic conduits further promotes magma differentiation, magma-crust assimilation, and subsequent crustal thickening. We further propose that transient magmatic activity may also occur during island arc infancy in the Izu-Bonin-Mariana convergent margin, until serpentinization of the fore-arc mantle develops and stabilizes.

Fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor

Slow fault slip includes a range of transient phenomena that occur over timescales longer than those of standard earthquakes. Slow slip events are often closely associated with swarms of tectonic tremor. Deep episodic tremor and slip close to the slab–mantle interface in subduction zones has been linked to high fluid pressures produced by dehydration of the subducting slab at greater depths. The slab–mantle interface is a fundamental chemical boundary, where mantle rocks are sheared and mixed with oceanic slab lithologies in a highly reactive environment to form serpentinite. Here we present field and microstructural observations from the plate boundary-scale crustal Livingstone Fault in New Zealand that suggest chemical reactions involving serpentinite can promote rock hardening and generate in situ fluid overpressures. We infer that these processes collectively can result in hydrofracturing and a transition from distributed creep to localized brittle failure and faulting. Serpentinite-related reactions occur over a wide range of pressure and temperature conditions that overlap with those in many forearc mantle wedges. We conclude that the release of fluids derived from such reactions may be an additional and widespread mechanism to generate high fluid pressure patches and brittle failure in the source region of deep tremor along the slab–mantle interface. Chemical reactions between slab and mantle rocks may lead to brittle failure where deep episodic tremor occurs in subduction zones, according to field and microstructural observations of a shear zone in New Zealand.

Formation of secondary olivine after orthopyroxene during hydration of mantle wedge: evidence from the Khantaishir Ophiolite, western Mongolia

Metaharzburgite and metadunite in the ultramafic body of the Naran Massif in the Khantaishir Ophiolite, western Mongolia, record multi-stage processes of serpentinization (antigorite, lizardite + brucite, then chrysotile). Bulk-rock chemistry and the compositions of primary olivine (P-olivine) and Cr-spinel suggest that the alteration occurred in the forearc mantle. In the metaharzburgite, a novel occurrence of fine-grained (10–50 μm) secondary olivine (S-olivine) takes the form of aggregates (a few millimeters across) with bands of antigorite. The S-olivine has higher Mg# values (0.96–0.98) than the P-olivine (Mg# = 0.92–0.94) and contains inclusions of clinopyroxene and magnetite. The P-olivine has been replaced by antigorite and magnetite. Mesh textures of lizardite + brucite are developed in both P- and S-olivine. The microtextures and chemical compositions of minerals suggest that S-olivine aggregates were formed by pseudomorphic replacement of orthopyroxene related to multi-stage hydration processes. Assuming the mantle wedge conditions beneath a thin crust, orthopyroxene was first replaced by S-olivine + talc at high temperatures (500–650 °C at ~ 0.5 GPa). With cooling to ca. 400–500 °C and fluid supply, talc transformed to antigorite with the release of silica. During this stage, P-olivine was also transformed to antigorite by consumption of silica released from orthopyroxene decomposition. At temperatures below 300 °C, lizardite + brucite ± magnetite formed from the remaining P- and S-olivine grains. The formation of S-olivine presented in this study contrasts with the commonly ascribed process of deserpentinization. Taking into account the geochemical data for the studied ultramafic rocks and those previously reported for mafic rocks, our results suggest that mantle wedge beneath thin crust was hydrated in response to continuous cooling and fluid supply from a subducting slab after subduction initiation.

Mantle Dynamics of the Eastern Mediterranean and Middle East: Constraints From P‐Wave Anisotropic Tomography

AbstractHigh‐resolution images of P‐wave anisotropic tomography beneath the Eastern Mediterranean and Middle East are determined by inverting a large number of high‐quality P‐wave arrival times. Our results clearly show along‐strike variations of subducting slabs in the Hellenic subduction zone. At least three high‐velocity anomalies are imaged beneath and behind the north Hellenides, which are associated with the subducting Hellenic slab at different stages, while in regions further south, a single slab is revealed that has subducted down to a depth of ~1,100 km. The slab feature is not prominent in the Cyprus subduction zone, but a fine‐scale high‐velocity structure is revealed beneath the central Anatolia, which may reflect the detached Cyprus slab. Trench‐parallel fast‐velocity directions occur in the forearc mantle wedge of the Hellenic subduction zone, which may reflect mantle flow induced by slab curvature. In the backarc north Aegean and westernmost Anatolia, dominant NNE‐SSW to N‐S fast‐velocity directions exist in the shallow mantle, which accord well with the extensional direction of present deformation at the surface, suggesting a vertically coherent deformation in the lithosphere. Our results reveal a large‐scale mantle flow beneath western Arabia, which is possibly related to the Zagros and Caucasus orogens and westward motion of the Anatolia lithosphere. The Arabian lithospheric plate is thinner beneath the Zagros than that under the Mesopotamian Foredeep, suggesting that a large portion of the crust was scrapped off from the subducted Arabian lithosphere during the Zagros orogen.

Porosity of metamorphic rocks and fluid migration within subduction interfaces

Large earthquakes break the subduction interface to depths of 60 to 80 km. Current models hold that seismic rupture occurs when fluid overpressure builds in link with porosity cycles, an assumption still to be experimentally validated at high pressures. Porosities of subduction zone rocks are experimentally determined under pressures equivalent to depths of up to 90 km with a novel experimental approach that uses Raman deuterium-hydrogen mapping. Natural rocks (blueschists, antigorite serpentinites, and chlorite-schists) representing a typical cross-section of the subduction interface corresponding to the deep seismogenic zone are investigated. In serpentinite, and to a smaller extent blueschist, porosity increases with deformation, whereas chlorite-rich schists remain impermeable regardless of their deformation history. Such a contrasting behavior explains the observation of over-pressurized oceanic crust and the limited hydration of the forearc mantle wedge. These results provide quantitative evidence that serpentinite, and likely blueschist, may undergo porosity cycles making possible the downdip propagation of large seismic rupture to great depths.

Seismic Anisotropy Evidence for Ductile Deformation of the Forearc Lithospheric Mantle in Subduction Zones

AbstractSeismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench‐parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab‐driven corner flow. Its formation mechanism is currently unclear. Here we present high‐resolution 3‐D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench‐parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity &gt; vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab‐mantle decoupling at a common depth of ~70 km. On the basis of the high‐resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench‐parallel shear wave splitting measurements dominant in subduction zones globally.

Subduction-Induced Fractionated Highly Siderophile Element Patterns in Forearc Mantle

Compositions of highly siderophile elements (HSEs) in forearc mantle have been little studied and effects of slab dehydration on their abundances in forearc mantle remains unclear. This study reports two different kinds of HSE patterns for peridotites from a New Caledonia forearc ophiolite. The Group-I samples show relatively flat patterns of Ir-group-platinum-group elements (IPGEs) and enrichment of Pt over Pd. Such patterns imply that interstitial sulfides were significantly removed through melt extraction, whereas sulfides enclosed within silicates were mostly unaffected. Meanwhile, Pt-Fe alloys were generated, resulting in suprachondritic Pt/Pd ratios. In contrast, the Group-II samples display convex HSE patterns and are depleted in all HSEs except for Ru, yielding strongly positive Ru anomalies. This indicates that both enclosed and interstitial sulfides were substantially consumed, whereas chromite was generated to stabilize Ru. Compared to abyssal peridotites, subduction-related peridotites commonly have stronger fractionation in the HSEs. Therefore, the HSE data of mantle peridotites are potentially able to discriminate the tectonic settings of ophiolites.

Iron isotopic composition of supra-subduction zone ophiolitic peridotite from northern Tibet

Abstract We present high-precision iron isotope data for harzburgites and constituting minerals from the Yushigou supra-subduction zone ophiolite in the North Qilian orogen of northern Tibet to provide insights into iron isotope behavior in subduction zones. The Yushigou harzburgites represent typical melting residues of silica-enriched forearc mantle wedge with significant melt extraction and minimal melt percolation. The δ56Fe ranges from −0.096‰ to 0.110‰ in olivine, from −0.111‰ to 0.105‰ in orthopyroxene and from −0.141‰ to 0.054‰ in spinel. Coexisting minerals fall on a line with a slope of 1 in the δ-δ plot, indicating iron isotope equilibrium between mineral pairs. Iron isotopic fractionation between olivine and orthopyroxene is minor, which is consistent with theoretical predictions and previous observations. Good iron isotopic equilibrium between olivine and orthopyroxene is a further indicator that silica enrichment in arc peridotites is inherited from their mantle source. In contrast, significantly negative iron isotopic fractionation between spinel and olivine contradicts with most previous constraints, which may be ascribed to Cr substitution in spinel. Bulk rock harzburgites have δ56Fe ranging from −0.097‰ to 0.076‰, the average of which (0.002 ± 0.020‰) is similar to average δ56Fe determined for abyssal peridotites. Based on this inference, the lower δ56Fe of arc magmas than MORB should be due to smaller discrepancy in Fe3+/ΣFe between arc magmas and arc peridotites when compared with MORB and abyssal peridotites. Our modelling demonstrates that this can be achieved if arc magmas are produced by redox-buffered melting of silica-enriched mantle wedge while MORB are produced by redox-unbuffered melting of the depleted mantle.

Sr, Nd, Pb and trace element systematics of the New Caledonia harzburgites: Tracking source depletion and contamination processes in a SSZ setting

The New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91–93 mol.%), high Mg# of orthopyroxene (0.91–0.93) and high Cr# of spinel (0.44–0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (DyN/YbN = 0.07–0.16) and fractionated LREE-MREE segments (LaN/SmN = 2.1–8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%–18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr–Hf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al2O3, and CaO-poor orthopyroxene (Al2O3 = 0.88–1.53 wt.%, CaO = 0.31–0.56 wt.%) and clinopyroxene with low Na2O (0.03–0.16 wt.%), Al2O3 (0.66–1.35 wt.%) and TiO2 (0.04–0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subduction-related melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic (−0.80≤εNdi≤+13.32) and negatively correlate with Sr isotopes (0.70257≤87Sr/86Sr ≤ 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.