Forearc Mantle Wedge Research Articles

Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

Serpentinization of the forearc mantle wedge in subduction zones: revisiting Roy D. Hyndman’s seminal contributions 25 years later

In this paper, we focus on the serpentinization of the forearc mantle wedge, just one of Roy Hyndman’s many contributions to our understanding of subduction zones. Over the past 25 years, numerous advances in geophysics, petrology, and geology clearly document that H2O-rich fluids, derived from the subducting plate, hydrate portions of the overlying mantle to form serpentinite. The extent of mantle-wedge serpentinization depends, to first approximation, on the thermal evolution of the subducting plate. Dehydration reactions in warm subducting slabs occur at shallow (<100 km) depth making relatively large amounts of H2O available for forearc mantle-wedge hydration; in cool subduction zones, dehydration reactions occur at greater depth and less H2O is available to directly hydrate the shallow mantle wedge. High-resolution seismological studies, complemented by numerical modelling, reveal that serpentinization in a subduction zone varies spatially, with strong evidence for a serpentinite layer at the base of the mantle wedge. Serpentinite mineralogy plays an important role in controlling the rheologic behaviour of the subduction plate interface, but appears not to control the downdip extent of large thrust earthquakes as originally proposed. Weak serpentinite along the base of the mantle wedge acts to mechanically isolate the forearc mantle wedge from induced corner flow, and serpentinized regions of the forearc mantle wedge may localize deformation. Rare mantle-wedge earthquakes may reflect the subvertical flow of fluids along fractures. Future work in well-instrumented subduction zones is expected to clarify the spatial distribution and extent of serpentinization in the forearc mantle wedge.

Evolving Subduction Zone Thermal Structure Drives Extensive Forearc Mantle Wedge Hydration

AbstractHydration of the subduction zone forearc mantle wedge influences the downdip distribution of seismicity, the availability of fluids for arc magmatism, and Earth's long term water cycle. Reconstructions of present‐day subduction zone thermal structures using time‐invariant geodynamic models indicate relatively minor hydration, in contrast to many geophysical and geologic observations. We pair a dynamic, time‐evolving thermal model of subduction with phase equilibria modeling to investigate how variations in slab and forearc temperatures from subduction infancy through to maturity contribute to mantle wedge hydration. We find that thermal state during the intermediate period of subduction, as the slab freely descends through the upper mantle, promotes extensive forearc wedge hydration. In contrast, during early subduction the forearc is too hot to stabilize hydrous minerals in the mantle wedge, while during mature subduction, slab dehydration dominantly occurs beyond forearc depths. In our models, maximum wedge hydration during the intermediate phase is 60%–70% and falls to 20%–40% as quasi‐steady state conditions are approached during maturity. Comparison to global forearc H2O capacities reveals that consideration of thermal evolution leads to an order of magnitude increase in estimates for current extents of wedge hydration and provides better agreement with geophysical observations. This suggests that hydration of the forearc mantle wedge represents a potential vast reservoir of H2O, on the order of 3.4–5.9 × 1021 g globally. These results provide novel insights into the subduction zone water cycle, new constraints on the mantle wedge as a fluid reservoir and are useful to better understand geologic processes at plate margins.

Genesis of high-Al chromitites from Xiaosuihe peridotite (Central Jilin Province, NE China): Implication for initial subduction of the Paleo-Asian ocean

This study presents whole-rock major- and trace-element data of serpentinites and mineral geochemistry of chromite/spinel from both serpentinites and associated podiform chromitites from the Xiaosuihe region in central Jilin Province, NE China, aimed to elucidate the genesis of serpentinites and podiform chromitites. The serpentinites exhibit low Al2O3/SiO2 ratios (0.01–0.02) and high Mg# (0.90–0.91), indicating their derivation from the fore-arc mantle wedge and protolith as harzburgite. Chromitite ores with semi-massive to disseminated textures occur as small pods. The chromite/spinel (Chr) present in both the podiform chromitite and serpentinite can be categorized into four distinct microstructural groups: (1) partially altered chromite, characterized by high-Al primary cores (Cr# = 0.44–0.49), surrounded by high-Cr porous rims; (2) porous chromite, featuring abundant pores enriched in Cr and Fe and depleted in Mg and Al, filled with chlorite; (3) zoned chromite consisting of high-Al cores surrounded by non-porous rims of Cr-magnetite (Cr-mag); and (4) homogeneous chromite, which is non-porous ferritchromite (Fe-Chr). The high-Al primary cores of the partly altered chromite are depleted in Cr but enriched in Mg, exhibiting MORB-like tholeiitic affinities. In contrast, the altered Chr displays low Mg# [Mg/(Mg + Fe2+)] along with elevated Cr# [Cr/(Cr + Al)] and Fe# [Fe3+/(Cr + Al + Fe3+)] values. Based on the above results, the high-Al cores of chromite within the chromitite originally formed through melt/fluid-rock interaction within the forearc mantle wedge. During the exhumation and post-magmatic processes, the reaction between high-Al Chr and olivine resulted in chlorite formation alongside secondary Chr enriched in Fe2+ and Cr content. Subsequently, the infiltration of hydrous fluid led to the development of Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within the original chromite during the retrograde process. The Xiaosuihe serpentinized harzburgites, along with associated chromitites, represent the composition of peridotite in the forearc mantle wedge that could have formed during the initial subduction of an oceanic island arc abutting the northern North China Craton during the Cambrian. Subsequently, these rocks underwent retrograde metamorphism from amphibolite (eclogite) to greenschist facies during their exhumation.

Beyond zircon fingerprinting: Zircon and TiO2 polymorphs constrain genealogy and evolution of the New Caledonian ophiolite

Ophiolitic mantle rocks play a crucial role in understanding deep geochemical cycling and the transfer of components associated with slab-mantle interactions. Geochemical and geochronological and geochemical analysis of zircon and TiO2 polymorphs (e.g., rutile) present within mantle rocks of the Eocene New Caledonian ophiolite reveal discrete magmatic and xenocrystic populations amongst harzburgite from Me Maoya and chromitite from Tiébaghi massifs. Most mineral grains examined have a xenocrystic origin and are concentrated particularly in the harzburgite. Source tracking involved the use of UPb dating and trace element analysis. They are inferred to have been recycled from the northern end of the Norfolk Ridge that formed the leading edge of a thin continental slab of Gondwana affinity which had previously rifted from eastern Australia together with other elements of Zealandia. The slab was drawn into an intra-oceanic subduction zone beneath the proto-Loyalty arc immediately before the emplacement of the forearc ophiolite today represented by the New Caledonian Peridotite Nappe. Geochemical and geochronological data of these xenocrystic grains support a scenario in which these minerals were recycled into the mantle wedge at moderate mantle depths (above 60–80 km depth) within the subduction channel. In contrast, Eocene UPb ages of zircon and rutile grains, mostly found within the Tiébaghi chromitite, imply their formation through magmatic processes. These minerals' distinctive geochemical characteristics (e.g., HFSE enrichment in rutile) likely indicate an association with late-stage percolation of relatively enriched fluids and melt. Overlapping ages of the Eocene zircon and rutile grains, ArAr pargasite ages, and Zircon UPb ages of cross-cutting dikes in the Tiébaghi chromitite provide evidence that it cooled at ca. 47–46 Ma. This study demonstrates that mineral geochemistry of mantle rocks, when combined with data from geochronology, mineralogy, and petrology, is critical to enhance our comprehension of forearc mantle wedge evolution. It also highlights the potential for studying recycled solid mineral phases, tracing their thermal evolution, and characterizing potential subducted source terranes.

Geochemistry, mineral chemistry and Re–Os isotopes of refractory peridotites of the North Balkhash ophiolite zone in the West Central Asian Orogenic Belt (Central Kazakhstan): multi-stage melt evolution of a late Precambrian forearc mantle

In this study we report on the petrography, major and trace element and mineral chemistry, platinum-group elements and Re–Os isotope systematics of depleted ultramafic rock suites from the Itmurundy Block in the North Balkhash ophiolite zone in Kazakhstan. Represented mainly by variably serpentinized harzburgites and dunites, our samples are characterized by low whole-rock Al 2 O 3 (0.33–0.86 wt%), CaO (0.51–0.86 wt%) and Na 2 O (0.07–0.25 wt%) concentrations, and high-Mg olivine (Fo = 91–92) and orthopyroxene (Mg# = 92–93) contents, together with moderately high spinel Cr-numbers (Cr# = 63–68). They are depleted in incompatible elements ( Σ REE, Nb, Sc) and enriched in compatible elements, such as Cr (up to 2817 ppm) and Ni (up to 2327 ppm), representing highly refractory mantle residues derived from a forearc mantle wedge. They underwent 19–23% hydrous partial melting to produce boninitic melts. 187 Os/ 188 Os values vary from 0.1202 to 0.12599, and 187 Re/ 188 Os ratios from 0.230 to 0.316. The Re–Os model ages (T MA ) and maximum Re depletion model age (T RD ) were calculated based on the obtained data. Re–Os isotope systematics suggests that the analysed peridotites formed in two stages: a first stage around 1.5 Ga and a later stage around 668–589 Ma. Supplementary material: A supplementary figure is available at https://doi.org/10.6084/m9.figshare.c.6845683 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Influences of subduction derived fluids and melt in the genesis of Nidar ophiolite peridotites, Ladakh Himalaya, India: Evidence from mineralogy, PGE and Nd isotopic compositions

The Nidar ophiolite is one of the well-preserved and almost complete ophiolite sections of the Neo-Tethyan oceanic lithosphere, obducted along the continental margin between the Indian and the Eurasian plate. This ophiolite sequence is mostly dominated by ultramafic rocks, consisting of forearc-related refractory, mainly harzburgite, dunite, and serpentinite, with minor intrusions of lherzolite, chromitites, and pyroxenites. In this present study, detailed mineralogical, whole rock geochemistry (major oxides, trace elements, PGE), and Nd isotopic composition of mantle-derived peridotites have been carried out to constrain the petrogenesis and melt evolution. These peridotites are depleted in nature due to the low modal composition of clinopyroxene, high forsterite content in olivine, and wide variation in Cr# and bulk rock chemistry, indicating variable degree of partial melting. The spoon-shaped rare earth element (REE) patterns indicate metasomatism by fluids derived from a subducting slab enriched in light REEs. Geochemical composition of the studied peridotites rocks is marked by high ratio of Al2O3/TiO2, LILE-LREE enrichment, HFSE depletion, and spoon-shaped chondrite-normalized REE patterns and (La/Sm)N > 1 and (Gd/Yb)N < 1, indicates some involvement of boninitic mantle melts and validate a subduction initiation process. The total PGE of the peridotites (ΣPGE = 33–337 ppb) is much more enriched than that of the primitive mantle and other ophiolite peridotites. The PGE distribution displays a concave upward pattern with higher PPGE/IPGE ratios (i.e., 0.11–1.45), suggesting that partial melting is not the only process for the evolution of the Nidar ophiolite peridotites. Enrichment of PPGE and incompatible elements (like LREE) and higher Pd/Ir ratio (0.69–8.26) indicates that these peridotites have undergone fluid/melt interaction in a supra-subduction zone (SSZ) tectonic setting. PGE concentrations of these depleted harzburgites and dunites, formed by partial melting of cpx–harzburgites in an SSZ that produced the boninitic-like melt. The enrichment of incompatible elements like the PPGE is mainly due to the circulation of fluids in the subduction zone, which leads to the PGE fractionation in mantle peridotites. Also, these peridotites have 143Nd/144Nd ratios (0.51148–0.51262) and εNd(t) (t = 140 Ma) values (i.e., +0.97 to −21.3), indicating derivation from depleted mantle sources within an intra-oceanic arc setting. The geochemical behavior exhibited by the Nidar ophiolite peridotites suggests the evolution of a highly depleted fore-arc mantle wedge significantly modified by various fluids and melts during subduction. The mineralogical, geochemical, and Nd isotopic composition of these peridotites and dunites mutually depict the diverse mantle compositions, suggesting insights into the interactions between the oceanic crust and mantle as well as associated geochemical cycling in an SSZ environment.

Seismic anisotropy and geodynamics of the East Japan subduction zone

Seismic anisotropy in the East Japan arc has been extensively investigated by conducting shear-wave splitting measurements, receiver-function analyses, and tomographic inversions of body-wave travel times and surface-wave dispersion data, which have provided a wealth of information on dynamic processes associated with active subduction of the Pacific plate. Measuring shear-wave splitting is popular and effective to detect seismic anisotropy, but it has poor depth resolution. This problem has been overcome by conducting 3-D anisotropic tomography, which has high-resolution in both lateral and vertical directions. Both P and S wave anisotropies are revealed in the crust, which are caused by alignment or preferred orientation of crustal minerals and stress-induced microcracks related to active faults. Trench-normal fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the back-arc mantle wedge, reflecting subduction-driven convection there. Trench-parallel FVDs appear in the forearc mantle wedge under the land area, which may reflect deformation that results in B-type olivine fabric. The forearc mantle wedge offshore may lack anisotropy, suggesting that it is stagnant and decoupled from the subducting slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. The most significant findings of the body-wave anisotropic tomography are its constraints on the slab anisotropy. The subducting Pacific slab exhibits mainly trench-parallel FVDs, which reflect shape-preferred orientation of crystals and cracks related to normal faults produced in the outer-rise area before the plate subduction, overprinting the fossil anisotropy that the Pacific plate gained when it was produced at the mid-ocean ridge. Trench-parallel intraslab fast velocity planes of anisotropy intersect the slab upper surface at high angles (∼45–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults in the upper part of the slab may cause large intraslab earthquakes (M ≥7.0) that take place frequently beneath the forearc area. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction.

Adjoint Tomography of Northeast Japan Revealed by Common-Source Double-Difference Travel-Time Data

AbstractWe present the wave equation-based adjoint tomography of northeastern Japan using common-source double-difference travel-time data. More than 30,000 high-quality first P-wave arrivals from 117 local earthquakes recorded by 713 seismic stations are included in the inversion, generating more than 350,000 double-difference travel times of first P-wave arrivals. In comparison with commonly used travel-time data, the common-source double-difference travel times are insensitive to source parameters and source-side structures but place enhanced constraints on receiver-side structures. As a result, these robust double-difference travel times generate a reliable VP model down to a depth of 120 km. Our inversion results clearly show crustal low-velocity anomalies beneath the volcanic arc. The subducting Pacific slab is illuminated as a landward-dipping high-velocity anomaly, and the arc magmatism in the mantle wedge is shown as low-velocity anomalies. Thanks to the newly operated Seafloor Observation Network (S-net), prominent high-velocity bodies are detected in the fore-arc mantle wedge at depths of 30–60 km. These abnormal high-VP bodies indicate a low degree of serpentinization in the fore-arc mantle wedge at low temperatures, correlating well with the previous finding that the uppermost mantle in the Kuril and Tohoku fore-arc is cold and dry. Our inversion results reveal reliable tectonic features in the subduction zone beneath northeastern Japan, suggesting that wave equation-based common-source double-difference travel-time adjoint tomography is an effective and robust method to illuminate detailed structures of the crust and uppermost mantle.

Large iron isotope fractionation during mantle wedge serpentinization: Implications for iron isotopes of arc magmas

Arc magmas are usually more oxidized than lavas from intraplate settings and spreading ridges. However, iron isotopes of arc magmas are generally lighter, and the mechanism is still controversial. The mantle wedge serpentinite provides new insights to resolve this issue. Here we show that mantle wedge serpentinites/serpentinized peridotites from the Mariana forearc mantle wedge have both heavy Fe isotopes (δ56Fe=−0.01–0.15‰) and high Fe3+/ΣFe values (0.50–0.88), which are positively correlated. This indicates coupled iron oxidation and iron isotope fractionation induced by serpentinization in the mantle wedge. The high Fe3+/ΣFe values and high SO3 contents of the mature serpentinites further suggest that the forearc mantle wedge is highly oxidized. Here we present a new mechanism for iron isotope fractionation and iron oxidation during mantle wedge serpentinization. Because iron solubility is low in the resultant high-pH fluids at the early stage of serpentinization, ferrous iron liberated during olivine serpentinization preferentially precipitates as serpentine, Fe-rich brucite, and magnetite in the serpentinite. After the full consumption of olivine during the late-stage serpentinization, ferrous iron becomes soluble and mobile in low-pH chlorine/sulfate-rich penetrating fluids, and is therefore released to the fluid with light Fe isotopes during progressive serpentinization, leaving the residual serpentinite with heavy iron isotopes. This suggests that mature serpentinites at the base of mantle wedge can act as a filter that sequesters ferric iron with heavy iron isotopes and liberates ferrous iron with light iron isotopes to the penetrating slab fluid, which migrate up to the above mantle wedge and even the mantle melting zone. The highly oxidized forearc mantle wedge serpentinites can also be dragged down to the subarc depth, releasing oxidizing fluids by dehydration. These processes may oxidize the arc mantle wedge coupled with light Fe isotopes, the fluxed partial melting of which consequently produces oxidized arc magmas with light Fe isotopes.

Solid-phase transfer into the forearc mantle wedge: Rutile and zircon xenocrysts fingerprint subducting sources

Voluminous flux of a hydrous component from subducting oceanic lithosphere into the forearc mantle and its contribution to arc magmas have long been recognised but the transfer of solid-phase materials has received less consideration. Although xenocrystic zircons are known from some arc magmas and ophiolitic chromitites their origins remain enigmatic. How and when such materials are transferred into the overlying wedge of mantle lithosphere and the length of their residency therein are unclear. Using zircons and rutiles recovered from ophiolitic rocks in New Caledonia, we demonstrate unambiguous association with subducting sediments and hence evidence for inter-plate transfer. The ‘fingerprints’ of age spectra for both minerals can be matched to lithospheric slab components subducted during Eocene time. Their occurrence in forearc harzburgite suggests that they were incorporated into their mantle wedge host at subduction depths beyond the onset of serpentinisation at ca. 300°C but before the ca. 600°C destabilisation temperature of the rutile U-Pb system. Relocation from slab to forearc mantle is therefore likely to have occurred at ca. 50-60 km depth within the subduction channel. Fluid-assisted conveyance of high field strength elements within solid-state accessory mineral phases into supra-subduction mantle wedges may represent a little-recognised but significant phenomenon that is both predictable and pervasive at convergent margins globally.

Cataclastic and crystal-plastic deformation in shallow mantle-wedge serpentinite controlled by cyclic changes in pore fluid pressures

Episodic tremor and slip (ETS) events in the forearc mantle wedge of a warm subduction zone may reflect mixed brittle–ductile deformation of serpentinite in association with high pore fluid pressures. To understand deformation mechanisms and processes occurring in the hydrated mantle wedge, we examined in the Sanbagawa metamorphic belt, SW Japan, an antigorite serpentinite shear zone derived from mantle wedge that was formed under pressure and temperature conditions that correspond to the ETS regions. The serpentinite underwent multiple extensional (mode I) and extensional–shear (mode I–II) failure events at supralithostatic pore fluid pressures (Pf). Such failure events led to drops in Pf (several MPa) and formation of a distributed ‘fault–fracture mesh’. Antigorite precipitation in the fracture openings contributed to an increase in Pf until the failure condition was reached again, and thereby antigorite kinetics controlled the recurrence interval of seismic events. We also suggest that under the low-Pf conditions that facilitate intracrystalline plasticity rather than cataclasis, the newly precipitated antigorite aggregates (localized along shear bands) deform by dislocation creep at a high strain rate and high shear stress, resulting in the transient, accelerated viscous creep that may characterize slow slip transients.

Multistage melt impregnation in Tethyan oceanic mantle: Petrochemical constraints from channelized melt flow in the Naga Hills Ophiolite

Ophiolitic sequences obducted onto continental margins allow field based observations coupled with petrochemical interrogations of upper mantle lithologies thereby aiding evaluation of compositional heterogeneity of oceanic mantle, depletion-enrichment events and geodynamic conditions governing oceanic lithosphere formation. The Naga Hills Ophiolite (NHO) suite preserves a segment of the Neotethyan oceanic lithosphere encompassing a package of mantle and crustal lithologies. This paper for the first time reports the occurrence of melt flow channels traversing the mantle section near Molen of the NHO and presents a comprehensive study involving chromite-spinel chemistry, bulk rock major, trace and PGE geochemistry to understand the petrogenesis and evolution in a geodynamic transition from mid oceanic ridge (MOR) to suprasubduction zone (SSZ). The spinel chemistry of peridotitic melt channels depicts both MOR-type and SSZ signatures underlining a transitional tectonic frame. Chromite chemistry and high Al2O3/TiO2 ranging from 15.98–35.70 in concurrence with low CaO/Al2O3 ranging from 0.03–0.53; and chondrite normalised LREE > MREE < HREE patterns confirm the influx of boninitic melts into the refractory mantle. The boninitic signature shared by melt channels and host rock invokes a geochemical and geodynamic transition from anhydrous melting of depleted mantle to hydrated fluid flux melting resulting in boninitic melts, that subsequently impregnate and refertilise the fore arc mantle wedge in a SSZ regime at the nascent stage of subduction. The high Ba/Nb, Ba/Th, and Ba/La for the studied peridotites highlight the influx of subduction derived fluids in the supra subduction mantle. Further higher Zr/Hf and Nd/Hf with respect to primitive mantle values in concurrence with lower Nb/Ta suggest progressive refertilisation due to fluid- and melt-driven metasomatism of the refractory fore arc mantle wedge. The chondrite normalised PGE patterns suggest positive Ir and Ru anomalies stipulating the source to be refractory while enriched Pt and Pd underpins the mobilisation of these elements by subduction derived fluids and melts. The elevated abundances of PPGEs than IPGEs as cited by PPGE/IPGE > 1; and Pd/Pt avg. 0.85 for melt channels and 0.84 for host peridotites indicate fluid-fluxed metasomatism of fore arc mantle wedge with a S-undersaturated trend coupled with boninitic affinity. The mineral, trace, REE and PGE chemistry collectively emphasizes that the mantle peridotites of the NHO formed in a transitional geodynamic tectonic setting caused by fore arc extension during subduction initiation followed by rejuvenation by subduction derived fluids and boninitic melts, which typically are of the SSZ tectonic regime. The harzburgitic melt channels and host rock are refractory in nature, reflecting multiple episodes of melt extraction of about 5–15% and ~10–20% respectively from a spinel peridotite mantle source. The occurrences of these melt channels indicate segregation and percolation of melt through porous and channelized network in upper mantle peridotites.

A continental forearc serpentinite diapir with deep origins: Elemental signatures of a mantle wedge protolith and slab-derived fluids at New Idria, California

The New Idria serpentinite body in central California has been interpreted as a diapir that was hydrated at depth in the forearc mantle wedge by aqueous fluids released from the subducted slab. This interpretation is primarily based on structural relationships and petrographic analyses of high-grade metabasaltic tectonic blocks entrained within the serpentinite. The geochemistry of the New Idria serpentinites is largely unknown other than a few boron isotope analyses that are consistent with slab-derived fluid compositions, but not definitive. In this contribution, we present whole-rock elemental analyses and petrographic examinations of a suite of ten serpentinite samples to constrain the tectonic origin of the New Idria serpentinite body. Major, minor, and trace element relationships reflect the signatures of depleted peridotitic protoliths typically associated with serpentinites from mantle wedge settings. These include high MgO/SiO2 vs. low Al2O3/SiO2, high MgO vs. low TiO2, Cr-rich chromites, and enriched Zr/Nb and Hf/Nb ratios. Unlike in abyssal serpentinites, U is not fluid-mobile and is strongly coupled to Th, with a U-Th relationship that is best explained by melt–rock interaction. Rare earth element fractionation trends are also consistent with melt-rock interaction. Fluid-mobile element (FME) concentrations (Cs, Ba, and Rb) are consistent with a slab-derived signature and are notably enriched, featuring some of the highest Cs and Ba concentrations ever reported in serpentinites. No petrographic or geochemical evidence was found to suggest that these enriched FME signatures resulted from systematic post-serpentinization fluid-rock processes or pre-serpentinization melt-rock interactions. In aggregate, the geochemical signatures of the New Idria serpentinites suggest a highly depleted mantle wedge protolith overprinted by melt-rock interactions and subsequent serpentinization by a slab-derived fluid, consistent with the forearc diapir hypothesis. Although subduction at New Idria ended around 25 Ma, geophysical evidence and simple geodynamic calculations suggest that the timing of the proposed diapiric rise plausibly coincides with the final stages of subduction in central California.

Postseismic geodetic signature of cold forearc mantle in subduction zones

A sharp thermal contrast between the cold forearc and the hot arc and backarc is considered fundamental to various subduction-zone processes. However, direct observational evidence for this contrast is rather limited. If this contrast is present, it must cause a rheological contrast in the mantle wedge: elastic in the forearc and viscoelastic in the arc and backarc for the timescale of earthquake cycles. Here we demonstrate that postseismic deformation following large subduction earthquakes provides independent evidence for the thermally controlled rheological contrast. Specifically, we show that seaward postseismic motion is deflected upward at the edge of the cold forearc mantle wedge, causing diagnostic uplift just seaward of the volcanic arc. From numerical simulations of postseismic deformation following the 2011 moment magnitude (Mw) 9 Tohoku-oki, 2010 Mw 8.8 Maule, 2007 Mw 8.4 Bengkulu and 1960 Mw 9.5 Chile earthquakes, together with a global synthesis of postseismic uplift measurements, we find that cold forearc mantle is present irrespective of the diversity in tectonic settings. Our findings also indicate that field surveys eight years after the 1960 Chile earthquake provided some of the earliest evidence for viscoelastic postseismic deformation. We suggest that the established link between long-term thermal processes and short-term earthquake cycle deformation is important to understanding subduction-zone dynamics. Deformation after large subduction earthquakes reflects the thermal contrast between the mantle wedge and its nose, according to numerical simulations and a synthesis of postseismic uplift data from subduction zones.

Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

Shear-wave anisotropy in Earth’s mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones.

Using Silica Activity to Model Redox-dependent Fluid Compositions in Serpentinites from 100 to 700 °C and from 1 to 20 kbar

AbstractSerpentinization is a metamorphic process that can stabilize highly reduced hydrogen-rich fluids. Previous measurements of elevated CH4 and H2 concentrations in ultramafic-hosted submarine springs indicate that active serpentinization occurs along mid-ocean ridge systems at seafloor pressures (∼&amp;lt;500 bar) and temperatures (∼&amp;lt;350 °C). Serpentinites also exist at higher pressures in subduction zones; for example, during retrograde hydration of the forearc mantle wedge and during prograde deserpentinization within the subducted slab. However, many studies demonstrating the thermodynamic stability of reduced serpentinite fluids have been limited to terrestrial seafloor conditions. To investigate the redox state of serpentinite fluids at elevated pressures, phase equilibria and fluid compositions were computed for 100–700 °C and 1–20 kbar using aqueous silica activity (aSiO2(aq)) as a governing parameter. Silica-sensitive, redox-buffering assemblages were selected to be consistent with previously proposed reactions: SiO2(aq)–fayalite–magnetite (QFM), SiO2(aq)–Fe-brucite–cronstedtite, SiO2(aq)–Fe-brucite–Fe3+-serpentine, plus the silica-free buffer Fe-brucite–magnetite. Fluid species are limited to simple, zerovalent compounds. For silica-bearing redox reactions, aSiO2(aq) is buffered by coexisting ultramafic mineral assemblages in the system MgO–SiO2–H2O. Silica activity and fO2 are directly correlated, with the most reduced fluids stabilized by the least siliceous assemblages. Silica activity and fO2 increase with pressure, but are more strongly dependent on temperature, leading to greater silica enrichment and more oxidized conditions along shallow, warm subduction paths than along steeper, colder paths. Reduced fluids with mCH4/mCO2 &amp;gt; 1 and fO2 below QFM are present only when serpentine is stable, and are favored along all subduction trajectories except shallow P–T paths at eclogite-grade. Values of mH2 and mCO/mCO2 depend strongly on P and T, but also on the choice of redox buffer, especially whether the Fe-serpentine component is cronstedtite or Fe3+-serpentine. Methane and H2S production are thermodynamically favored throughout the P–T range of the serpentinized forearc mantle and in other settings with similar conditions; for example, deep planetary seafloors. The model offers a generalized technique for estimating the redox state of a fluid-saturated serpentinite at elevated P and T, and yields results consistent with previous petrographic and thermodynamic analyses. High-pressure serpentinization may be an important source of reduced species that could influence prebiotic chemistry, support microbial life in the deep biosphere or in deep planetary oceans, or promote greenhouse warming on early Earth.

Experimental constraint on grain-scale fluid connectivity in subduction zones

The dihedral angle (θ) between olivine and aqueous fluid is a critical parameter in identifying the grain-scale fluid connectivity which controls the distribution and migration of aqueous fluids in subduction zones besides physical properties of mantle wedges. Recent magnetotelluric observations have suggested the occurrence of significant fluid circulation in deep fore-arc regions, which can be explained by infiltration of saline fluid with a low θ in the mantle wedge (Huang et al., 2019). Along with the salt component, non-polarized gas such as CO2, is a crucial constituent of subduction zone fluids. CO2 is known to increase the olivine–fluid θ under conditions in which the olivine does not react with CO2, which is in contrast to the effect of NaCl on θ. For a better understanding of the connectivity of multicomponent fluid in the mantle wedge, we experimentally constrained θ in olivine + H2O–CO2 fluid and olivine + H2O–CO2–NaCl (multicomponent) fluid systems at 1–4 GPa and 800–1100°C. For the H2O–CO2 system, we found that CO2 tends to increase θ at 1 GPa and 800–1100°C and at 2 GPa and 1100°C. In contrast, CO2 reduced θ even below 60° at relatively high-pressure (P) and low-temperature (T) conditions, in which the olivine partly reacts with CO2 to form magnesite and orthopyroxene (opx). The consumption of non-wettable CO2 components in aqueous fluid alone cannot explain θ lower than those in a pure H2O system. Additional experiments on olivine–magnesite + H2O and olivine–opx + H2O systems showed that the presence of magnesite or opx decreased the olivine–fluid θ, which implies that coexisting minerals affect the olivine–fluid interfacial energy by changing the fluid chemistry. The results of the multicomponent system showed that the effect of NaCl on θ is much more significant than that of CO2. Strikingly, θ was smaller than 60° in all the magnesite- and opx-bearing multicomponent systems. Our results suggest that slab-derived fluid can infiltrate into the deep fore-arc mantle wedge through an interconnected network even in a CO2-bearing multicomponent system at pressures above 2 GPa, which facilitates significant fore-arc fluid circulation. The contrasting effects of aqueous fluid and silicate melt on the seismic wave velocity in a wide condition may allow for the possibility of mapping partial melt in the mantle wedge.

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

AbstractWe investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40Ar/39Ar dates of amphibole and phengite and U‐Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu‐Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole‐eclogite facies conditions occurred as a single episode at 165–157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to ~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one‐time event led to exhumation of amphibole‐eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist‐facies metamorphism of the Red Ant schist of the Sierra Nevada foothills.

Semi-brittle deformation of antigorite serpentinite under forearc mantle wedge conditions

Serpentinization of the mantle wedge immediately above a subducting slab is a key process controlling the strength contrast between the slab and the overriding nominally anhydrous (i.e., olivine-rich) mantle. To assess the influence of serpentinite rheology on the degree of decoupling between the slab and mantle wedge, we conducted axial compression tests on partially foliated antigorite serpentinite using a modified Griggs-type apparatus at confining pressures (Pc) of 1.0–2.5 GPa, temperatures (T) of 300–600 °C, and a strain rate of 1.5 × 10−5 s−1. With increasing temperature, the dependence of maximum differential stress (i.e., strength) on Pc changes from positive to flat, suggesting an increasingly important contribution from plastic deformation. Microstructural observations show that increasing temperature promotes a transition in deformation mechanisms from distributed inter- and intra-granular microcracking to comminution and kinking localized within a through-going shear zone. The ductile-to-brittle transition can be attributed to the operation of thermally activated processes such as dislocation glide and grain boundary sliding in aligned antigorite grains. Comparison with strength data for dunite demonstrates that in the semi-brittle regime, the strength contrast between antigorite and olivine is < 2, which suggests that a thin antigorite layer will have only a weak decoupling effect at the slab–mantle wedge interface.