Cycle In Subduction Zones Research Articles

Experimental constraints on serpentinite carbonation in the presence of a H2O–CO2–NaCl fluid

Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have inferred considerable hydrothermal alteration in plate bending faults, opening the possibility of significant C storage in the slab mantle. However, there is a lack of quantitative determination of C uptake in serpentinized mantle rocks. Here, we experimentally constrain serpentinite carbonation in H2O–CO2–NaCl fluids to estimate C uptake in hydrated mantle rocks. We find that serpentinite carbonation results in the formation of talc and magnesite along the serpentinite surface. The presence of porous reaction zones (49.2% porosity) promotes the progress of carbonation reactions through a continuous supply of CO2-bearing fluids to the reaction front. Added NaCl effectively decreases the serpentinite carbonation efficiency, particularly at low salinities (< 5.0 wt%), which is likely attributed to the reduction in fluid pH and the transport rate of reactants, and the increase in magnesite solubility. Based on previous and our experiments, we fit an empirical equation for the reaction rate of serpentinite carbonation. Extrapolation of this equation to depths of plate bending fault systems suggests that serpentinite carbonation may contribute to an influx of up to 7.3–28.5 Mt C/yr in subduction zones. Our results provide new insights into serpentinite carbonation in environments with high fluid salinities and potentially contribute to the understanding of the C cycle in subduction zones.

Mo-Mg isotopes trace the role of serpentinite in generating arc magmatism

Serpentinite-derived components play an important role in chemical cycling in subduction zones. However, it is difficult to identify recycled serpentinite in subarc mantle due to the overprinting of other slab components, and how serpentinite contributes to arc magmatism is poorly understood. Here we report Mo and Mg isotopic compositions of Early Paleozoic Qushiang arc mafic rocks in the East Kunlun orogen. The Qushiang mafic rocks have variable δ98Mo values of −0.50‰ to 0.78% and δ26Mg values of −0.31‰ to −0.05‰. These features, combining with arc-type trace element distribution patterns, enriched SrNd isotopes, and variable zircon HfO isotopes, suggest the incorporation of different subducting components in their mantle source. Some mafic rocks show high δ98Mo values together with low Mo/Ce ratios and enriched SrNd isotopes, suggesting the contribution of subducted sediment (SSD)-derived melts. Additionally, light Mo isotopes are also identified in mafic rocks, which are closely related to depleted SrNd isotopes. These features are mainly inherited from the subducted dehydrated altered oceanic crust (SAOC)-derived melts. More importantly, the coexisting and significantly high δ26Mg and δ98Mo values of these rocks could not be explained by the SSD or SAOC components, but require serpentinite components in the subarc mantle source. Our study reveals that combined Mo and Mg isotopic studies are potential tools for tracing serpentinite recycling, and provides new insight into the mechanisms by which different subducting components contribute to arc magmatism.

Molybdenum Isotopic Signatures of Forearc Serpentinites: Origin and Contribution to the Subduction Zone Mo Cycle

AbstractSerpentinites, as important reservoirs for volatile and fluid‐mobile elements, have been increasingly recognized as playing a critical role in global geochemical cycles. However, direct evidence for their dominance in the molybdenum (Mo) cycle in subduction zones has been elusive. Here, we address this issue by comprehensively investigating the Mo isotope systematics of serpentinites and accompanying metabasites recovered from the Mariana forearc. Forearc serpentinites under low‐pressure conditions (blue‐ and lizardite‐serpentinites) show substantial enrichments in Mo/Ce ratios (0.42 to 27.34) with high δ98/95Mo values (−0.03‰ to 2.48‰), which arise from not only the incorporation of shallow forearc slab‐derived fluids but also late low‐temperature metasomatism by seawater during clast exhumation. Subducted oceanic crust represented by metabasites has low Mo/Ce ratios (0.004 to 0.049) and δ98/95Mo values (−1.13‰ to −0.61‰), indicating that the loss of Mo during oceanic crust alteration in the early or before subduction stage tends to decrease the Mo isotopic compositions. Combined Mo‐Sr‐Pb isotope systematics indicate that the high δ98/95Mo values in some arc lavas cannot be ascribed simply to the incorporation of slab‐derived fluids at subarc depths; instead, the injection of forearc serpentinite‐derived fluid into their mantle source is noteworthy. We propose that the mobilization of Mo by shallow slab‐derived fluids during the early subduction stage generated forearc lizardite‐bearing serpentinites enriched in heavy Mo isotopes. Subsequently, dehydration and melting of these serpentinites, dragged by the subducting slab down to the subarc region, might have contributed to the Mo systematics in arc lavas. This study demonstrated that forearc serpentinites may be important reservoirs for arc lavas with high Mo isotopic compositions and may contribute to the Mo cycling in subduction zones.

Different effects of serpentinite-derived and slab-derived fluids on arc magmatism in the Chinese Eastern Tianshan: evidence from Mg–B isotopic systematics

Fluids are important media for elemental cycling in subduction zones, but the compositions of such fluids remain obscure, especially those derived from dehydration of serpentinite. To better understand the role of subduction-zone fluids in arc magmatism of the Chinese Eastern Tianshan (CET), we present Mg and B isotopic data for the arc diorites (e.g. Kalatage and Nanshankou). Our results show that the Kalatage diorites have higher δ 26 Mg (−0.23 to −0.13‰) and δ 11 B (−0.04 to +1.08‰) values than the corresponding mantle values, whereas the Nanshankou diorites display relatively lower δ 26 Mg (−0.26 to −0.19‰) and δ 11 B (−10.7 to −2.97‰) values. Because high-temperature magmatic and post-magmatic processes have limited effects on Mg and B isotope fractionation of our samples, their distinct Mg–B isotopic signatures inherited from the mantle sources should be caused by subduction-related metasomatism. Because neither altered oceanic crust (AOC) directly melting nor the incorporation of subducted sediments could induce significant Mg–B isotope fractionation in these rocks, we infer that their variable δ 26 Mg and δ 11 B values were probably associated with subduction fluids that were derived from different sources. Specifically, dehydrated fluids from serpentinites possess high δ 26 Mg and δ 11 B values, which resulted in the heavy Mg–B isotopic compositions of the Kalatage diorites, whereas mantle metasomatized by fluids from AOC and subducted sediments were responsible for the Nanshankou rocks with relatively low δ 26 Mg and δ 11 B values. Taking into account the different effects of serpentinite-derived and slab-derived fluids in the formation of arc magmatism, subduction-zone fluids can be probably an important factor in changing the Mg–B isotope composition of mantle sources. Supplementary material : Supplementary data tables and parameters used in models are available at https://doi.org/10.6084/m9.figshare.c.6702686

Some thoughts about eclogites and related rocks

Abstract. The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure–temperature (P–T) conditions; determined ages related to the prograde, metamorphic peak and retrograde stages; and calculated time-integrated rates of cooling and exhumation for eclogites and related rocks, including blueschists, from orogenic belts worldwide. Improvements to single mineral thermometers and new developments in elastic barometry using inclusions of one mineral in another (e.g. quartz and/or zircon in garnet), coupled with ongoing innovations in petrochronology and diffusion modelling, presage a new age for eclogite studies in which detailed quantification of metamorphic conditions and timescales will be linked to an improved understanding of processes at all scales. Since the turn of the century, numerical modelling of subduction zone and rock exhumation processes has become increasingly important. As a result, subduction and exhumation are quite well understood, but the volume of continental crust subducted to and returned from mantle conditions and the amount lost to the mantle are largely unknown. We have generated sufficient data to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the rare occurrence of orogenic eclogites and the absence of blueschists prior to the late Neoproterozoic and the emergence of plate tectonics on Earth. Since the turn of the century, the assumption that metamorphic pressure is lithostatic has come under increasing scrutiny. Whether local variations in stress extrapolate to the crustal scale and, if so, whether the magnitude of the calculated deviations from lithostatic pressure can be generated and sustained in mechanically heterogeneous rock units remains contentious. Could the paradigm of subduction of continental lithosphere to mantle depths be simply an artefact of the lithostatic assumption? Fluid cycling in subduction zones and understanding the role of fluids in the generation of intermediate-depth earthquakes remain important topics of current research. Dry (H2O-absent) conditions are unlikely around the peak of ultrahigh-pressure (UHP) metamorphism or during exhumation, due to dehydroxylation of nominally anhydrous minerals and breakdown of hydrous minerals at P–T conditions in the realm of supercritical fluid and hydrous melt. Indeed, the presence of melt may be necessary to facilitate the exhumation of HP and UHP tectonometamorphic rock units. Finally, our ability to interrogate inclusions in superdeep diamonds should lead to a better understanding of how the deep interior and surface are linked in the context of Earth as a fully coupled system.

Progressive lawsonite eclogitization of the oceanic crust: Implications for deep mass transfer in subduction zones

Abstract Lawsonite eclogites are major hosts of H2O and trace elements and thus key for long-term deep element cycling in subduction zones. Existing cycling models suggest that the subducting oceanic crust transforms to lawsonite-eclogite assemblages; yet the scarcity of lawsonite eclogites in the rock record questions to what extent the oceanic crust transforms to lawsonite-eclogite assemblages during subduction. Here, we use petrological modeling coupled with geodynamic calculations for a typical subduction zone to show that the occurrence of lawsonite eclogites is controlled by the maturation of the subduction zone. We find that lawsonite eclogite does not form in infant subduction zones; with time, the oceanic crust forms lawsonite assemblages, but prograde heating obliterates lawsonite except in rocks exhumed prior to such heating. Lawsonite-eclogite assemblages in the oceanic crust form and survive prograde metamorphism only in mature and cold subduction zones but still necessitate specific characteristics during exhumation to preserve lawsonite. We show that the stability of lawsonite in mature subduction zones is hindered by hybridization between the mafic crust and the overlaying mantle wedge material; by contrast, lawsonite proportion increases with intense seafloor alteration and Ca-Al metasomatism. These latter processes are thus key for enhancing recycling. We argue that lawsonite-driven mass cycling to the deep mantle is important in mature subduction zones, but the role of lawsonite eclogite in carrying mass deep into Earth in the infant stage of a subduction zone is minor.

Anhydrite solubility enhanced by CaO in silicate melts: Implications for sulfur cycling in subduction zones

The capacity of slab-derived melt in mobilizing sulfur from the slab to the mantle wedge is essential for understanding global S cycling in subduction zones of the present or ancient Earth. The promotive role of the CaO constituent in the silicate melts, provided chiefly by subducting sediments, in anhydrite dissolution in silicate melts has been noted before. Nevertheless, the dependence of SCAS, sulfur concentration in the melt at anhydrite saturation, on melt CaO concentration and the underlying dissolution mechanism have yet to be well constrained. Using a piston-cylinder apparatus, we carried out five series of phase equilibrium experiments between anhydrite and felsic melts to obtain constraints on the effect of melt CaO concentration, water concentration, temperature (T), and Ca/(Ca+Mg) ratios on SCAS at 1–3 GPa and 1050–1350 °C. Electron probe microanalyses of quenched products confirmed that the dissolved sulfur was predominantly present as SO42−. The experimental results show that SCAS increased significantly with increasing melt CaO and Ca/(Ca+Mg) ratios at a fixed degree of silicate melt polymerization (NBO/T = 0.43 ± 0.01). The SCAS also increased with increasing T and dissolved water concentration, and the effect of water was non-linear. Previous SCAS models could not capture well enough our new experimental data, therefore a new SCAS parameterization was developed using previous and our new experimental data. With our new results and model, we find that slab melts with high CaO concentration could efficiently transport S from the slab to the mantle wedge in the Archean. Slab melts in modern hot subduction zones can carry more than 1800 ppm sulfur, raising S concentration in the mantle wedge to the lower end of the observed level by metasomatism. In modern intermediate or colder subduction zones, however, slab melt is not an effective agent of sulfur transfer even at CaO concentration >10 wt%.

Sulfur Isotope and Trace Element Systematics in Arc Magmas: Seeing through the Degassing via a Melt Inclusion Study of Kyushu Island Volcanoes, Japan

Abstract Sulfur is a minor element in magmas but one of the major volatile elements released in volcanic systems, from the magma to the fluid phase upon ascent. Not only are sulfur gasses potentially toxic for humans and plants, they are also involved in causing drastic climate changes after major volcanic eruptions. Therefore, studies are carried out by the geoscience community to assess the magmatic sulfur flux by looking at the sulfur content and isotopes in erupted products, with the ultimate aim of improving understanding of the sulfur cycle in subduction zones. Kyushu Island in Japan hosts 25 volcanoes, among which 11 are active and represent a natural hazard for the local population. It is perhaps the most suitable site for the study of the sulfur cycle for its availability of recent volcanic deposits and its many highly monitored volcanoes. We investigated sulfur and sulfur isotope compositions of the magma source of Kyushu Island arc using olivine-hosted melt inclusions in mafic tephras and lavas, from eight volcanoes (nine Holocene samples) going from Northern Kyushu with Oninomi, Yufu, Kuju, and Aso, to Southern Kyushu volcanoes such as Kirishima-Ohachidake, Kirishima-Shinmoedake, Sumiyoshi-ike, and Kaimondake, and one back arc volcano, Fukue-Onidake. We measured major, trace and volatile elements and S isotopes (δ34S) in melt inclusions. Magma composition recorded in the inclusions ranges from basalt to andesite (SiO2 ranging from 40.3 to 60.7 wt%). For each edifice, we identified the least degassed and least differentiated compositions based on volatile and trace element systematics and selected the melt inclusions closest to their primitive melts. Comparing these primitive magmas, Sr/Y underlines a compositional dichotomy between volcanoes from northern (Sr/Y &amp;gt; 20) and southern Kyushu (Sr/Y &amp;lt; 20), separated by a non-volcanic area corresponding to the subduction of the Kyushu-Palau ridge. The δ34S in melt inclusions range from −0.32 ± 0.79‰ to +9.43 ± 0.47‰ (2σ) and trace the source of the magma from the different volcanoes, rather than degassing or crustal fractionation processes. δ34S is not fractionated by the nature of the fluid (aqueous or melt) metasomatizing the mantle wedge, therefore it is not the first-order factor controlling the sulfur isotope variations. Instead, this study illustrates the need for a heavy δ34S component, likely sulfate from seawater, contained in the agent that metasomatized the mantle beneath the arc. If such an observation is confirmed in other subduction zones, sulfur isotopes in melt inclusions may be an effective way to trace seawater input into the mantle beneath

Seismic Anisotropy and Intraslab Hydrated Faults Beneath the NE Japan Forearc

AbstractThe water cycle in subduction zones is closely related to the generation of large earthquakes and arc magmas. Outer‐rise hydrated faults subducting with the oceanic lithosphere are suspected to be important for the water cycle. However, geophysical evidence for the intraslab hydrated faults beneath forearc is scarce. Here we present high‐resolution P‐wave anisotropic tomography of the Tohoku forearc derived from arrival‐time data recorded at both onshore and offshore seismic stations. Trench‐parallel intraslab fast velocity planes of anisotropy are revealed that intersect the slab upper surface at high angles (∼45°–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults may cause large intraslab earthquakes. The hydrated‐fault associated water entering a large near‐trench asperity in the megathrust zone could have triggered the great 2011 Tohoku‐oki earthquake (Mw 9.0).

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H2O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H2O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified.Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones.

The sulfur concentration at anhydrite saturation in silicate melts: Implications for sulfur cycle and oxidation state in subduction zones

Magmatic sulfur plays an important role in affecting mantle oxidation state, volcanic eruption, formation of ore deposits, and global climate change. To better understand the sulfur cycle in subduction zones and to constrain the sulfur concentration at anhydrite saturation (SCAS) in subducting slab-derived silicate melts, forty-three experiments were conducted at 0.5–5 GPa and 900–1200 °C using a piston cylinder and a multi-anvil apparatus. The experimentally produced silicate melts are rhyodacitic to rhyolitic in composition, and the measured SCAS values range from 170 to 3500 ppm. The SCAS values increase with increasing temperature and the water and CaO content of the silicate melts, but the effect of pressure varying from 0.5 to 5 GPa is negligible. Using our new and all available literature SCAS data (n = 252), we tested the accuracy of all previous SCAS models that were calibrated for predicting SCAS in silicate melts at various conditions. We find that the Z–T model (Zajacz and Tsay, 2019) works as the greatest SCAS model in capturing all SCAS data with a mean and median absolute error of 5% and 4%, respectively. The success of the Z–T model in capturing all SCAS data demonstrates its robustness in predicting SCAS in silicate melts relevant for magmatism in subduction zones. Applying the Z–T model to slab melting reveals that slab-derived silicate melts of global subduction zones can dissolve 130–1200 ppm S6+, but they cannot contribute enough sulfur to explain the estimated sulfur abundance (200–500 ppm) in the metasomatized sub-arc mantle, which thus requires the addition of sulfur by slab-derived aqueous fluids. The addition of slab S6+ can cause oxidation of the sub-arc mantle in an fO2 range of FMQ+0.5 to FMQ+2, consistent with the fO2 values observed for the metasomatized sub-arc mantle peridotites. However, during partial melting of the metasomatized sub-arc mantle, S2− would play as a reducer and the fO2 of primitive arc basalts cannot be higher than FMQ+0.5 to FMQ+1, which is consistent with the sub-arc mantle fO2 inferred from the V–Sc, Fe–Zn, V–Ga, and Cu–Re systematics of primitive arc basalts. The fO2 above FMQ+1 of arc basalts may thus be obtained during magmatic differentiation in the lithosphere. We finally modeled the fate of S6+ during the differentiation of parental arc basalts with fO2 varying from FMQ+0.5 to FMQ+1.5 in a thickened continental arc setting. We find that significant fractions of S6+ in the parental arc basalts are converted into S2− and lost in sulfides during arc magmatic differentiation, and the estimated 400 ppm sulfur in Earth’s continental crust implies that Earth’s continental crust cannot have formed from arc basalts with fO2 significantly higher than FMQ+0.5 to FMQ+1.

Morphological impacts of the Chilean megathrust earthquake Mw 8.8 on coastal wetlands of high conservation value

The subduction earthquake Mw = 8.8 on February 27th, 2010 which affected Chile's south-central coast (37°S) produced a co-seismic uplift of ~1.4 m in coastal Tubul-Raqui wetland area, with abrupt morphological changes. In order to determine the magnitude of the changes, in this area of high conservation value, salt marsh morphological features were identified and quantified by mapping the changing extent of the coverage of morphological features before (2009) and after (2011–2012) the tectonic disturbance. Rectified satellite images of seven study sites were created for the three years using Google Earth images and processed in ArcGIS. The results indicate a total decline of 31.7% in the area of the morphological features and the emergence of 1.25 km2 of dried area; salt pans and tidal creeks were severely affected, with more than 90% loss. In contrast, there was a slight recovery of the Tubul and Raqui main river channel (12%) and the tidal channels (8.5%) between 2011 and 2012, mainly in the area near the river mouth. The salt marsh (cover by Spartina densiflora) showed slight variation after the co-seismic uplift (14.6%), demonstrating high tolerance in the face of high-impact natural disturbances. The changes and later evolution may be explained mostly by the action of the seismic cycle in subduction zones. Continuing to monitor state of the recovery of the salt marsh and other similar environments may help to understand the true role that the seismic cycle plays in the dynamics of coastal ecosystems, as well as its incidence in the recovery process.

Fluid-mediated selective dissolution of subducting carbonaceous material: Implications for carbon recycling and fluid fluxes at forearc depths

Subduction of crustal C governs the long-term global C cycling. The role of carbonates recycling in subduction zones and the related dissolution of C at various depths have been the subject of a large body of literature over the last decades. Much less is known about the contribution of carbonaceous material (CM) to the deep C cycling in subduction zones. This paper presents natural evidence for intense fluid-mediated leaching of CM in pelitic schists at high-pressure/low-temperature conditions relevant to the forearc region of subducting slabs. Manifestations of such process were identified along fluid pathways at various scales in the blueschist-facies subduction complexes of both Alpine Corsica and the Western Alps. Microstructural, whole-rock and Raman analyses across a selected metasomatic aureole were used to quantify the amount and mechanisms of C loss during fluid-rock interaction. In samples affected by intense fluid infiltration, >90% of the initial CM was removed from the rock. Microstructural and micro-Raman data indicate selective leaching of disordered CM relative to nearly crystalline graphite. The collected data allowed constraining the magnitude of fluid fluxes required to bleach the studied CM-bearing lithologies at different P-T-fO2 conditions, which corresponds to rather high time-integrated fluid fluxes in the order of ~106 m3/m2. In settings of large-scale fluid channelization, such as along regional-scale, lithological/tectonic boundaries or at the top of the subducted sedimentary pile, intense dissolution of subducted CM is expected. This process may thus exert a negative feedback on the sink of C phases into the deep mantle over the geological timescales and contribute to the release of isotopically light C from subducting slabs in forearc regions.

Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle

Knowing the phase relations of carbon-bearing phases at high-pressure (HP) and high-temperature (HT) condition is essential for understanding the deep carbon cycle in the subduction zones. In particular, the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones, which is poorly explored. Here we summarized the phase relations of carbon-bearing phases (calcite, aragonite, dolomite, magnesite, graphite, hydrocarbon) in HP metamorphic rocks (marble, metapelite, eclogite) from the Western Tianshan subduction zone and high-pressure experiments. During prograde progress of subduction, carbonates in altered oceanic crust change from Ca-carbonate (calcite) to Ca,Mg-carbonate (dolomite), then finally to Mg-carbonate (magnesite) via Mg–Ca cation exchange reaction between silicate and carbonate, while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones. Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone. The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ − 1.9 to FMQ − 2.5 ​at its metamorphic peak (maximum P-T) conditions. In comparison with redox conditions of other subduction zones, Western Tianshan has the lowest oxygen fugacity. Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition, which is also confirmed by high-pressure experimental simulation. Based on petrological observation and high-pressure simulation, a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed, and its effect on deep carbon cycle in subduction zones is further discussed.

Deep carbon cycle in subduction zones

The carbon cycle between the deep Earth and the atmosphere (i.e., the deep carbon cycle) can significantly affect the global climate on both long and short time scales. Although carbon in the deep Earth can be released to the atmosphere in many ways, plate subduction is the only pathway for the return of carbon from the surface to the deep Earth. Owing to diversity in the forms of carbon and the special physicochemical property of carbonates, the behavior of carbon and carbonates in subduction zones significantly affects the products of subduction processes, the oxygen fugacity in subduction zones, and the activation and migration of elements during the crust-mantle interaction. Therefore, the carbon cycle in subduction zones plays an important role in maintaining a habitable climate by regulating the atmospheric CO2 concentration, which significantly affects the global climate, and in causing fundamental changes in the physical and chemical properties of the mantle that result in a heterogeneous mantle. In this study, we review and discuss previous studies and scientific problems regarding the carbon cycle in subduction zones from four aspects: observation and tracing of the carbon cycle, migration and variation of carbon during subduction, carbon flux, and the effect of the carbon cycle.

Magnesium isotopic composition of metasomatized upper sub-arc mantle and its implications to Mg cycling in subduction zones

Dehydration of subducting oceanic crust and underlying serpentinized mantle is potentially the most important source of fluids for element cycling between surface-processed materials and the interior of the Earth. Magnesium (Mg) isotopes -have been proposed as a promising tracer of dehydration of serpentinites because of their high Mg contents and distinctive Mg isotopic signatures relative to the mantle, which is difficult to identify using other isotopes. To constrain the primary controls on Mg isotope cycling in subduction zones, here we analyze a suite of well-characterized sub-arc mantle xenoliths from the Avacha volcano in southern Kamchatka arc, Russia. They are comparable in modal and/or chemical composition to serpentinized fore-arc harzburgites and sub-arc harzburgite xenoliths from the western Pacific. Despite evidence for ubiquitous slab-related fluid metasomatism, the 23 spinel harzburgite xenoliths and four pyroxenite metasomatic veins display mantle-like δ26Mg values from −0.30 to −0.21‰. Mineral separates have similar δ26Mg values, with an average of −0.26 ± 0.04‰ (2SD, n = 17) for olivine, −0.23 ± 0.04‰ (2SD, n = 17) for orthopyroxene, and −0.24 ± 0.10‰ (2SD, n = 5) for late-stage interstitial clinopyroxene. The lack of a clear slab-derived δ26Mg signature in the Avacha sub-arc peridotites is probably due to their high MgO contents and low fluid/rock mass ratios during flux melting and metasomatism. We also compare published δ26Mg data for four volcanic arcs distributed worldwide. This inter-arc comparison reveals that the thermal structure of a subduction zone appears to exert a strong control on the efficiency of Mg isotope cycling from the downgoing slab to the mantle wedge and subsequently to the arc magmas; those arcs that permit large-scale dehydration of Mg-rich serpentinized slab mantle, coupled with available channelized fluid pathways, are more likely to produce isotopically diverse arc magmas.

Sulfur isotope compositions of pyrite from high-pressure metamorphic rocks and related veins (SW Tianshan, China): Implications for the sulfur cycle in subduction zones

A detailed petrographic, microstructural and geochemical investigation of pyrites from four eclogites / blueschists and their veins in the Tianshan (NW China) have revealed the occurrence of four types of pyrites: δ34S values of the pyrites range from −24.7 to +13.2‰, whereby three populations can be distinguished that have distinct δ34S ranges corresponding to different paragenetic stages. Py1 pyrite grains contain micro- to nanoscale inclusions of pyrrhotite, chalcopyrite, bornite, sphalerite, galena, arsenopyrite, cobalt-pentlandite, enargite, titanite, biotite, barite, and have δ34S values of −3.8 to +2.7‰ (average δ34S = −0.6 ± 0.9‰), which is inherited from a magmatic and hydrothermal fluid input during alteration of the oceanic crust. Py2 pyrite occurs as inclusions within the cores of garnet where it coexists with fine-grained inclusions of rutile, glaucophane and rarely with omphacite. Thus, these pyrite inclusions likely formed during prograde evolution. Py2 is too small to be analyzed by SIMS and thus no isotopic compositions could be determined. The Py3 pyrite grains contain eclogite-facies mineral inclusions, such as omphacite, glaucophane, epidote, rutile and lawsonite, and are inferred to have formed during the late prograde evolution close to or at peak eclogite-facies conditions. Py3 pyrite display positive δ34S values ranging from +3.6 to +13.2‰ (average δ34S = +9.7 ± 2.8‰), suggesting the source of sulfur was either anhydrite incorporated during seafloor alteration or 34S-enriched sulfide from lower sections of the oceanic lithosphere. Py4 pyrite occurs as irregular patches, isolated euhedral to subhedral grains or as rims around Py1 or Py3 pyrites, associated with magnetite and ilmenite. Py4 grains have δ34S values of −24.7 to −3.9‰ (average δ34S = −5.0 ± 2.1‰), with the most likely source of sulfur being 32S-enriched pelagic sediments that in the Tianshan are interlayered with the metavolcanic lithologies and underwent amphibolite facies metamorphism during exhumation. This study provides insight into the sulfur cycle within subduction zones. We document that during subduction and exhumation fluids with distinct sulfur isotopic compositions driven by dehydration reactions in the downgoing slab cause metasomatism in the surrounding lithologies. In particular, we suggest that at least locally the plate interface is characterized by sediment-derived fluids that are characterized by 32S-enriched sulfur and comparatively higher oxygen fugacities.

Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

AbstractSubduction is a key component of Earth's long‐term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution‐precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ34S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high‐pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed‐system (re)crystallization from protolith‐inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ34S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ34S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self‐diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate‐bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab‐derived fluids.

Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts – Implications for sulfur cycle in subduction zones

To add to our understanding of sulfur cycle in subduction zones in general and constrain the effects of slab-released sulfate (SO42−) on the magma genesis and sulfur transport from sub-arc mantle to the arc volcanoes in particular we carried out an experimental study. High pressure-temperature, piston-cylinder experiments were carried out in Au-Pd capsules at 0.5–3 GPa and 1050–1325 °C to investigate (a) the effect of variable sulfur concentration from 0 to ~2 wt%, dissolved as SO42− on the stability field of a primary arc basalt with ~4 wt% H2O and determine (b) the sulfur content at anhydrite saturation (SCAS) of hydrous mafic magmas. Speciation of sulfur in the silicate melt was confirmed to be SO42− by S Kα X-ray peak position using electron microprobe. S-free hydrous clinopyroxene liquidus at 2 GPa is ~25 °C hotter than the hydrous clinopyroxene liquidus with ~0.1 wt% S in the liquidus melt as SO42− and the liquidus depression with further S-enrichment to anhydrite saturation (~2 wt% S) can be fitted by a power function ΔT (°C) = 26.52(±3.48)(Smelt in wt%)0.24(±0.06). Anhydrite-saturated experiments show that SCAS increases with increasing temperature and CaO content of melt and decreases with increasing SiO2 content of the melt. Previous SCAS models based mostly on lower P-T experiments and/or on silicic melt compositions can't capture our new experimental SCAS data. A new SCAS parameterization was developed using previous and our new experimental data. Calculations using our new parameterization and assuming 200–500 ppm S in the arc mantle show that <10% hydrous melting of mantle wedge would exhaust anhydrite, if present. Therefore, anhydrite even if present is expected to be exhausted by partial melting in the mantle wedge and parental basaltic melt will extract similar amount of S via mantle melting irrespective of the presence of sulfide or sulfate at subsolidus conditions. The S content, as dissolved SO42−, of hydrous arc basalts produced by 10–30% melting will be 500–4000 ppm, which is comparable to the melt inclusion S contents from various arcs. The sulfate undersaturated basalts may assimilate crustal sulfate and lead to high observed SO2 flux at the arcs, known as the “excess S”.

Chilean megathrust earthquake recurrence linked to frictional contrast at depth

Fundamental processes of the seismic cycle in subduction zones, including those controlling the recurrence and size of great earthquakes, are still poorly understood. Here, by studying the 2016 earthquake in southern Chile—the first large event within the rupture zone of the 1960 earthquake (moment magnitude (Mw) = 9.5)—we show that the frictional zonation of the plate interface fault at depth mechanically controls the timing of more frequent, moderate-size deep events (Mw 8.5). We model the evolution of stress build-up for a seismogenic zone with heterogeneous friction to examine the link between the 2016 and 1960 earthquakes. Our results suggest that the deeper segments of the seismogenic megathrust are weaker and interseismically loaded by a more strongly coupled, shallower asperity. Deeper segments fail earlier (~60 yr recurrence), producing moderate-size events that precede the failure of the shallower region, which fails in a great earthquake (recurrence >110 yr). We interpret the contrasting frictional strength and lag time between deeper and shallower earthquakes to be controlled by variations in pore fluid pressure. Our integrated analysis strengthens understanding of the mechanics and timing of great megathrust earthquakes, and therefore could aid in the seismic hazard assessment of other subduction zones. The recurrence time of megathrust earthquakes in Chile may be controlled by frictional contrasts at depth, according to analyses of stress build-up and release related to the December 2016 southern Chile earthquake.