Subduction Zone Magmatism Research Articles

Sulfur partitioning between aqueous fluids and felsic melts at high pressures: Implications for sulfur migration in subduction zones

Sulfur (S), an essential volatile in subduction zone magmatism, exhibits higher solubility in aqueous fluids compared to silicate melts. Despite its importance, the partitioning of S between aqueous fluids and silicate melts under the conditions of subduction zone, critical for magma generation and evolution, remains poorly understood. To address this knowledge gap, we performed piston-cylinder experiments at a temperature of 950 ℃ and pressures of 1 and 2 GPa, investigating the effects of various parameters including oxygen fugacity, melt composition, fluid composition (salinity) and pressure on S partitioning between aqueous fluid and silicate melt (DSfluid/melt). Our results indicate that the DSfluid/melt is always large (> > 1), and S prefers to enter the aqueous fluid at high pressures. However, the DSfluid/melt decreases with increasing pressure from 1 to 2 GPa. Specifically, under reducing conditions (Ni-NiO buffer), DSfluid/melt decreased from 147 ± 40 to 20 ± 2, whereas under moderately oxidizing conditions (Re-ReO2 buffer), it decreased from 27 ± 1 to 20 ± 2. These results stress the strong affinity of S for aqueous fluids at high pressures. Together with the great capacity for S dissolution in the H2O-rich magma within the deep Earth, fluid-saturated felsic magma efficiently transports substantial amounts of S from deep to shallow regions in subduction zone settings. This process plays a crucial role in the formation of giant porphyry deposits and provides a potential source of excess S released during explosive volcanic eruptions.

Parental magma and mantle source compositions of chromitites in the Mesoarchean Nuggihalli greenstone belt, India: Evidence for Archaean subduction zone magmatism

We present comprehensive petrological and geochemical analyses of chromitites from the 3.3-3.1 Ga Nuggihalli greenstone belt in Western Dharwar Craton, India. Our findings provide insights into the primary magma and mantle source compositions associated with these Archean oceanic lithosphere chromitites. Major and trace element data from chromites and clinopyroxenes in the chromitites, combined with thermobarometry and hygrometry calculations, and numerical modeling, indicate that the Nuggihalli chromites are characterized by high-Cr chromites (Cr# = 68.1-78.6) with low TiO 2 (0.19-0.29 wt. %), Al 2 O 3 (9.10-13.13 wt. %) and Ga (4.24-10.37 ppm). These chemical signatures are comparable to those of chromites from high-Cr podiform chromitites and boninites. Clinopyroxenes are mainly augites with high Mg# (93-95). Equilibrated melts with the clinopyroxenes from the Nuggihalli chromitites have high Mg# values (79-86), similar to ancient komatiitic or picritic magmas (Mg# = 75-90). Thermobarometry and hygrometry calculations indicate that the clinopyroxenes likely formed at temperatures of 1194-1221℃ and pressures of 0.7-5.8 kbar with high water contents (∼2.4-2.6 wt. %), exceeding water contents of mid-ocean basalts (MORB) and oceanic island basalts (OIB). Numerical modeling of rare earth elements indicates moderate degrees of partial melting (1% to ∼10%) of a hydrated spinel lherzolite mantle source (water content: ∼478-5408 ppm). The parental magmas of the 3.3-3.1 Ga Nuggihalli chromitites are characterized by enrichment of water contents (∼2.4-2.6 wt. %) and depletion of high field strength elements (HFSEs, e.g., Nb and Hf), which are similar to boninites and arc magmas. Furthermore, the rock associations, including ultramafic rocks with chromitites, high-Mg basalts, and sheets of gabbro-anorthosites in the Nuggihalli greenstone belt, are similar to the rock assemblages of ophiolites, which could represent a relic of a Mesoarchaean oceanic lithosphere formed in a forearc setting.

Disentangling the Roles of Subducted Volatile Contributions and Mantle Source Heterogeneity in the Production of Magmas Beneath the Washington Cascades

AbstractThe compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and the amount and composition of slab‐derived materials. This contribution presents the volatile (H2O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively spans nearly the entire range of arc magmas globally. All volcanic centers have magmas with H2O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of variably saline fluid and/or slab partial melt. Magmas from Glacier Peak in northern Washington have unusually high Th/Yb ratios that are similar to Lassen region basalts, indicating possible contributions of “subcreted” metasediments that geophysical data suggest are not present beneath central Oregon and southern Washington. This data set adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Origin and multi-episodic melt/fluid interactions revealed by the subducted serpentinites in the North Qilian Belt, NW China: Implications for the compositional heterogeneity of the fossil oceanic mantle

In nature, serpentinites act as reservoirs of water and fluid-mobile elements and play an essential role in arc magmatism, fluid metasomatism, and elemental circulation in subduction zones. This paper presents new mineral and whole-rock geochemical analyses of the Qingshuigou serpentinite massif and discusses its petrogenesis, tectonic setting, and mantle metasomatism. The chemical composition (e.g., REE, Y, Mg, and Al) and petrological observations (e.g., pseudomorph and hourglass textures) of serpentines demonstrate that the serpentinites’ protoliths belong to the peridotites. Their degrees of partial melting were estimated to be 15–20 % using Yb, Cr, Ti, and HREEs, indicating that these peridotite protoliths were residual in origin after high degrees of mantle melting. They were relics of the subducted lithospheric mantle in the subduction zone, based on their geochemical similarities with subducted serpentinites. Notably, the Qingshuigou serpentinites show several geochemical fingerprints of melt-rock interactions, such as the U-shaped REE patterns, elevated Zr/Ti, Zr/Hf, and Nb/Ta ratios; this led us to propose that the peridotite protoliths of the serpentinites were not merely melting remnants but were significantly modified by melt metasomatism. Also, they experienced two stages of fluid/rock interactions in the subduction zone: early-stage serpentinization at shallow depths and lizardite/chrysotile to antigorite transition at an elevated depth. Combined with the published data of ophiolitic peridotites from the North Qilian Belt, the compositional heterogeneity of these peridotites and associated serpentinites might be associated with the nature of melt/fluid metasomatism under different geodynamic backgrounds.

The relative contribution of mantle and continental crustal sources to primitive arc magmas: Insights from the early Palaeozoic Famatinian - Puna arc

Subduction zone magmatism plays a crucial role in driving the exchange of chemical elements between Earth's interior and surface. This survey focuses on the compositional analysis of primitive mafic rocks within an ancient Early Palaeozoic subduction-related magmatic belt. The primary objective is to assess the petrological processes involved in the cycling of chemical elements from the Earth's surface, through the slab and mantle reservoirs, to the crustal arc magmatic systems. By examining the composition of primitive mafic rocks and utilizing inverse modelling techniques, we estimate the composition of the ambient mantle source, which appears to have undergone depletion due to melt extraction before being metasomatized by slab agents. The inverse approach reveals that fluid-mobile elements (Th, U, La, Ce, and Sr) were primarily derived from the slab contribution and suggests that subduction had influenced the abundances of Nb and Ta in the modified mantle source. Conversely, Zr, Hf, and Ti were sourced from the ambient mantle. However, the available mineral/melt partition coefficients used in inverse modelling may overestimate the contribution of Sr and Nb from the slab. Instead, forward modelling is limited by the incomplete preservation of ancient volcanic arc geology. The forward mixing models that consider the interaction between slab-released sedimentary-derived melts and the ambient depleted mantle can explain the observed variations in Nd and Sr isotopic ratios in primitive mafic rocks. Two-component mixing shows that a small mass fraction (≤1.5 wt%) of average Cambrian continental sediments subducted beneath the Early Ordovician arc into a MORB-depleted mantle source account for the abundance of sedimentary-supplied elements such as Th, La, and Ba, as well as radiogenic isotope ratios of 143Nd/144N and 87Sr/86Sr in the primitive mafic magmas. The chemical and isotopic composition of deep-seated primitive mafic rocks reveals a decoupling between incompatible trace elements, which display minimal variation, and radiogenic isotopic ratios, which exhibit a wide range and reflect contributions from continental crustal reservoirs. To explain the observed variations in trace elements and isotopes, we propose a simple model involving simultaneous bulk assimilation (A) and imperfect fractional crystallization (IFC) in an open igneous system, where crystal accumulation and assimilation occur concurrently. Especially, deep crustal physicochemical interaction of primitive arc magmas with supracrustal host rocks may mask the expected clear signature of subarc mantle petrological processes. Our assessment of the Famatinian-Puna arc, an ancient subduction zone, concurs with estimates from active volcanic arcs, underscoring the general applicability of this study case to subduction zone magmatism.

Hot subduction in the southern Paleo-Asian Ocean: Insights from clinopyroxene chemistry and Sr–Nd–Hf–Pb isotopes of Carboniferous volcanics in West Junggar

The chemical evolution and pressure–temperature conditions of subduction zone magmatism along ancient suture zones in orogenic belts can provide important information regarding plate convergence processes in paleo-oceans. Carboniferous magmatism in West Junggar is key to understanding the tectonothermal and subduction history of the Junggar Ocean, which was a branch of the Paleo-Asian Ocean, as well as the accretionary processes in the southwestern Central Asian Orogenic Belt (CAOB). We undertook a geochronological, mineralogical, geochemical, and Sr–Nd–Hf–Pb isotopic study of volcanic rocks from the Baikouquan area of West Junggar. We used these data to determine the petrogenesis, mantle source, and pressure–temperature conditions of these magmas, and further constrain the subduction and tectonic history of the Junggar Ocean. The studied volcanic rocks yielded zircon U–Pb ages of 342–337 Ma and are characterized by enrichments of large-ion lithophile elements (LILEs), and depletions in high-field-strength elements (HFSEs), indicative of an island arc affinity. The volcanic rocks have positive ƐNd(t) (5.83–7.04) and ƐHf(t) (13.47–15.74) values, 87Sr/86Sr(t) ratios of 0.704023–0.705658, and radiogenic 207Pb/204Pb(t) and 208Pb/204Pb(t) ratios at a given 206Pb/204Pb(t) ratio, indicative of a depleted mantle source contaminated by subduction-related materials. Geochemical modeling calculations indicate that ≤1% of a subduction component comprising fluid and sediment melt could have generated the source of the parental melts of the Baikouquan volcanic rocks. Clinopyroxene phenocrysts in the volcanic rocks are classified as high- and low-Ti clinopyroxene, and pressure–temperature calculations suggest the host rocks formed at high temperatures (∼1300 °C) and shallow to moderate depths (<2 GPa). The magma was probably generated by hot and hydrous melting in a mantle wedge in response to subduction of young, hot oceanic lithosphere. The present results, combined with published data, suggest that the Baikouquan volcanic rocks record a transition in tectonic setting from normal cold to anomalous hot subduction of young oceanic lithosphere close to a mid-ocean ridge. This indicates ridge subduction began shortly after 337 Ma. Our results provide new insights into the tectonomagmatic evolution during intra-oceanic subduction prior to ridge subduction.

Coexisting Nb-enriched basalts and arc volcanic rocks in the northern Qiangtang Terrane, China: Implications for the effects of ambient mantle on subduction zone magmatism

The petrogenesis of Nb-enriched basalts (NEBs) is controversial. NEBs are generally considered to form by melting of either a mixed enriched and depleted mantle source or a mantle wedge metasomatized by adakitic melts. Here we present geochronological, petrological, geochemical, and Sr-Nd-Os isotope data for Early Permian volcanic rocks from the Tuotuohe area in the northern Qiangtang Terrane, China. The studied volcanic rocks can be divided into two groups: arc basalts and basaltic andesites in the lower sequence and NEBs in the upper sequence. Zircon grains from one basaltic andesite sample in the lower sequence yielded a concordia age of 302.0 ± 1.1 Ma. The geochemical and isotopic characteristics suggest that the NEBs were derived from a mixed source consisting of enriched mid-oceanic ridge basalts (MORB)- to oceanic-island basalt-like ambient mantle and slab-derived fluids, whereas the arc basalts and basaltic andesites originated from a source made of the MORB-type ambient mantle modified by slab-derived melts. We propose for the first time that the Nb enrichment of the NEBs was most likely inherited from high-Nb ambient mantle and is independent of the addition of slab-derived components. In addition to slab-derived components, the nature of the ambient mantle is also an important factor controlling the diversity of arc magmatism. Our new findings show that the Late Carboniferous−Early Permian magmatism in the northern Qiangtang Terrane was related to northward subduction of the Paleo-Tethys Ocean, suggesting that subduction might have occurred in at least the Early Permian rather than in the previously thought Middle Permian.

Tectonic deformation at the outer rise of subduction zones

SUMMARY Fluids associated with subducting slabs play a crucial role in regulating the dynamics of water discharge, subsequent arc magmatism and intermediate-depth earthquakes in subduction zones. The incoming slab mantle hydration is primarily determined by deep normal faulting due to plate bending at the trench. However, the controlling factors on the outer rise faulting pattern, and the correlation between the inherited outer rise deformation and the intermediate-depth earthquakes, remain to be understood. Here we present high-resolution viscoelasto-plastic numerical models of free subduction for slab bending-related faulting prior to subduction. Our model results show that plastic weakening and friction coefficient of the slab mantle exhibit a significant impact on fault pattern, while plate age and elasticity have a minimal bearing for mature slabs. The brittle bending faults result in large positive pressure gradients in the vertical direction, facilitating seawater infiltrating into the subducting slabs, which corroborates previous numerical models. The faults reaching 15–30 km beneath the Moho coincide with the width of the double seismic zone in subduction zones. We anticipate that water pumped into the slab mantle along the faults, with decreasing water content along the depth, can explain the relatively sporadic lower plane earthquakes.

A Role for Crustal Assimilation in the Formation of Copper-Rich Reservoirs at the Base of Continental Arcs

Abstract Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up to ~730 µg g–1) hosted by sulfide phases, providing direct evidence for the formation of lower crustal copper-rich reservoirs. The Acadian cumulates reveal a wide range of δ34S values, from –4.9‰ in the ultramafic rocks to 8‰ in the most evolved mafic rocks. The negative δ34S values observed in the most primitive and copper-rich cumulates (avg –3‰) reflect the assimilation of isotopically light sulfur from surrounding sulfidic and graphite-bearing metasedimentary rocks (δ34S of –19 to –12‰), whereas the more evolved cumulates with positive δ34S signatures may have formed from different magma batches that experienced less sediment assimilation. The assimilation of these reducing metasedimentary rocks caused a critical drop in oxygen fugacity (~DFMQ –2.5 to –1.9; FMQ = fayalite-quartz-magnetite buffer) in the evolving magmas, ultimately leading to extensive sulfide saturation and the consequent formation of copper-rich subarc cumulates. Assimilation-driven sulfide saturation may be a common process at the root of thickened arc crusts that triggers the formation of lower crustal copper-rich reservoirs, which play a pivotal role in the fate of copper during arc magmatism. Thus, deeply buried reducing metasedimentary crustal material at the base of continental arcs can act as a barrier to the magmatic flux of chalcophile elements and may play a crucial role in the genesis and distribution of porphyry copper deposits.

The Bazman and Taftan volcanoes of southern Iran: Implications for along-arc geochemical variation and magma storage conditions above the Makran low-angle subduction zone

Bazman and Taftan are two Late Miocene-Quaternary volcanoes of the Makran continental arc formed by the subduction of Arabian oceanic lithosphere beneath southern Eurasia. This study compares their bulk rock compositions as well as major and trace element contents of major mineral phases. Bazman is composed of a geochemically heterogeneous suite of low to medium-K basic to acidic compositions, while Taftan exposes more homogenous mainly andesitic rocks of medium to high-K affinity. Both volcanoes are characterized by the enrichment of LILE compared to HFSE and depletion of Nb, Ta, and Ti, reminiscent of subduction zone magmatism. At a given SiO2 content, the Taftan volcanics have higher concentrations of incompatible elements and, La/Yb, Th/Yb, and Sr/Y ratios (adakitic signature), here interpreted as being related to the more enriched mantle source, higher contribution of subducted sediments in magma genesis and thicker continental crust beneath Taftan. Thermobarometric calculations based on mineral chemistry (amphibole, orthopyroxene, clinopyroxene, and plagioclase) indicate pre-eruptive T = ∼1050–800 °C, P = 250–100 MPa and H2Omelt > 3.5 wt%. Bazman lavas record slightly lower P and T, and higher H2Omelt, suggesting a shallower magma reservoir and thus a more developed magma plumbing system. Trace element abundances in minerals show consistent distinctions between the two volcanoes, e.g., amphibole is REE-enriched in Taftan over Bazman samples. The features of trace element compositions and zoning patterns of minerals (amphibole, plagioclase and, pyroxene) along with available major element data are suggestive of both recharged magmatic systems and convective self-mixing magma chambers.

Molybdenum isotopic constraint from Java on slab inputs to subduction zone magmatism

Molybdenum isotope is a diagnostic tracer for crustal and mantle components in arc magmatism. However, the mechanism of Mo isotopic variation in arc magmas is still debated, e.g., input of different subduction components into the mantle wedge versus isotopic fractionation during dehydration of subducted slab. Here we present whole-rock Mo-Sr-Nd-Hf-Pb isotopic data for the Continental Arc Basalt (CAB) and Back Arc Basalt (BAB) from Java, Indonesia, to investigate the role of slab inputs in Mo isotopic variation of the Sunda arc magmatism. The CAB samples have variable K2O contents (0.44–2.49 wt%) and are mainly classified as calc-alkaline series, while the BAB samples are shoshonitic with markedly high K2O contents (2.12–6.90 wt%) relative to the CABs. The Java CABs and BABs have similar Mo isotopic compositions (δ98/95Mo = −0.65 to −0.07‰ and −0.66 to −0.07‰, respectively, relative to NIST SRM3134), suggesting that such a significant Mo isotopic variation should not be caused solely by the isotopic fractionation during the subduction. Instead, δ98/95Mo values of the Java basalts positively correlate with Pb isotopic ratios. This implies that the Mo isotopic variations in the Java arc rocks should result from the metasomatism in the mantle wedge by hybrid agents, including varying proportions of melts from subducted sediments (with heavy Mo isotope) and melts from the subducted altered upper oceanic crust (SAOC) (with light Mo isotope). The light Mo isotope of the Java arc rocks, compared with the Mariana arc basalts, suggests that melts from the SAOC have much lighter Mo isotopic compositions than the components from the lower oceanic crust. Thus, Mo isotope has great potential to distinguish the components from the subducted upper and lower oceanic crust. The Java CABs show along-arc variations in Mo-Sr-Nd-Hf-Pb isotopes, which is related closely with the thermal status of the subducted slab. Upwelling of the asthenosphere due to the slab tearing beneath the Java arc might have enhanced the partial melting of subducted sediments nearby the slab window. The complicated subduction system in the Sunda arc has strongly controlled the geochemical composition of arc magmas, which changes with input of different subduction components into the mantle wedge along arc.

Carbonatitic pockets in intra-ocean arc volcanics (Qilian orogen): Petrogenesis and implications for carbon recycling in subduction zones

Carbonatites in island arcs are crucial for understanding the processes of magmatism and carbon recycling in subduction zones. Here we report carbonatitic pockets in Ordovician intra-oceanic island-arc volcanic rocks and in Ocean island basalts (OIB) of Cambrian ophiolites in the South Qilian Accretionary Belt, NW China. All these carbonatitic pockets are composed of relatively pure calcites, some of which have accessory minerals of apatite, sphene, hematite, magnetite and allanite. The 87Sr/86Sr(i) ratios of calcite range from 0.703833 to 0.706732, concentrated at about 0.705, which is close to the composition of Enriched Mantle 1 (EMI) affected by fluids from the subduction zone. The C-O isotopic compositions show trends that evolve away from primary carbonatite (δ13CPDB = −9.0 ~ −1.0 ‰, 18OSMOW = 10.1– 16.5 ‰), probably caused by calcite fractional crystallization. High Sr, Th, U, Pb and low Nb, Zr, Hf, Ti contents and 87Sr/86Sr(i) ratios indicate that these carbonatitic pockets derived from carbonatite magma, and may have been altered by subduction-zone fluids. Carbonatitic pockets in ankaramites represent primary mantle-derived carbonate melts/liquids with low REE (rare earth elements) abundances (<10 ppm) and δ13CPDB. Elevated REE contents and LREE/HREE (light / heavy rare earth elements) ratios suggest that carbonatitic pockets in high-Al andesite may have experienced fractionation crystallization. The EMI-type carbonatitic pockets occurrence in the two different rock assemblages may reflect an interaction between an oceanic arc and an oceanic plateau, deriving from the recycling of Proterozoic marine carbonates through the lower mantle.

Making Andesites and the Continental Crust: Mind the Step When Wet

ABSTRACT Andesites are iconic of subduction zone magmatism. Yet intermediate magmas (57–66 wt % SiO2) are less abundant than generally thought in arc settings. A comparison of experimental hydrous liquid lines of descent, melt inclusions and bulk-rock compositions demonstrates the importance of polybaric crystallization–differentiation in producing the compositional range and dictating the relative abundance of arc melts, but also highlights the preponderant role of mixing (sensu lato) in producing andesitic magmas. Based on their P2O5 contents, at least 74% of the arc magmas with around 64 wt % SiO2 are inferred to be mixing products. In addition to their surprisingly low abundance, andesitic melt inclusions are characterized by relatively low H2O, Al2O3, ± Na2O contents compared to the ranges measured in mafic and silicic melt inclusions. These compositional characteristics suggest that there is a sweet spot for the production of andesitic melts delimited by the low-pressure stability limit of amphibole (&amp;lt;150 MPa) and the adiabatic ascent path of mafic melts, but that this low-pressure differentiation pathway plays a minor role in the production of silicic arc magmas that principally form along high-pressure hydrous liquid line of descents (&amp;gt;700 MPa) before decompression. The compositional bimodality recorded by the melt inclusions and in well-preserved intra-oceanic arc crustal sections is a fundamental characteristic of differentiation in transcrustal arc magmatic systems, with important consequences for the chemical evolution of the continental crust. We propose that the overall bimodality shown by arc melts does not relate to a compositional gap in the differentiation mechanisms but results from a combination of (1) the disparity in volume of differentiated magmas produced by low and high-pressure crystallization–differentiation and (2) the strong nonlinearity of the high-pressure liquid lines of descent in composition–temperature–crystallinity space related to crystallization of amphibole-rich assemblages. In this context, the compositional characteristics shared by andesitic magmas and the continental crust principally depict the central role of mixing and mass balance processes in producing andesitic compositions. The step in differentiation efficiency encountered by hydrous magmas entering the amphibole stability field at high pressure plays an important role in defining the silicic component involved in these scenarios.

Continental crust recycling in ancient oceanic subduction zone: Geochemical insights from arc basaltic to andesitic rocks and paleo-trench sediments in southern Tibet

The recycling of continental crust via trench sediment subduction and subduction erosion is well established in modern oceanic subduction zones, and this is fundamental to subduction zone magmatism and mantle heterogeneity. Although a similar process likely took place in ancient oceanic subduction zones, its evaluation is difficult because most geological records were erased by subsequent continental collision and post-collisional reworking processes. To address this issue, a comprehensive geochemical study was conducted for Mesozoic basaltic to andesitic plutonic rocks and surviving contemporaneous paleo-trench sediments from the Himalayan orogen in southern Tibet. These plutonic rocks show variably enriched Sr-Nd-Pb-Hf isotope compositions with εNd(t) values varying from −5.6 to +4.7, which are neither positively nor negatively correlated with Mg# values. This indicates that they were not affected by crustal contamination but rather inherited from heterogeneous magma sources. There are consistently continuous variations in the Sr-Nd-Pb-Hf isotope space among the paleo-trench sediments, the arc plutonic rocks, and basaltic rocks from the Yarlung Tsangpo ophiolite suites, indicating that the binary mixing between the paleo-trench sediment-derived melts and the ambient mantle dictated the radiogenic isotope compositions of the arc igneous rocks. These qualitative interpretations are further verified by quantitative geochemical examination of the compositional links in Nd and Hf isotopes between the paleo-trench sediments and arc rocks. As a consequence, the present study has for the first time identified and systematically demonstrated the key role of paleo-trench sediments in constraining the continental crust recycling as well as the origin and compositional heterogeneity of arc igneous rocks above ancient oceanic subduction zones.

Geochemical analysis of magmatic rocks from Shyok Suture Zone (SSZ) Trans-Himalaya, NW India: Insights for geodynamic evolution of the terrane

The present study discusses the geochemistry of the Early Cretaceous Shyok and Nubra volcanics from the Shyok Suture Zone (SSZ), Trans-Himalaya. Geochemically, the studied volcanic rocks belong to calc-alkaline magma series and vary in composition from basalt-basaltic andesite-andesite-dacite. The SSZ rocks are enriched in light rare earth elements (LREE: La, Ce, Nd) and large ion lithophile elements (LILE: Rb, Ba, U, K, Pb and Sr) while depleted in high field strength elements (HFSE: Nb, P and Ti), analogous to subduction zone magmatism. Their perturbed LILE concentrations coupled with higher 87Sr/86Sr ratios reflect possible influence of post-crystallization processes. Trace element modelling reflects variable degrees of partial melting (~5–30%) of a metasomatised spinel peridotite mantle source followed by fractional crystallization of clinopyroxene and amphibole along with olivine and plagioclase. The Shyok and Nubra volcanics depict positive εNd(t = 110 Ma) values (+2.3 to +5.2), slightly lower than the contemporary Chalt fore-arc boninites (+6.3 to +8.0) and adjoining intra-oceanic island arc rocks along the Indus Tsangpo Suture Zone (ITSZ) such as Nidar ophiolite (+6.8 to +8.7), Shergol gabbros (+5.1 to +8.4) and Dras volcanics (+5.8 to +7.0), therefore reflecting depleted mantle characteristics. In contrast to the ITSZ rocks, the Shyok and Nubra volcanics have enriched LREE-LILE characteristics, indicating their derivation from isotopically depleted and elementally (LREE-LILE) enriched mantle wedge sources. Thus, based on the geochemical and isotopic signatures of the SSZ rocks and their association with continentally derived sediments, we suggest an Andean-type arc system as opposed to ITSZ rocks and associated oceanic sediments, representing intra-oceanic arc system.

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.

Petrologic evolution of boninite lavas from the IBM Fore-arc, IODP Expedition 352: Evidence for open-system processes during early subduction zone magmatism

Abstract Boninite samples from several intervals within Hole U1439C, recovered during IODP Expedition 352, show highly variable mineral chemistries that imply complex crystallization histories. Small pyroxene grains show oscillatory zoning with cores and zones ranging from pigeonite to augite. Late crystallizing augite has highly variable Al2O3 contents (1.9–13.7 wt%) and Ca-Tschermak component contents (3–13 mol%), which reflect disequilibrium conditions. Large, euhedral, low-Ca pyroxene (i.e., enstatite/clinoenstatite) crystals exhibit complex sector and oscillatory zoning patterns. Cr-rich spinel is found as inclusions both in olivine and low-Ca pyroxene. Early crystallized olivine phenocrysts have embayed and reacted margins, and some early crystallized olivines exhibit zoning. A few olivines have multiple zones, with both normal and reverse zoning between Fo86–92. Olivine xenocrysts also have embayed and reacted margins; however, xenocrysts do not exhibit chemical zoning patterns and have consistent Fo88 compositions. Disequilibrium crystallization of pyroxene rims reflects rapid cooling during an eruption. Sector zoning in low-Ca pyroxenes is the result of crystallization during periods of moderate undercooling between mixing events. Oscillatory, normal, and reverse zoning in olivine and pyroxene appears to have formed in response to multi-stage magma mingling or mixing processes, which introduced additional Ca, Fe, Ti, and Al into parental boninitic melts. The presence of olivine xenocrysts and orthopyroxene indicate equilibrium at 2–4 kbar (Whattam et al. 2020) indicates that boninite magma mixing events likely occurred within shallow magma chambers containing olivine-rich cumulate piles. Large mixing events probably destabilized the magma chamber, resulting in devolatilization and eruption. In contrast, small mixing events lacked the energy to destabilize the chamber, resulting in repeated compositional oscillations in minerals affected by multiple small mixing events.

Partial melting of a depleted peridotite metasomatized by a MORB-derived hydrous silicate melt – Implications for subduction zone magmatism

Recent geodynamic models and geothermometers suggest that slabs in intermediate to hot subduction zone cross the water-saturated basalt solidus, indicating that hydrous silicate melts are important agents of mass transfer from slab to mantle wedge beneath arcs. Yet the effects of basaltic crust-derived hydrous melt fluxing on mantle wedge melting are poorly known. Here we present the melting phase relations of a depleted peridotite + a MORB-derived hydrous silicate melt at a melt:rock mass ratio of 0.1 and 0.05 (3.5 and 1.7 wt.% H2O, respectively) to simulate fluid-present partial melting of a depleted peridotite, which has been metasomatized by a hydrous silicate melt derived from subducting basaltic crust. Experiments were performed at 2–3 GPa and 900–1250 °C in a piston cylinder, using Au and Au75Pd25 capsules. Amphibole (7–10 wt.%) is stable up to 1000 °C at 2 and 3 GPa coexisting with an assemblage dominated by olivine and opx and with minor fractions of cpx and garnet at 3 GPa. The apparent fluid-saturated solidus of our bulk composition is located at 1000–1050 °C, coinciding with the exhaustion of amphibole at 2 and 3 GPa. Amphibole is exhausted between 0 and 5 wt.% melting at 2 and 3 GPa and dominates the melting reactions in this melting interval along with opx, generating SiO2 and Al2O3-rich, and FeO*- and MgO-poor primitive andesites under fluid-saturated conditions. The melting reactions during low-degree, fluid-saturated melting are incongruent, consuming opx and producing olivine + SiO2-rich melts and is observed over a wide range of starting compositions and pressures from this study and others. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 – 4.2 wt.% H2O can reproduce the major oxide spread and trends of primitive arc magmas globally. From this comparison, it is clear that differences solely in the pressure of hydrous mantle melting, where the partial melts are fluid-under saturated, can account for the first order trends observed in experimental and natural data, with differences in temperature and composition contributing to the compositional spread within these trends. The ubiquity of andesite genesis over a wide range of pressures and bulk compositions during aqueous fluid-saturated melting suggests that the relative rarity of primitive andesitic melt flux through the crust could be related to the fact that such melts are only produced at the base of the mantle wedge where temperatures are relatively low. As fluid-saturated andesitic melts ascend into the hotter core of the mantle wedge, they are likely consumed by higher-degree, fluid-undersaturated melting generating more common hydrous basaltic melts.

In situ determination of sulfur speciation and partitioning in aqueous fluid-silicate melt systems

Current knowledge of sulfur behaviour in magmas is based exclusively on ex situ analyses. Here we report the first in situ measurement of sulfur speciation and partitioning between coexisting aqueous fluids and silicate melts. These data were acquired using Raman spectroscopy in a diamond anvil cell at 700 °C, 0.3–1.5 GPa, and oxygen fugacity in the vicinity of the sulfide-sulfate transition, conditions relevant to subduction zone magmatism. Results show that sulfate and sulfide are dominant in the studied systems, together with the and radical ions that are absent in quenched fluid and silicate glass products. The derived fluid/melt partition coefficients for sulfide and sulfate are consistent with those from available ex situ data for shallow crust conditions (<10 km), but indicate stronger partitioning of these sulfur species into the silicate melt phase with increasing depth. In contrast, both radical ions partition at least 10 times more than sulfate and sulfide into the fluid phase. Thus, by enhancing the transfer of sulfur and associated chalcophile metals from melt into fluid upon magma degassing, and may be important players in the formation of economic metal resources within the redox window of the sulfate-sulfide transition in subduction zone settings.

The geodynamic history of the Famatinian arc, Argentina: A record of exposed geology over the type section (latitudes 27°- 33° south)

The Famatinian arc, over central-western Argentina, is one of the few examples of exposed crustal arc cross-section in the world. This Paleozoic magmatic arc and orogenic system on the Earth's surface offers unique insights into the nature of whole-arc processes, continental crust generation, and preservation. A detailed account of birth, growth, and closure is integrated throughout the region, and the entire geodynamic history of the Famatinian arc is presented. The Famatinian magmatic arc grew during a single-cycle episode that spanned a few tens of million years. Upon waning of the Cambrian Pampean orogen, an Upper Cambrian – Lower Ordovician marginal and open sea basin developed mostly on the recently stabilized Cambrian crystalline crust and was flooded by a turbidite wedge. Typical subduction zone magmatism resumed outboard of the Pampean orogen in the Lower Ordovician. After about 20 My of magmatism driven by subduction zone dynamics at plate scale, magmatism waned and stopped with the entry of a Laurentia-rifted continental microplate in the subduction zone. A full phase of a mountain-building process accompanied the continent-arc collision. The orogenic collapse occurred during the Devonian, ending the Famatinian system after nearly 150 My of magmatic arc, plate convergence, and continent-arc collision development on the proto-Andean Gondwana margin.