Subduction-zone Dehydration Research Articles

Tourmaline composition probes serpentinite-derived fluid mobility in subduction zones

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low XMg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ11B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low XMg (<0.6), relatively high trace element concentrations and δ11B values of less than −10 ‰, whereas the overgrowths exhibit extremely high XMg values (>0.99), low trace element concentrations and high δ11B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-11B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high XMg > 0.85 and δ11B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.

Fluid-mediated Cu and Zn isotope fractionation in subduction zones and implications for arc volcanism: Constraints from high pressure veins within eclogites in the Dabie Orogen

High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δ65Cu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δ66Zn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δ65Cu: −0.71 vs. 1.84‰, δ66Zn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δ65Cu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δ65Cu with redox-sensitive ratios of Fe3+/ΣFe and (Eu/Eu*)N in those veins. Additionally, the higher δ66Zn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

Deciphering Contribution of Recycled Altered Oceanic Crust to Arc Magmas Using Ba‐Sr‐Nd Isotopes

AbstractAltered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc. Based on a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes, we confirm that AOC‐derived fluid (or hydrous melt), rather than sediment‐derived melt or fluid, is responsible for the Sr‐Nd isotope decoupling (i.e., 87Sr/86Sr is “excessively” enriched relative to 143Nd/144Nd when compared to the “normal” mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr‐Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC‐derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr‐Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr‐Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones.

Epitactic magnetite growth in fluid inclusions as driving force for olivine oxidation coupled with hydrogen production at high pressure

We disclose the crystallisation evolution of magnetite-bearing multiphase inclusions hosted in metamorphic olivine of harzburgites from the Cerro de Almirez (Betic Cordillera, Spain), which have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration of former serpentinites. The chemical exchange between inclusion fluid and olivine started soon after entrapment, at peak P-T conditions of 1.6–1.9 GPa and 650–700 °C, and continued during cooling along the retrograde path, with the coexistence of olivine and magnetite with orthopyroxene, chlorite, talc, antigorite and the destabilisation of olivine and antigorite into brucite and low-temperature chrysotile serpentine, as recognised by Raman analyses. Thermodynamic modelling and mass balance calculations demonstrate that the water component of fluid trapped in the inclusions of metamorphic olivine is expected to trigger the oxidation of the fayalite component in olivine, producing a mineral assemblage made of magnetite + orthopyroxene and molecular hydrogen, where the elemental redox processes are Fe2+ of olivine that oxidizes to Fe3+ and H+ of water that reduces to H2. Probable H2 trapped in the olivine host close to the inclusion wall has been detected by Raman spectroscopy. To corroborate its presence, we performed quantitative mass spectrometry analyses of the fluid phase trapped in the multiphase inclusions and of the olivine crystals hosting the inclusions, revealing that 1 kg of olivine matrix contains 6.2 ± 0.1 mmol of H2. We identify two synergistic driving forces of the whole process, which has the peculiarity to produce molecular hydrogen at apparently oxidising conditions: i) the building up of an epitaxial interface between olivine and magnetite, and ii) the olivine ability to trap H2 at high pressures. The olivine + H2O system of these natural microreactors simulates a process of oxidation of the mantle olivine by water, with production of H2 at pressure and fO2 conditions (FMQ + 2) at which water reduction is considered an unlike mechanism.

Decoupling of inorganic and organic carbon during slab mantle devolatilisation

Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

The intrinsic nature of antigorite breakdown at 3 GPa: Experimental constraints on redox conditions of serpentinite dehydration in subduction zones

Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than mid-oceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multi-anvil apparatus. These experiments aim to reproduce the different stages of H2O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic high oxygen fugacity (fO2) of serpentinite during dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high XMg numbers setting up the oxygen fugacity to high values, between 3.1 and 4.1 log units above QFM (Quartz–Fayalite–Magnetite buffer). Hematite is observed concomitantly with high XMg in olivine, of 0.94–0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown which occurs above 800 °C, the oxygen fugacity decreases to 3.7 due to the decrease of the XMg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, fO2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.

Commensurate Growth of Magnetite Microinclusions in Olivine under Mantle Conditions

Magnetite-bearing multiphase solid inclusions hosted in metamorphic olivine have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration ...

Into the deep and beyond: Carbon and nitrogen subduction recycling in secondary peridotites

Understanding the volatile cycles at convergent margins is fundamental to unravel the Earth's evolution from primordial time to present. The assessment of fluid-mobile and incompatible element uptake in serpentinites via interaction with seawater and subduction-zone fluids is central to evaluate the global cycling of the above elements in the Earth's mantle. Here, we focus on the carbon (C), nitrogen (N) and C isotope compositions of chlorite harzburgites and garnet peridotites deriving from subduction-zone dehydration of former oceanic dehydration of serpentinite – i.e., metaperidotites (Cima di Gagnone, Swiss Central Alps) with the aim of evaluating the contribution of these rocks to the global C-N cycling. These ultramafic rocks, enclosed as lenses in a metasedimentary mélange, represent the destabilization of antigorite and chlorite at high-pressure/temperature (P/T) along a slab-mantle interface. Chlorite- and garnet-bearing rocks have similar ranges in C concentration ([C] = 210 – 2465 ppm and 304 – 659 ppm, respectively), with one magnesite-bearing chlorite harzburgite hosting 11000 ppm C. The average N concentrations ([N]) of the garnet peridotites (54 ± 15 ppm, one standard deviation indicated) are higher than those of the chlorite harzburgites (29 ± 6 ppm). The δ13C of total C (TC) and total organic C (TOC) values of the Gagnone metaperidotites range from -12.2 to -17.8‰ and from -27.8 to -26.8‰, respectively, excluding the magnesite-bearing chlorite harzburgites with higher values of -7.2‰ (TC) and -21.2‰ (TOC). The [C] of these rocks are comparable to those of serpentinites form modern and ancient oceanic environments and with [C] of high-P serpentinites. However, the lack of preserved serpentinite precursors makes it difficult to determine whether release of H2O during high-P breakdown of antigorite and chlorite is coupled with significant C release to fluids. The δ13C values appear to reflect mixing between seawater-derived carbonate and a reduced C source and a contribution from the host metasedimentary rocks ([C] = 301 ppm; [N] = 33 ppm; TC δ13C = -24.4‰; TOC δ13C = -27.0‰) cannot be completely excluded. The C-O isotope composition of the carbonate in magnesite-bearing chlorite harzburgites is compatible with progressive devolatilization at oxidized conditions, whereas the signatures of the majority of the other Gagnone samples appear to reflect different degree of interaction with sedimentary fluids. The [N] of the Gagnone metaperidotites are higher than those of oceanic and subducted serpentinites and show a range similar to that of high-P antigorite-serpentinites from mantle wedges. This enrichment is compatible with fluid-mediated chemical exchange with the surrounding metasedimentary rocks leading to strong modification of the Gagnone metaperidotites' geochemistry during prograde subduction along the slab-mantle interface. Comparing the δ13C data reported in this study with published δ13C values for diamonds, we suggest that the volatile recycling via Gagnone-like metaperidotites in subduction zones could contribute to deep-Earth diamond genesis and in particular to the formation of blue boron (B)-bearing diamonds. Our results highlight that the subduction of secondary peridotites evolved along the slab-mantle interface is a viable mechanism to inject volatiles into the deep mantle, particularly in hotter geothermal regimes such as the ones active during the early Earth's history.

An experimental study of trace element mobility during dehydration of lawsonite blueschist along different P-T paths: Implications for geochemical heterogeneity of Earth’s mantle

Dehydration in subduction zones is an important process that causes portioning of elements between downgoing slab and overlying mantle wedge. When subducted into Earth’s deep mantle, dehydrated oceanic crust may generates critical geochemical heterogeneity. Trace elements in oceanic crust are liberated from subducting slab by dehydration. However, trace elements mobilities may vary along different subduction P-T paths. In order to constrain the mobility of trace elements during dehydration along different subduction P-T paths, we carried out three groups of high-pressure experiments on a natural lawsonite blueschist at 4 and 5.5 GPa with T ranging from 650 to 900 °C. We found that large ion lithophile elements (LILEs) are significantly released during dehydration along both hot and cold subductions. However, some water-bearing residual minerals (e.g. lawsonite and Al-10 Å phase) are stable along a cold subduction P-T path and can carry significant amounts of Rb, Sr, Th, U and light rare earth elements (LREEs) into deep mantle. The mobilities of radioactive parent and radiogenic daughter (e.g. U-Pb, Th-Pb, Rb-Sr and Sm-Nd) are rather different during dehydration along different subduction P-T paths. Along both hot and cold subduction paths, Pb shows significantly greater mobility than U and Th. However, the mobile behavior of Rb-Sr and Sm-Nd totally opposite along cold and hot subduction P-T paths. We conclude that recycling of oceanic crust in mantle through cold subduction can lead to a geochemically heterogeneous mantle more efficiently than along a hot subduction.

Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones. Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

Experimental insight into redox transfer by iron- and sulfur-bearing serpentinite dehydration in subduction zones

Dehydration of antigorite in subduction zones releases a large amount of aqueous fluid and volatile elements, which can potentially oxidize the mantle wedge. The redox capacity of three synthetic serpentinites with variable Fetotal, Fe3+ and S− contents is investigated using XANES spectroscopy at both, Fe and S K-edges. Experiments are performed between 450 and 900 °C, at 2 GPa and fO2∼QFM-2; conditions similar to those encountered in subduction zones.Redox reactions in the synthetic serpentinites, which involve Fe and S can be summarized as follows: 1) the reduction of (S−)-pyrite into (S2−)-pyrrhotite (∼450 °C), with ∼4.4 mg/g of the sulfur degassed most likely as H2S, 2) the consumption of magnetite that reacts with antigorite to form Fe-rich olivine (<500 °C), 3) the reduction of (Fe3+)-antigorite into (Fe2+)-antigorite (∼580 °C), occurring about 100 °C below the temperature of antigorite breakdown, 4) the main (Fe2+)-antigorite breakdown that forms olivine and enstatite (∼675 °C), and 5) the decomposition of minor amounts of (Fe2+/3+)-clinochlore (∼800 °C). The bulk Fe3+/Fetotal ratio is found to decrease with run temperature from ∼0.82–0.97 depending on the hydrous starting material, down to 0.1–0.2 in the high-temperature anhydrous assemblages.The evolution of mineral modes and Fe3+/Fetotal with temperature in our synthetic samples shows similar trends to what has been reported in serpentinite rocks collected, for example, along a metamorphic transect in the western Alps. We show that a large amount of O2-equivalent – up to 10 mol/kg of rock – can be generated at temperature around 450 °C due to the presence of oxides and sulfides such as magnetite and pyrite. Owing to the poor capacity of aqueous fluid to transfer redox conditions, we surmise that this O2-equivalent is “consumed” at the scale of the lithospheric-mantle top which is partially serpentinized and therefore bear strong redox gradients.

Zr-in-rutile thermometry of eclogites from the Karakaya Complex in NW Turkey: Implications for rutile growth during subduction zone metamorphism

Eclogites occur as a tectonic slice within a metabasite-phyllite-marble unit of the Karakaya Complex in northwest Turkey. The high-pressure mineral assemblage in eclogite is mainly composed of garnet+omphacite+glaucophane+epidote+quartz. Trace element characteristics of rutile and Zr-in-rutile temperatures were determined for eclogites from the Karakaya Complex. Core-rim analyses of rutile grains yield remarkable trace element zoning with lower contents of Zr, Nb and Ta in the core than in the rim. The variations in Zr, Nb and Ta can be ascribed to growth zoning rather than diffusion effects. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration in subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are dominated by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.The Zr contents of all rutile grains range between 81 and 160ppm with an average of 123ppm. The Zr-in-rutile thermometry yields temperatures of 559–604°C with an average temperature of 585°C for eclogites from the Karakaya Complex. This average temperature suggests growth temperature of rutile before peak pressure during the subduction. However, some rutile grains have higher Zr contents in the outermost rims compared to the core. Zr-in-rutile temperatures of the rims are about 20°C higher than those of the cores. This suggests that the outermost rims would have grown from a distinct fluid at higher temperatures than that of the cores. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical, which suggests that eclogites within the Karakaya Complex belong to the same tectonic slice and underwent similar metamorphic evolution.

Fluid-related inclusions in Alpine high-pressure peridotite reveal trace element recycling during subduction-zone dehydration of serpentinized mantle (Cima di Gagnone, Swiss Alps)

Serpentinites release at sub-arc depths volatiles and several fluid-mobile trace elements found in arc magmas. Constraining element uptake in these rocks and defining the trace element composition of fluids released upon serpentinite dehydration can improve our understanding of mass transfer across subduction zones and to volcanic arcs. The eclogite-facies garnet metaperidotite and chlorite harzburgite bodies embedded in paragneiss of the subduction melange from Cima di Gagnone derive from serpentinized peridotite protoliths and are unique examples of ultramafic rocks that experienced subduction metasomatism and devolatilization. In these rocks, metamorphic olivine and garnet trap polyphase inclusions representing the fluid released during high-pressure breakdown of antigorite and chlorite. Combining major element mapping and laser-ablation ICP-MS bulk inclusion analysis, we characterize the mineral content of polyphase inclusions and quantify the fluid composition. Silicates, Cl-bearing phases, sulphides, carbonates, and oxides document post-entrapment mineral growth in the inclusions starting immediately after fluid entrapment.Compositional data reveal the presence of two different fluid types. The first (type A) records a fluid prominently enriched in fluid-mobile elements, with Cl, Cs, Pb, As, Sb concentrations up to 103 PM (primitive mantle), ∼102 PM Tl, Ba, while Rb, B, Sr, Li, U concentrations are of the order of 101 PM, and alkalis are ∼2 PM. The second fluid (type B) has considerably lower fluid-mobile element enrichments, but its enrichment patterns are comparable to type A fluid.Our data reveal multistage fluid uptake in these peridotite bodies, including selective element enrichment during seafloor alteration, followed by fluid–rock interaction along with subduction metamorphism in the plate interface melange. Here, infiltration of sediment-equilibrated fluid produced significant enrichment of the serpentinites in As, Sb, B, Pb, an enriched trace element pattern that was then transferred to the fluid released at greater depth upon serpentine dehydration (type A fluid). The type B fluid hosted by garnet may record the composition of the chlorite breakdown fluid released at even greater depth.The Gagnone study-case demonstrates that serpentinized peridotites acquire water and fluid-mobile elements during ocean floor hydration and through exchange with sediment-equilibrated fluids in the early subduction stages. Subsequent antigorite devolatilization at subarc depths delivers aqueous fluids to the mantle wedge that can be prominently enriched in sediment-derived components, potentially triggering arc magmatism without the need of concomitant dehydration/melting of metasediments or altered oceanic crust.

Garnet growth as a proxy for progressive subduction zone dehydration

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO 2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ∼1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3–5.9 wt% water in hydrated MORB and 1.8–3.1 wt% water in pelite, representing 42%–100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM.On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.

The Petrology and Geochemistry of St. Helena Alkali Basalts: Evaluation of the Oceanic Crust-recycling Model for HIMU OIB

We present major element, trace element, and petrographic data on alkali basalts from St. Helena, and examine the geochemical characteristics of a recycled component involved in the source of HIMU (Pb/ Pb420·5) ocean island basalts. Petrographic and compositional variations in the St. Helena basalts are best explained by the combined effect of fractional crystallization and accumulation of phenocrysts. Primary melt compositions are estimated by correcting for the effects of crystal^liquid differentiation by reconstructing the order of crystallization and the relative amount of fractionated phases.This calculation indicates that the St. Helena alkali basalts are derived from a common primary magma with 14^20 wt % MgO. Simple partial melting of fertile mantle peridotite, depleted mid-ocean ridge basalt (MORB)-source mantle, or garnet pyroxenite fails to produce the St. Helena primary melt. Instead, this primary melt can be reproduced if there are contributions from ancient recycled oceanic crust and depleted peridotite [(Rb/ Nb)PM1⁄4 0·38^0·80]. Subducted sediment can be excluded to explain the low (Rb, Ba, U)/Nb and Ce/Pb of St. Helena basalts. Geochemical modeling using major and trace element abundances, together with Sr, Nd, Pb, and Hf isotope ratios, indicates that the St. Helena primary melt can be formed by 1^2% melting of a peridotitic source that was refertilized by a small amount (8^18%) of melt derived from recycled oceanic crust. This source has a similar trace element pattern to modern normal (N)-MORB, but element abundances are 0·1^0·2 times N-MORB values. The calculated recycled crust has a wide range of present-day Pb isotopic ratios (Pb/ Pb of 21·7^79·3 and Pb/ Pb of 40·8^89·3), Sr/ Sr of 0·7018^0·7028, Nd/Nd of 0·51274^0·51285, and Hf/Hf of 0·28262^0·28293 after a residence time of 1·2^2·8 Gyr. Rb, Ba, Pb, Sr, and light rare earth element abundances in the recycled crust are depleted compared with modern N-MORB, whereasTh, U, Sm, and Nd abundances fall within the range of compositional variations in modern N-MORB. The trace element compositions of the recycled oceanic crust can be explained by element behavior during seafloor alteration and subduction zone dehydration of oceanic crust. Therefore, recycling of ancient subducted oceanic crust is a potential process for producing the St. Helena HIMU basalts.

The origin of enriched mantle beneath São Miguel, Azores

Lavas from the island of São Miguel, Azores Archipelago have long been known to display large radiogenic isotopic variability, that ranges from “depleted” isotopic signatures (e.g. high ε Nd ∼ +5) in the west, typical of many ocean island basalts, to more “enriched” compositions (e.g. low ε Nd ∼ +1) in the east. Here, we further characterise the geochemistry of lavas from this remarkable locality, focussing on the nature and origin of the enriched source. Our new isotope data define a striking, linear array in Nd and Hf isotope space that points towards an unusual, enriched composition below the mantle array. This distinctive Hf–Nd isotope signature is associated with elevated values of all three radiogenic Pb isotope ratios. Although the enriched component has certain geochemical similarities to both terrigenous sediments and some samples of the continental mantle lithosphere, such comparisons do not stand closer examination. In the absence of a clear, modern analogue we explore the isotope evolution of some simple, model melt compositions to investigate plausible means of producing an appropriate enriched component. Nd–Hf isotope characteristics provide the tightest constraints and can be reproduced by an ancient (∼3 Ga), modest-degree melt (∼2%) from a garnet peridotite source. Currently, modest-degree melts from garnet-bearing sources are found forming some major oceanic islands. Subduction, isolation and later mixing of small amounts (<5%) of such basaltic material with more ubiquitous ambient mantle can account for the isotopic characteristics of the enriched São Miguel source. Yet the incompatible element ratios of the enriched São Miguel lavas do not show “recycled” signatures of near-surface alteration nor subduction zone dehydration. Thus, we infer that the enriched component was originally under-plated basalt, intruded into oceanic mantle lithosphere rather than forming the island edifice itself. Since the extreme isotope compositions of São Miguel reflect unextraordinary, albeit ancient, magmatic fractionation, the general rarity of such signatures indicates the efficiency of mantle processes in homogenising or hiding similar sources.

Correction to “Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B – Implications for seawater‐crust exchange budgets and Sr‐ and Pb‐isotopic evolution of the mantle”

[1] This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater-ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open-ocean sites 417/418 likely reflects prolonged fluid-rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1–2 Ga, if on the order of 80–90% Pb, 40–55% Rb, 40% Sr, and 35–40%U are removed during partial dehydration in subduction zones.

Origin of ocean island basalts: A new perspective from petrology, geochemistry, and mineral physics considerations

Consideration of petrology, geochemistry, and mineral physics suggests that ancient subducted oceanic crusts cannot be the source materials supplying ocean island basalts (OIB). Melting of oceanic crusts cannot produce high‐magnesian OIB lavas. Ancient oceanic crusts (&gt;1 Ga) are isotopically too depleted to meet the required values of most OIB. Subducted oceanic crusts that have passed through subduction zone dehydration are likely to be depleted in water‐soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb) relative to water‐insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). Melting of residual crusts with such trace element composition cannot produce OIB. Oceanic crusts, if subducted into the lower mantle, will be &gt;2% denser than the ambient mantle at shallow lower mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser than the ambient peridotitic mantle, perhaps by as much as ∼15%. Neither in the solid state nor in melt form can bulk oceanic crusts subducted into the lower mantle return to upper mantle source regions of oceanic basalts. Deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting volatiles and incompatible elements as a result of metasomatism taking place at the interface between the low‐velocity zone and the cooling and thickening oceanic lithosphere. These metasomatized and recycled deep portions of oceanic lithosphere are the most likely candidates for OIB sources in terms of petrology, geochemistry and mineral physics.

Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B – Implications for seawater‐crust exchange budgets and Sr‐ and Pb‐isotopic evolution of the mantle

This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater‐ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open‐ocean sites 417/418 likely reflects prolonged fluid‐rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1–2 Ga, if on the order of 80–90% Pb, 40–55% Rb, 40% Sr, and 35–40%U are removed during partial dehydration in subduction zones.