Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation

© The Author(s), [year]. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Walters, J. B., Cruz-Uribe, A. M., & Marschall, H. R. Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation. Geochemical Perspectives Letters, 13, (2020): 36-41, doi:10.7185/geochemlet.2011.Oxygen fugacity (fO2) is a controlling factor of the physics of Earth’s mantle; however, the mechanisms driving spatial and secular changes in fO2 associated with convergent margins are highly debated. We present new thermodynamic models and petrographic observations to predict that oxidised sulfur species are produced during the subduction of altered oceanic crust. Sulfur loss from the subducting slab is a function of the protolith Fe3+/ΣFe ratio and subduction zone thermal structure, with elevated sulfur fluxes predicted for oxidised slabs in cold subduction zones. We also predict bi-modal release of sulfur-bearing fluids, with a low volume shallow flux of reduced sulfur followed by an enhanced deep flux of sulfate and sulfite species, consistent with oxidised arc magmas and associated copper porphyry deposits. The variable SOx release predicted by our models both across and among active margins may introduce fO2 heterogeneity to the upper mantle.We thank James Connolly for modelling support and Peter van Keken for providing updated P–T paths for the Syracuse et al. (2010) models. The manuscript benefited from the editorial handling by Helen Williams and from constructive reviews of Maryjo Brounce, Katy Evans, and an anonymous reviewer. JBW acknowledges Fulbright and Chase Distinguished Research fellowships. This work was supported by NSF grant EAR1725301 awarded to AMC

Boron isotope analysis of silicate glass with very low boron concentrations by secondary ion mass spectrometry

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of International Association of Geoanalysts for personal use, not for redistribution. The definitive version was published in Geostandards and Geoanalytical Research 39 (2015): 31-46, doi:10.1111/j.1751-908X.2014.00289.x.Here we present an improved method for the determination of the boron isotopic composition of volcanic glasses with boron concentrations of as low as 0.4–2.5 μg g−1, as is typical for mid-ocean ridge basalt glasses. The analyses were completed by secondary ion mass spectrometry using a Cameca 1280 large-radius ion microprobe. Transmission and stability of the instrument and analytical protocol were optimised, which led to an improvement of precision and reduction in surface contamination and analysis time compared with earlier studies. Accuracy, reproducibility (0.4–2.3‰, 2 RSD), measurement repeatability (2 RSE = 2.5–4.0‰ for a single spot with [B] = 1 μg g−1), matrix effects (≪ 0.5‰ among komatiitic, dacitic and rhyolitic glass), machine drift (no internal drift; long-term drift: ~ 0.1‰ hr−1), contamination (~ 3–8 ng g−1) and machine background (0.093 s−1) were quantified and their influence on samples with low B concentrations was determined. The newly developed set-up was capable of determining the B isotopic composition of basaltic glass with 1 μg g−1 B with a precision and accuracy of ± 1.5‰ (2 RSE) by completing 4–5 consecutive spot analyses with a spatial resolution of 30 μm × 30 μm. Samples with slightly higher concentrations (≥ 2.5 μg g−1) could be analysed with a precision of better than ± 2‰ (internal 2 RSE) with a single spot analysis, which took 32 min.This study was financially supported by the NSF ocean sciences program (OCE grant 1232996 to Dorsey Wanless and HRM).2015-06-1

Geochemical evidence for mélange melting in global arcs

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 3 (2017): e1602402, doi:10.1126/sciadv.1602402.In subduction zones, sediments and hydrothermally altered oceanic crust, which together form part of the subducting slab, contribute to the chemical composition of lavas erupted at the surface to form volcanic arcs. Transport of this material from the slab to the overlying mantle wedge is thought to involve discreet melts and fluids that are released from various portions of the slab. We use a meta-analysis of geochemical data from eight globally representative arcs to show that melts and fluids from individual slab components cannot be responsible for the formation of arc lavas. Instead, the data are compatible with models that first invoke physical mixing of slab components and the mantle wedge, widely referred to as high-pressure mélange, before arc magmas are generated.This work was supported by the NSF (EAR-1119373 to S.G.N., EAR-1427310 to S.G.N. and H.R.M., and EAR-1348063 to H.R.M. and G. Gaetani) and Woods Hole Oceanographic Institution–Ocean Exploration Institute (to H.R.M. and G. Gaetani)

Lithium, Beryllium and Boron in High-Pressure Metamorphic Rocks from Syros (Greece)

In this study, high-pressure metamorphic rocks from the island of Syros (Greece) that are interpreted as parts of subducted oceanic crust, equilibrated at about 1.5-2.0 GPa and 500 °C were analysed. A first group of samples (Group 1) includes rocks that preserved the parageneses formed at the pressure peak of metamorphism, while a second group of samples (Group 2) represents metamorphic reaction zones formed at the contacts between contrasting lithologies. Additional to bulk-rock analyses, in-situ analyses of Li, Be and B abundances and B isotope ratios were performed using secondary ion mass spectrometry (SIMS). A new method was developed for B analysis at low concentrations using SIMS, which enables a reduction of boron contamination to levels close to or even below the detection limit of 2 ng/g. Group-1 samples contribute information on the impact of dehydration of oceanic crust on whole-rock abundances of different trace elements. The abundances of Li and Be do not correlate with H2O contents and are in the same range as in fresh and altered oceanic crust, suggesting that most of the Li and Be is not released with hydrous fluids. In contrast, B concentrations and B/Be ratios are correlated to the H2O contents of the rocks. Group-2 samples provide information on the effects of metasomatism of rocks during exhumation. Li and Be show high abundances in many samples, suggesting an enrichment during metasomatism. The enrichment of B is controlled by the occurrence of tourmaline. Tur-bearing samples display very high B/Be ratios, while Tur-free samples show low B concentrations and B/Be ratios. These results demonstrate that Li is probably a good tool for tracing metasomatic enrichment processes, while B is enriched only in the case of tourmaline formation. The SIMS study resulted in sets of inter-mineral partition coefficients for Li, Be and B for 15 different minerals, derived on the basis of in-situ analyses of coexisting phases in natural rock samples. These sets provide information on the behaviour of the light elements in different lithologies within subducting slabs, and they are essential for modelling of trace-element and isotope fractionation during subduction and dehydration of oceanic crust. In addition, the hosts of Li, Be and B were quantified with respect to the whole rock budgets. Modelling of trace element release from progressively dehydrating rocks was performed for Li, Be and B, which show contrasting behaviour during fluid/rock interaction processes. In principle, the presented model offers the possibility of a quantification of trace-element release from the slab in any lithology along any reasonable P-T path. Tourmaline grains of two metasedimentary and one metabasic rock were analysed in-situ for their chemical and B isotopic compositions. The ?11B values of prograde and peak metamorphic tourmaline range from -1.6 to +2.8 ‰ and are significantly higher than values reported in the literature for high-pressure meta-sedimentary tourmaline, demonstrating that a clear distinction between altered oceanic crust and oceanic sediments is not possible on the basis of B isotopes. Samples investigated in this study display heterogeneous sedimentary sources of tourmaline detrital grains with ?11B between -10.7 ‰ and +3.6 ‰ in a single sample. High-pressure blocks enclosed on the island of Syros are rimmed by blackwalls containing abundant tourmaline, with an unusually high ?11B values, exceeding +18 ‰ in all investigated samples, reaching a unique value of +28.4 ‰ in one sample. Blackwalls formed during exhumation of the rocks at a depth of 20-25 km. Estimated P-T conditions are ~ 0.6-0.75 GPa and 400-430 °C. They were produced by the influx of external hydrous fluids that probably originated in the subsequently subducting slab. The exceptionally high ?11B values are explained by interaction of the tourmaline-forming fluids with material composing the exhumation channel. The calculated model demonstrates that fluids are rapidly modified in both trace-element and isotopic composition during their migration through the material overlying the subducting slab. The formation of tourmaline at the contact between mafic or felsic high-pressure blocks and ultramafic matrix may also occur on the slab-mantle interface during subduction. If this is the case, the formation of blackwall tourmaline has a significant impact on the geochemical cycle of B in subduction zones, as it is fixing heavy B in large quantities in the slab within a highly stable mineral. Trace elements are only selectively incorporated into the blackwall tourmaline. The light elements Li and Be, the HFSE, the REE, Y and the LILE all show very low concentrations. First row transition metals and Sr, Pb and Ga are incorporated into dravite, demonstrating abundances in the same order as in the respective whole rocks, and in paragenetic rock-forming minerals. Hence, tourmaline is not strongly fractionating these elements in any way

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Taylor & Francis for personal use, not for redistribution. The definitive version was published in International Geology Review 59 (2017): 702-720, doi:10.1080/00206814.2016.1233834.In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰). Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.Support of this project was partly provided by National Science Foundation grant EAR-0711355 to GEB.2017-10-1

Isotopic compositions of sulfides in exhumed high-pressure terranes: Implications for sulfur cycling in subduction zones

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 20(7), (2019): 3347-3374, doi:10.1029/2019GC008374.Subduction 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.All isotopic data and analysis locations are detailed in the supporting information accompanying this article. The authors would like to thank B. Monteleone and M. Yates for assistance with SIMS and EPMA analyses, respectively. J. Selverstone is thanked for providing samples and D. Whitney for providing additional field context. The authors would also like to thank J. Alt, C. LaFlamme, and an anonymous reviewer for their thoughtful and thorough reviews, as well as careful editorial handling by J. Blichert‐Toft. This project was funded by National Science Foundation Grant EAR 1725301 awarded to A. M. C. and a Geological Society of America grant to J. B. W.2019-12-1

Syros Metasomatic Tourmaline: Evidence for Very High-δ11B Fluids in Subduction Zones

High-pressure (HP) metamorphic blocks enclosed in a mafic to ultramafic matrix from a mélange on the island of Syros are rimmed by tourmaline-bearing reaction zones (blackwalls). The B isotopic composition of dravitic tourmaline within these blackwalls was investigated in situ by secondary ion mass spectrometry. Boron in these tourmalines is unusually heavy, with δ11B values exceeding +18‰ in all investigated samples and reaching an extreme value of +28·4‰ in one sample. Blackwalls formed during exhumation of the HP mélange at a depth of 20-25 km at temperatures of 400-430°C, by influx of external hydrous fluids. The compositions of the fluids are estimated to be in the range of 100-300 μg/g B with δ11B values of +18 to +28‰. The high δ11B values cannot be explained by tourmaline formation from unmodified slab-derived fluids. However, such fluids could interact with the material in the exhumation channel on their way from the dehydrating slab to the site of tourmaline formation in the blackwalls. This could produce exceptionally high δ11B values in the fluids, a case that is modelled in this study. The model demonstrates that subduction fluids may be effectively modified in both trace element and isotopic composition during their migration through the material overlying the subducting slab. Blackwall tourmaline from Syros has a large grain size (several centimetres), high abundance, and an exceptionally high δ11B value. The formation of tourmaline at the contact between mafic or felsic HP blocks and their ultramafic matrix involved fluids released during dehydration reactions in the subducting slab. It forms a heavy-boron reservoir in hybrid rocks overlying the subducting slab, and may, thus, have a significant impact on the geochemical cycle of B and its isotopes in subduction zone

Effects of fluid-rock interaction on 40Ar/39Ar geochronology in high-pressure rocks (Sesia-Lanzo Zone, Western Alps)

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 126 (2014):475-494, doi:10.1016/j.gca.2013.10.023.in situ UV laser spot 40Ar/39Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40Ar/39Ar ages of phengite cores span a range of ∼20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 Ma and 77-74 Ma, respectively. The younger cores have on average lower B contents (∼36 mg/g) than the older ones (∼43-48 mg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable d11B values (-18 to -10 ‰), indicating the lack of km scale B homogenization during HP crystallization. Overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40Ar/39Ar ages than the respective cores. They also show variable effects of heterogeneous excess 40Ar incorporation and Ar loss. One acceptable inverse isochron age of 77.1 ±1.1 Ma for rims surrounding older cores (82.6 ±0.6 Ma) overlaps with the second period of core crystallization. Compared to the phengite cores, all rims have lower B and Li abundances but similar d11B values (-15 to -9 ‰), reflecting internal redistribution of B and Li and internal fluid buffering of the B isotopic composition during rim growth. The combined observation of younger 40Ar/39Ar ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains. Strongly deformed samples from a several km long, blueschist-facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40Ar/39Ar ages (64.7 to 68.8 Ma), yielding an inverse isochron age of 65.0 ±3.0 Ma. Almost complete B and Li removal in mylonitic phengite is due to leaching into a fluid. The B isotopic composition is significantly heavier than in phengites from the weakly deformed samples, indicating an external control by a high-d11B fluid (d11B = +7 ±4 ‰). We interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone. This event also caused partial resetting of the Ar isotope system and further B loss in more permeable rocks of the adjacent unit. We conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40Ar/39Ar data in partially metasomatized rocks.Funding of this work by the Deutsche Forschungsgemeinschaft (grant KO-3750/2-1) is gratefully acknowledged

Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Codillo, E., Klein, F., Dragovic, B., Marschall, H., Baxter, E., Scambelluri, M., & Schwarzenbach‬, E. Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones. Geochemistry Geophysics Geosystems, 23, (2022): e2021GC010206, https://doi.org/10.1029/2021gc010206.Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass-transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high-pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk-rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg-metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg-rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si-metasomatism of serpentinite in subduction zones is limited by pressure-dependent increase in the silica activity buffered by the serpentine-talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P-T conditions. Alternatively, a different set of mineral assemblages, such as serpentine- or chlorite-rich rocks, may be controlling the coupling-decoupling transition of the plate interface.M. Scambelluri acknowledges the Italian Ministry of Research MUR for granting the PRIN project n. 2017ZE49E7. This research was funded by NSF-OISE (Office of International Science & Engineering, Petrology & Geochemistry) PIRE, Award #1545903, and the WHOI Ocean Ventures Fund

Tectonic settings of continental crust formation:Insights from Pb isotopes in feldspar inclusions in zircon

International audienceMost crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238U/204Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206Pb/204Pb = 16.88 ± 0.08, 1σ) that indicate it was derived from source rocks with low U/Pb ratios (∼0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206Pb/204Pb = 19.11 ± 0.23, 1σ) indicative of a source with higher U/Pb ratios (∼0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time