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

A Possible Detection of Occultation by a Proto-planetary Clump in GM Cephei

GM Cep in the young (~ 4 Myr) open cluster Trumpler 37 has been known to be an abrupt variable and to have a circumstellar disk with very active accretion. Our monitoring observations in 2009–2011 revealed the star to show sporadic ?are events, each with brightening of . 0.5 mag lasting for days. These brightening events, associated with a color change toward the blue, should originate from an increased accretion activity. Moreover, the star also underwent a brightness drop of ~ 1 mag lasting for about a month, during which the star became bluer when fainter. Such brightness drops seem to have a recurrence time scale of a year, as evidenced in our data and the photometric behavior of GM Cep over a century. Between consecutive drops, the star brightened gradually by about 1 mag and became blue at peak luminosity. We propose that the drop is caused by obscuration of the central star by an orbiting dust concentration. The UX Orionis type of activity in GM Cep therefore exemplifies the disk inhomogeneity process in transition between grain coagulation and planetesimal formation in a young circumstellar disk

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)

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-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

Extreme magnesium isotope fractionation at outcrop scale records the mechanism and rate at which reaction fronts advance

Isotopic fractionation of cationic species during diffusive transport provides novel means of constraining the style and timing of metamorphic transformations. Here we document a major (~1‰) decrease in the Mg isotopic composition of the reaction front of an exhumed contact between rocks of subducted crust and serpentinite, in the Syros mélange zone. This isotopic perturbation extends over a notable length-scale (~1 m), implicating diffusion of Mg through an intergranular fluid network over a period of ~100 kyr. These novel observations confirm models of diffusion-controlled growth of reaction zones formed between rocks of contrasting compositions, such as found at the slab-mantle interface in subduction zones. The results also demonstrate that diffusive processes can result in exotic stable isotope compositions of major elements with implications for mantle xenoliths and complex intrusions

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