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

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

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)

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

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

Prospective Associations between Religiousness/Spirituality and Depression and Mediating Effects of Forgiveness in a Nationally Representative Sample of United States Adults

The present investigation examines the prospective associations of religiousness/spirituality with depression and the extent to which various dimensions of forgiveness act as mediating mechanisms of these associations. Data are from a nationally representative sample of United States adults who were first interviewed in 1998 and reinterviewed six months later. Measures of religiousness/spirituality, forgiveness, and various sociodemographics were collected. Depression was assessed using the Composite International Diagnostic Interview administered by trained interviewers. Results showed that religiousness/spirituality, forgiveness of oneself and others, and feeling forgiven by God were associated, both cross-sectionally and longitudinally, with depressive status. After controlling for initial depressive status, only forgiveness of oneself and others remained statistically significant predictors of depression. Path analyses revealed that religiousness/spirituality conveyed protective effects, prospectively, on depression by way of an indirect path through forgiveness of others but not forgiveness of oneself. Hence, forgiveness of others acts as a mechanism of the salutary effect of religiousness/spirituality, but forgiveness of oneself is an independent predictor. Conclusions regarding the continued development of this type of research and for the treatment of clients with depression are offered

Oxidative stress, NADPH oxidases, and arteries

Atherosclerosis and its major complications –myocardial infarction and stroke– remain major causes of death and disability in the United States and world-wide. Indeed, with dramatic increases in obesity and diabetes mellitus, the prevalence and public health impact of cardiovascular diseases (CVD) will likely remain high. Major advances have been made in development of new therapies to reduce the incidence of atherosclerosis and CVD, in particular for treatment of hypercholesterolemia and hypertension. Oxidative stress is the common mechanistic link for many CVD risk factors. However, only recently have the tools existed to study the interface between oxidative stress and CVD in animal models. The most important source of reactive oxygen species (and hence oxidative stress) in vascular cells are the multiple forms of enzymes nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase). Recently published and emerging studies now clearly establish that: 1) NADPH oxidases are of critical importance in atherosclerosis and hypertension in animal models; 2) given the tissue-specific expression of key components of NADPH oxidase, it may be possible to target vascular oxidative stress for prevention of CVD

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