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1. Experimental calibration of an Fe3+/Fe2+-in-amphibole oxybarometer and its application to shallow magmatic processes at Shiveluch Volcano, Kamchatka

   https://doi.org/10.2138/am-2022-8031  

   Goltz, Andrea E. ; Krawczynski, Michael J. ; McCanta, Molly C. ; Dyar, M. Darby ( November 2022 , American Mineralogist)

   Abstract Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes. 

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2. Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

   https://doi.org/10.1029/2024GC011542  

   Beaudry, Patrick ; Sverjensky, Dimitri A ( July 2024 , Geochemistry, Geophysics, Geosystems)

   Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H₂O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logfO₂from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atfO₂conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ³⁴S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke³⁴S‐rich subducted lithologies.

    

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3. Mobilization of S during subduction via metamorphic redox reactions

   Cruz-Uribe, AM ; Walters, JB ; Marschall, HR ( August 2018 , Goldschmidt Abstracts)

   The large range in oxidation states of sulfur (-II to +VI) provides it with a large oxidation potential in rocks, even at relatively low concentrations. Most importantly, the transition from sulfide to sulfate species in rocks and silicate melts occurs in the same approximate fO2 region (for a given temperature) as the transition from ferrous to ferric iron, and reduced S species can coexist with oxidized Fe and vice versa. The result is a large potential for reactions involving sulfur to oxidize or reduce Fe in silicate minerals, since Fe only occurs in two oxidation states (+II and +III). In order for sulfur to be released during slab dehydration, sulfur in sulfide must be converted into an easily dissolved species, such as SO42− or H2S, through either oxidation or reduction. We propose that oxidation of sulfur in sulfide follows the generalized reaction: 8Fe3+SiaOb(OH)c +S2− = 8Fe2+SidOe +SO42− +(H2O)f (1) In this type of reaction, sulfur participates in the dehydration of greenschist- or blueschist-facies hydrous silicates during transition to the eclogite facies: ferric Fe in Fe-bearing silicates (chlorite, amphibole, epidote) is reduced to ferrous Fe in anhydrous ferromagnesian silicates (pyroxene, garnet). At the same time, the reaction consumes sulfide by oxidation of S2− to produce SO42−, which is readily dissolved in the fluid produced during dehydration. Additionally, a similar redox reaction could oxidize sulfur by reducing ferric Fe in oxides. It is important to note that one mole of S has the same redox potential as 8 moles of Fe. The molar ratio of 8 moles of Fe per 1 mole of S translates to a mass ratio of approximately 14; therefore, small concentrations of sulfur can have a large impact on reduction/oxidation of the silicate assemblage. Our observations show that sulfide minerals that can be identified as primary or related to the peak metamorphic stage are rare in eclogites and restricted to inclusions in garnet, consistent with reaction (1). Thermodynamic modeling is currently underway to assess the influence of sulfur on the phase equilibria of silicate phases during high pressure metamorphism. 

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4. Shock experiments on basalt—Ferric sulfate mixes and their possible relevance to the sulfide bleb clusters in large impact melts in shergottites

   https://doi.org/10.1111/maps.13770  

   Rao, M. N. ; Nyquist, L. E. ; Asimow, P. D. ; Ross, D. K. ; Sutton, S. R. ; See, T. H. ; Shih, C. Y. ; Garrison, D. H. ; Wentworth, S. J. ; Park, J. ( December 2021 , Meteoritics & Planetary Science)

   Large impact‐melt pockets in shergottites contain both Martian regolith components and sulfide/sulfite bleb clusters that yield high sulfur concentrations locally compared to bulk shergottites. The regolith may be the source of excess sulfur in the shergottite melt pockets. To explore whether shock and release of secondary Fe‐sulfates trapped in host rock voids is a plausible mechanism to generate the shergottite sulfur bleb clusters, we carried out shock recovery experiments on an analog mixture of ferric sulfate and Columbia River basalt at peak pressures of 21 and 31 GPa. The recovered products from the 31 GPa experiment show mixtures of Fe‐sulfide and Fe‐sulfite blebs similar to the sulfur‐rich bleb clusters found in shergottite impact melts. The 21 GPa experiment did not yield such blebs. The collapse of porosity and local high‐strain shear heating in the 31 GPa experiment presumably created high‐temperature hotspots (~2000 °C) sufficient to reduce Fe³⁺to Fe²⁺and to decompose sulfate to sulfite, followed by concomitant reduction to sulfide during pressure release. Our results suggest that similar processes might have transpired during shock production of sulfur‐rich bleb clusters in shergottite impact melts. It is possible that very small CO presence in our experiments could have catalyzed the reduction process. We plan to repeat the experiments without CO.

    

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5. Post-melting oxidation of highly primitive basalts from the southern Andes

   https://doi.org/10.1016/j.gca.2020.01.042  

   Tassara, S. ( January 2020 , Geochimica et cosmochimica acta)

   null (Ed.)

   The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on the fO2 of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution of fO2 during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg#
   92.5, and Ni
   5000 mgg1). These features offer a unique opportunity to explore the fO2 conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and m-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined the fO2 based on the olivine-spinel equilibrium, yielding average values DFMQ + 1.3 ± 0.4 (1r). Second, we constrained the fO2 conditions of melt inclusions using Fe m-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe m-XANES data indicate that the melt inclusions were trapped in average at DFMQ +2.5 ± 0.5 (1r). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe m-XANES data, yielding average DFMQ values of +2.6 ± 0.3 (1r). In order to test the potential effects of other postentrapment modifications of the melt inclusions that could have affected the fO2 prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailing fO2 conditions during their entrapment. Correlations between melt inclusions major element composition and their fO2 determined by Fe m-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect to fO2 during their early geochemical evolution. Our data indicate a complex fO2 early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive melts fO2 values to that of their mantle source. 

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