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1. Deformation and Strength of Mantle Relevant Garnets: Implications for the Subduction of Basaltic-rich Crust

   https://doi.org/10.2138/am-2021-7587  

   Vennari, C; Lin, F; Kunz, M; Akaogi, M; Miyagi, L; Williams, Q (July 2021, American Mineralogist)

   null (Ed.)

   Garnet is an important mineral phase in the upper mantle as it is both a key component in bulk mantle rocks, and a primary phase at high-pressure within subducted basalt. Here, we focus on the strength of garnet and the texture that develops within garnet during accommodation of differential deformational strain. We use X-ray diffraction in a radial geometry to analyze texture development in situ in three garnet compositions under pressure at 300 K: a natural garnet (Prp60Alm37) to 30 GPa, and two synthetic majorite-bearing compositions (Prp59Maj41 and Prp42Maj58) to 44 GPa. All three garnets develop a modest (100) texture at elevated pressure under axial compression. Elasto-viscoplastic self-consistent (EVPSC) modeling suggests that two slip systems are active in the three garnet compositions at all pressures studied: {110}<1-21 11> and {001}<110>. We determine a flow strength of ~5 GPa at pressures between 10 to 15 GPa for all three garnets; these values are higher than previously measured yield strengths measured on natural and majoritic garnets. Strengths calculated using the experimental lattice strain differ from the strength generated from those calculated using EVPSC. Prp67Alm33, Prp59Maj41 and Prp42Maj58 are of comparable strength to each other at room temperature, which indicates that majorite substitution does not greatly affect the strength of garnets. Additionally, all three garnets are of similar strength as lower mantle phases such as bridgmanite and ferropericlase, suggesting that garnet may not be notably stronger than the surrounding lower mantle/deep upper mantle phases at the base of the upper mantle. 

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2. Phase transformation of ferric-iron-rich silicate in Earth’s mid-mantle

   https://doi.org/10.2138/am-2024-9410  

   Lv, Mingda; Zhu, Shengcai; Liu, Jiachao; Hu, Yi; Zhu, Feng; Lai, Xiaojing; Zhang, Dongzhou; Chen, Bin; Dera, Przemyslaw; Li, Jie; et al (July 2024, American Mineralogist)

   Abstract Incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. In MgSiO3-Fe2O3 mixtures, recent experiments indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction measurements show that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 GPa and 24.7 GPa at 300 K. Diffraction observations are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. These observations combined with multi-anvil experiments suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. Dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction. 

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3. Accessory phases as recorders of subduction redox: Sulfide–oxide– silica equilibria during high-pressure metamorphism

   Walters, JB; Cruz-Uribe, AM; Marschall, HR (December 2018, 2018 Fall Meeting, AGU)

   Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

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4. 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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5. Trace element redistribution during rehydration of eclogite via sulfide-silicate reactions

   Cruz-Uribe, AM; Walters, JB; Marschall, HR (December 2018, 2018 Fall Meeting, AGU)

   Sulfide breakdown during subduction releases oxidizing fluids that transport chalcophile and siderophile elements (CSE) such as Ni ,Co, and As. These fluids are reincorporated into high-pressure rocks such as eclogites during exhumation and rehydration along the slab-mantle interface. Evidence for these rehydration reactions takes the form of large sulfide (pyrite, pyrrhotite, chalcopyrite) grains (up to 5 mm) associated with hydrous Fe3+-bearing minerals. Here we present results of trace element determination by LA-ICP-MS coupled with mass balance calculations for sulfide-silicate reactions in rehydrated eclogites from the Mariánské Lázně Complex and Moldanubian Zone, Bohemian Massif, Czech Republic. One key texture observed in these rocks is the breakdown of garnet + omphacite in the presence of fluid to produce hornblende + diopside + plagioclase + pyrite. This rehydration reaction involves the oxidation of Fe2+ in garnet to Fe3+ in hornblende. In order to oxidize the iron from the garnet, we propose that sulfate is brought into the rock by an infiltrating fluid, where it is reduced to form pyrite, consistent with the observed textures. Trace element analyses reveal the Co distribution within rehydrated eclogite: Co is measurable in garnet (~50 μg/g), omphacite (~26 μg/g), hornblende (~80 μg/g), and pyrite (~5000 μg/g). Mass balance calculations suggest that of the total amount of Co present in the rehydration products, only ~35 % can be supplied by the breakdown of garnet and omphacite, leaving ~65 % of the Co to be supplied by another source. Average concentrations of Ni are: in garnet (1–4 μg/g), omphacite (~57 μg/g), hornblende (~90 μg/g), and pyrite (~2500 μg/g). Mass balance calculations suggest that of the total amount of Ni present in the rehydration products, ~70 % comes from the breakdown of garnet + omphacite, with the other 30 % supplied external to this reaction. Arsenic is not present in the silicate minerals, but is in the 10s of μg/g range in pyrite, and must be supplied externally to the rock, likely from a fluid. We conclude that the fluids released from subducting slabs carry sulfate and CSEs, which infiltrate the slab-mantle interface and eventually make their way into the sub-arc mantle, where they can be incorporated into the arc magmatic system. 

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