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1. Chemical and mechanical controls on metasomatic assimilation of eclogite blocks in high-pressure host gneiss

   Menold, C.A.; Grove, M.; Macris, C.A. (December 2019, AGU Fall Meeting 2019)

   Mica- and garnet-rich selvages are often developed around eclogitized mafic blocks within felsic gneiss in HP to UHP metamorphic terranes. The development of these metasomatic features ranges from readily identified reaction zones between the eclogite and host gneiss to shear zones where the spatial relationships between eclogite blocks and host gneiss are completely obscured. Block-selvage relationships within the Luliang Shan HP/UHP belt (North Qaidam, China) and the Tso Morari UHP terrane (NW Himalaya, India) approximate end members of the selvage preservation process. Here we apply whole-rock and incompatible trace element compositions coupled with B and O isotopic data in white mica to constrain the relationship of metasomatism vs. deformation during selvage formation. Within the Luliang Shan, extensive fluid flow formed thick, compositionally hybridized phengite- and garnet-bearing selvages between eclogite (SiO2 ~ 50%) and quartzofeldspathic gneiss (SiO2 ~ 80%). The Luliang Shan HP selvages have intermediate SiO2 and range from 5-10 m in thickness as "halos" around spheroidal eclogite blocks. Volatile enrichment at near-UHP conditions in the selvage is indicated by enrichment of Li, Cs, Ba, Ar, and δ18O and very light δ11B values in phengite. The retrograde muscovite from the host gneiss is low in Li, Cs, Rb, and Sr but possess remarkably high B concentrations (up to 3000 ppm) and positive δ11B values that are best explained by interaction with fluids devolatilized from accreted sediments within cooler regions of the subduction zone. Alternatively, the Tso Morari UHP terrane features boudinaged discoids of eclogite encased within highly strained quartzofeldspathic gneiss. Whole rock major element sampling performed normal to the foliation reveal consistently high SiO2 (78-80%). Highly variable degrees of metasomatic recrystallization occur within the phengite-rich rocks spatially associated with eclogite. The selvage rocks exhibit heterogeneous degrees of enrichment in Li, Be, B, and Ba and yield δ11B values of -4 to -6‰ typical of undevolatilized oceanic and continental crust. We conclude that fluid-mediated metasomatic reaction between eclogite and gneiss at Tso Morari is sheared out into lenses that are incorporated into, and heterogeneously distributed throughout, the host gneiss. 

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2. Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

   https://doi.org/10.1029/2023GC011389  

   Kang, Patricia; Martin, Laure_A_J; Vitale_Brovarone, Alberto; Whitney, Donna_L (April 2024, Geochemistry, Geophysics, Geosystems)

   Abstract During the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high‐pressure/low‐temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ‐measured δ¹⁸O of lawsonite and garnet in blueschist‐ to eclogite‐facies rocks from 10 subduction zones that represent various field settings, including mélanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct δ¹⁸O depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 ± 0.2–14.8 ± 1.1‰) is isotopically lighter than in metasediment (20.6 ± 1.4–24.1 ± 1.3‰) but heavier than in metagabbro (4.0 ± 0.4–7.9 ± 0.3‰). The extent of δ¹⁸O fractionation was evaluated for lawsonite–fluid and lawsonite–garnet pairs as a function of temperature (T). Results demonstrate that variations of >1.7‰ in lawsonite and >0.9‰ in garnet are not related to changingT. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite‐bearing metasomatite. Results indicate that δ¹⁸O_Lwsand δ¹⁸O_Grtrecord interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that δ¹⁸O_Lwsis a useful indicator of subduction fluid‐rock interactions. 

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3. Amphibolization of the Tso Morari UHP eclogites: a record of fluid infiltration at amphibolite-facies during uplift in the subduction channel

   https://doi.org/10.1002/essoar.10509538.1  

   Pan, R.; Macris, C. A.; Menold, C. A. (January 2021, Transactions American Geophysical Union)

   Ultra-high pressure (UHP) metamorphism of the Tso Morari coesite-eclogite during burial in NW Himalaya has been intensively studied over the past several decades. However, amphibolite-facies metamorphism and accompanying metasomatism occurring at lower-crustal depths in the Tso Morari terrane are less well-constrained. In this study, we characterize the eclogite amphibolization and related metasomatic fluids by systematically sampling and analyzing the eclogites at the core of an eclogite boudin and the amphiblolized eclogite (amphibolite) at the rim. Integrated techniques including modal mineralogy, mineral chemistry, whole-rock geochemistry, Mössbauer spectroscopy, and thermodynamic modelling are used to constrain the fluid-induced eclogite amphibolization and associated fluid behaviors. Petrographic observations show that infiltration of an external fluid caused complete amphibolite-facies overprinting of the eclogites at the boudin rim. This is recorded petrographically as increased modal proportions of amphibole, biotite, epidote, plagioclase, and calcite in the amphibolites. The infiltrating fluid caused increased K2O and CO2 concentrations and higher bulk-rock Fe3+/ΣFe ratio for the amphibolites, as well as increased LILE (e.g., K, Rb, Cs, Sr, Ba) and ratios of Ba/Rb and Cs/Rb. Phase equilibria modelling using P–T–M(H2O) pseudosections on the amphibolite and the surrounding gneiss indicate that the fluid infiltration occurred at 9.0–12.5 kbar and ~608 °C with >2.6–3.1 mol % H2O infiltration. The abrupt increase of bulk-rock Fe3+/ΣFe ratio from 0.192 to 0.395 near the boudin rim indicate that this phase of fluid most likely derived from the mixing of dehydrated host orthogneiss and/or metasediments during uplift at the amphibolite-facies zone in the subduction channel. This study also demonstrates the need for using careful petrographic observations and geochemical analysis in parallel with thermodynamic modelling to achieve realistic results. 

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4. Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

   https://doi.org/10.1029/2019GC008374  

   Walters, Jesse B.; Cruz‐Uribe, Alicia M.; Marschall, Horst R. (July 2019, Geochemistry, Geophysics, Geosystems)

   Abstract 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. 

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5. What controls the mobility of rare earth elements (REE) in critical mineral deposits in acidic vs. alkaline hydrothermal fluids?

   https://doi.org/10.7185/gold2023.19801  

   Gysi, Alexander; Hurtig, Nicole; Waters, Laura; Harlov, Daniel; Miron, George Dan; Migdissov, Artaches; Zhu, Chen; Kulik, Dmitrii (July 2023, European Association of Geochemistry)

   (Per)alkaline complexes and carbonatites evolve through a complex sequence of magmatic-hydrothermal processes. Most of them are overprinted by late auto-metasomatic processes which involves the mobilization, fractionation and/or enrichment of critical elements, such as the rare earth elements (REE) [1]. However, our current ability to predict the behavior of REE in high temperature aqueous fluids and interpret these natural systems using geochemical modeling depends on the availability of thermodynamic data for the REE minerals and aqueous species. Previous experimental work on REE solubility has focused on acidic aqueous fluids up to ~300 °C and considered chloride, fluoride and sulfate as important ligands for their transport [2]. However, magmatic-hydrothermal systems that form these critical mineral deposits may cover a wider range of fluid chemistries spanning acidic to alkaline pH as well as temperatures and pressures at which the fluids are supercritical. A few recently published studies have shown that other ligands (e.g., REE carbonates and/or combined fluoride species) could become important in near-neutral to alkaline fluids [3,4], and that REE mobility can also be increased in saline alkaline fluids reacted with fluorite [5]. Here we present new hydrothermal REE hydroxyl/chloride speciation data and REE phosphate/hydroxide minerals [6,7], calcite and fluorite solubility experiments as a function of pH, salinity and temperature. We use an integrated approach to link a wide array of experimental techniques (solubility, calorimetry, and spectroscopy) with thermodynamic optimizations using GEMSFITS [8], and present the development of a new experimental database for REE and its integration into the MINES thermodynamic database (https://geoinfo.nmt.edu/mines-tdb). The latter permits simulating hydrothermal fluid-rock interaction and ore-forming processes in critical mineral deposits to better understand the behavior of REE during metasomatism. 

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