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1. CSEs in high pressure rocks from exhumed terranes

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

   Despite their geochemical and economic importance, very little work has focused on the behavior of subducted chalcophile and siderophile elements (CSE). Here we present the first survey of these elements in metasediments, metabasites, and hybrid mélange rocks from exhumed terranes worldwide. These samples represent greenschist- to eclogite-facies conditions. EPMA X-ray maps display significant Co, Ni, and As zoning in pyrite; however, no zoning was observed in pyrrhotite or chalcopyrite. In situ LA- ICP-MS analyses of sulfides reveal Co, Ni, Cu, Zn, As, Pb, and Cr at concentrations above 10 μg/g, whereas Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, Tl, and Bi are typically below 1 μg/g. Pyrite is enriched in Co, As, Zn, and Cr relative to pyrrhotite and chalcopyrite, whereas pyrrhotite contains abundant Ni. Pyrite is also enriched in Cu relative to pyrrhotite. Amphiboles and phyllosilicates were found to contain up to hunderds of μg/g of Ni, Cr, and Zn, and tens of μg/g of Co and Ga. In eclogites, Co in silicates mainly occurs in garnet, whereas Ni, Ga, and Zn occur in pyroxene. Both phases contain similar concentrations of Cr and Ge. Most silicates were found to have less than 1 μg/g of Cu; Cu in garnet was below detection, and As was below detection in all silicates. Contrasting behavior of Co and Ni is displayed in hybrid mélange samples. Transects of pyrite in chlorite schists show no correlation between these two elements, consistent with the hightened fluid mobility of Co over Ni observed in hydrothermal ore deposits. In one glacophane-omphacite rock, Co and Ni are anti-correlated. This behavior may be explained by alternation between fluid-buffered conditions, in which cobalt is mobile, and rock-buffered conditions, in which reactions with silicates release Ni. Matrix sulfides are absent in most eclogite-facies samples. Sulfide breakdown during subduction likely drives the release of As, Pb, and Cu into fluids that flux the overlying mantle, whereas both silicates and sulfides may contribute Co to these fluids. The elements Cr, Zn, Ga, and Ge likely persist into the eclogite facies, but may also be released during silicate dehydration. 

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2. A corundum reference material for oxygen isotope analysis by secondary ionization mass spectrometry

   https://doi.org/10.1039/D3JA00229B  

   Schmidt, Sebastian; Hertwig, Andreas; Schmitt, Axel Karl; Cionoiu, Katharina; McKeegan, Kevin D.; Bindeman, Ilya; Di Rocco, Tommaso; Pack, Andreas (February 2024, Journal of Analytical Atomic Spectrometry)

   A synthetic laser ruby crystal (HD-LR1) is introduced as a new matrix-matched reference material for secondary ionization mass spectrometry (SIMS) analysis of oxygen isotopes in corundum. Laser fluorination isotope ratio mass spectrometry (LF-IRMS) bulk analyses of multiple mg-sized fragments are homogenous, averaging δ18O = +18.40 ± 0.14‰ (95% confidence interval, n = 23) and Δ′17O = −0.368 ± 0.005‰ (as deviation from slope 0.528 for δ′17O vs. δ′18O at 95% conf., n = 11) relative to Vienna Standard Mean Ocean Water (V-SMOW). SIMS spot analyses show homogeneous O-isotopic values at the ng-scale independent of the location in the HD-LR1 single crystal and in four different crystallographic orientations. However, sample surface topography as an artefact of polishing corundum embedded in epoxy creates excess variability in δ18O within ∼100 μm from the edges of the grains. HD-LR1 is a chemical pure crystal with only Cr as a trace component detected at 276 μg g−1 by EPMA, whereas Be, often introduced in artificial gem enhancement, is <0.002 μg g−1 based on SIMS analyses. Therefore, HD-LR1 can also be used as a reference material for Cr, or as a blank for other trace element analyses of corundum by SIMS or LA-ICP-MS. 

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3. Crustal fluid contamination in the Bushveld Complex, South Africa: An analogue for subduction zone fluid migration

   https://doi.org/10.1080/00206814.2020.1795734  

   Benson, Erin; Connolly, James A.; Boudreau, Alan E. (August 2020, International Geology Review)

   null (Ed.)

   Crystallization of the 2.06 Ga Bushveld magma formed a 9 km (maximum) sequence of ultramafic and mafic rocks that generated a large volume of country fluid as it thermally metamorphosed a 3+ km section of previously unaltered underlying sedimentary rocks of the Transvaal sequence – a geometry similar to that seen as subducting lithospheric slabs are heated by overlying mantle rocks. The presence of a diatreme (breccia pipe) and other large, pipe-like features in the Bushveld Complex located proximal to diapiric upwelling of the basement rocks suggest that overpressured fluids generated during dehydration of the footwall sediments are focused by the diapiric structures such that the country fluids rapidly penetrate the Bushveld rock. A re-examination of existing stable and radiogenic isotopic evidence is consistent with contamination of Main Zone magmas by 1–2% country fluid. Numeric modelling of the footwall dehydration similarly shows that most of the country fluids will be confined to pipe-like channels as it percolates into the Bushveld sill. Modelling also suggests that the maximum extent of the metamorphic aureole was reached at about the same time that the Main Zone began to crystallize. It is proposed that rapid inflation of the Bushveld sill induced the sudden and catastrophic expulsion of overpressured country fluids to both generate the diatreme and contaminate the Main Zone magma, resulting in the Main Zone enrichment in crustal stable and radiogenic isotopic signatures (Sr, Nd, O and others). By analogy, it is also suggested that hydration melting in the mantle wedge is episodically driven by similar sudden influxes of slab fluids that are able to retain their geochemical and isotopic character by rapid channelled influx. This can be aided by flow focusing at diapirs structures at the upper slab-mantle contact. 

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4. Evidence and timing for multiple cooling mechanisms in a long-lived subduction zone from the Easton metamorphic suite in the northwest Cascades

   Mulcahy, S.R.; Schermer, E.L.; Cordova, J.; Gates, E. (January 2023, Geological Society of America Abstracts with Programs)

   Numerical models of subduction initiation and observations of exhumed subduction complexes indicate that the early stages of subduction are characterized by rapid cooling followed by a prolonged steady thermal state that can last tens of millions of years. Several mechanisms are proposed to drive cooling in subduction zones and include thermal relaxation, exhumation, and underplating, but determining the relative contribution of each mechanism in the history of an exhumed subduction complex can be difficult. The Easton metamorphic suite in the Northwest Cascades of Washington is a Jurassic-Cretaceous subduction complex that records the subduction and accretion of distinct units within a thermally maturing nascent subduction zone. The region preserves an inverted metamorphic sequence with metamorphic temperatures and ages that decrease structurally downward from an early accreted metamorphic sole to younger regionally extensive blueschist facies units. In the metamorphic sole Grt±Cpx amphibolite was metamorphosed at 750-800 C at 1.0 GPa prior to 167 Ma. The amphibolite is underlain by a high temperature Grt-Ab-Gln blueschist that was metamorphosed at ~530 C and 1.0 GPa at 165 Ma. The contact between the units is gradational and the general lack of deformation suggests initial cooling to lower temperatures may have been caused by cooling of the overall subduction zone. Retrograde Lws-Ep-Gln-Ms assemblages suggest that cooling of both the amphibolite and high-grade blueschist units to below 400-500 C was caused by exhumation to 0.7 GPa by 157 Ma. In the regionally extensive blueschist units, Ep-Ab-Chl-Ms±Grt±Lws phyllite was metamorphosed at 430-450 C and 0.7 GPa by 149 Ma. Retrograde fabrics in the phyllite record similar temperatures to peak metamorphic conditions in an underlying Ep-Ab-Gln/Act greenschist/blueschist unit that was accreted and metamorphosed at ≤350 C by ≤140 Ma. The contact between the phyllite and greenschist is marked by a high strain mylonite zone and the combined observations suggest that cooling of the phyllite was driven by underplating of the younger greenschist unit. The observed assemblages and fabrics within the Easton metamorphic suite record cooling as the result of thermal relaxation, underplating, and exhumation at distinctly different times in the subduction history. 

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5. Evaluating the Potential of Rhyolitic Glass as a Lithium Source for Brine Deposits

   https://doi.org/10.5382/econgeo.4866  

   Ellis, B. S.; Szymanowski, D.; Harris, C.; Tollan, P.M.E.; Neukampf, J.; Guillong, M.; Cortes-Calderon, E. A.; Bachmann, O. (January 2022, Economic Geology)

   Abstract Lithium is an economically important element that is increasingly extracted from brines accumulated in continental basins. While a number of studies have identified silicic magmatic rocks as the ultimate source of dissolved brine lithium, the processes by which Li is mobilized remain poorly constrained. Here we focus on the potential of low-temperature, post-eruptive processes to remove Li from volcanic glass and generate Li-rich fluids. The rhyolitic glasses in this study (from the Yellowstone-Snake River Plain volcanic province in western North America) have interacted with meteoric water emplacement as revealed by textures and a variety of geochemical and isotopic signatures. Indices of glass hydration correlate with Li concentrations, suggesting Li is lost to the water during the water-rock interaction. We estimate the original Li content upon deposition and the magnitude of Li depletion both by direct in situ glass measurements and by applying a partition-coefficient approach to plagioclase Li contents. Across our whole sample set (19 eruptive units spanning ca. 10 m.y.), Li losses average 8.9 ppm, with a maximum loss of 37.5 ppm. This allows estimation of the dense rock equivalent of silicic volcanic lithologies required to potentially source a brine deposit. Our data indicate that surficial processes occurring post-eruption may provide sufficient Li to form economic deposits. We found no relationship between deposit age and Li loss, i.e., hydration does not appear to be an ongoing process. Rather, it occurs primarily while the deposit is cooling shortly after eruption, with δ18O and δD in our case study suggesting a temperature window of 40° to 70°C. 

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