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  1. 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 providemore »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.« less
    Free, publicly-accessible full text available January 1, 2023
  2. Abstract Dense, glassy pyroclasts found in products of explosive eruptions are commonly employed to investigate volcanic conduit processes through measurement of their volatile inventories. This approach rests upon the tacit assumption that the obsidian clasts are juvenile, that is, genetically related to the erupting magma. Pyroclastic deposits within the Yellowstone-Snake River Plain province almost without exception contain dense, glassy clasts, previously interpreted as hyaloclastite, while other lithologies, including crystallised rhyolite, are extremely rare. We investigate the origin of these dense, glassy clasts from a coupled geochemical and textural perspective combining literature data and case studies from Cougar Point Tuff XIII, Wolverine Creek Tuff, and Mesa Falls Tuff spanning 10 My of silicic volcanism. These results indicate that the trace elemental compositions of the dense glasses mostly overlap with the vesiculated component of each deposit, while being distinct from nearby units, thus indicating that dense glasses are juvenile. Textural complexity of the dense clasts varies across our examples. Cougar Point Tuff XIII contains a remarkable diversity of clast appearances with the same glass composition including obsidian-within-obsidian clasts. Mesa Falls Tuff contains clasts with the same glass compositions but with stark variations in phenocryst content (0 to 45%). Cumulatively, our results supportmore »a model where most dense, glassy clasts reflect conduit material that passed through multiple cycles of fracturing and sintering with concurrent mixing of glass and various crystal components. This is in contrast to previous interpretations of these clasts as entrained hyaloclastite and relaxes the requirement for water-magma interaction within the eruptive centres of the Yellowstone-Snake River Plain province.« less