Partitioning and isotopic fractionation of lithium in mineral phases of hot, dry rhyolites: The case of the Mesa Falls Tuff, Yellowstone

Autor: Julia Neukampf, Tomáš Magna, Olivier Bachmann, Ben S. Ellis, Oscar Laurent
Přispěvatelé: Géosciences Environnement Toulouse (GET), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)
Rok vydání: 2019
Předmět:
Zdroj: Chemical Geology
Chemical Geology, Elsevier, 2019, 506, pp.175-186. ⟨10.1016/j.chemgeo.2018.12.031⟩
Chemical Geology, 506
ISSN: 0009-2541
Popis: Studying the inventory and distribution of lithium (Li) in rhyolitic magmas, and the potential Li isotope fractionation during progressive crystallisation is crucial to understand the formation of economic Li deposits. Lithium concentrations and isotopic compositions of co-existing groundmass glass, quartz, sanidine, plagioclase, clinopyroxene, fayalite, and orthopyroxene from the Mesa Falls Tuff (Yellowstone) were determined using LA-ICP-MS and MC-ICP-MS. The highest Li contents were obtained from quartz-hosted melt inclusions (avg. 244 ppm) and groundmass glass (avg. 43 ppm). Lithium concentrations are lower in mineral phases, decreasing from quartz (avg. 18 ppm), plagioclase (avg. 17 ppm), fayalite (avg. 16 ppm), clinopyroxene (avg. 11 ppm), orthopyroxene (avg. 8 ppm) to sanidine (avg. 6 ppm). Lithium isotopic compositions of bulk tuff samples and major mineral phases reveal a range in δ7Li values (per mil deviation of 7Li/6Li ratio relative to L-SVEC reference material), spanning ~10‰ between the co-existing phases and decreasing from groundmass glass (6.5–7.5‰) to feldspars (plagioclase from −2.3 to −1.4‰; sanidine from −0.6 to −0.1‰). Lithium concentration profiles across sanidine and plagioclase crystals show a decrease towards the crystal rims, which is not observed in the other mineral phases of the Mesa Falls Tuff. The variations identified in feldspar are decoupled from both the major and all other trace element compositions, precluding an origin related to mafic recharge and magma mixing. We propose that they result from Li loss from the melt to a vapour or fluid phase taking place immediately prior to, or synchronously with, eruption. This is supported by the observation that Li contents in quartz-hosted melt inclusions are a factor of six greater than those found in groundmass glass while the concentrations of immobile trace elements are identical. The low-Li rims on sanidine and plagioclase crystals observed in the high-silica Mesa Falls Tuff rhyolite are potential candidates to preserve Li isotopic disequilibrium as has been observed in less evolved systems. The variation in Li abundances within feldspar crystals hints at the potential for kinetic isotopic fractionation to play a role in the reported isotopic compositions of bulk separates. The large elemental database of Li concentrations from the MFT allows us to calculate apparent partition coefficients between groundmass glass and co-existing minerals. These data are compared to fifteen other ignimbrites from the Yellowstone-Snake River Plain province to provide a coherent picture of Li partitioning in hot and dry rhyolites with Li most compatible in quartz and olivine, and least compatible in sanidine.
Databáze: OpenAIRE
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