Coarse-grained xenoliths of hibonite + grossite + Mg-Al-V spinel from Cretaceous pyroclastic rocks on Mt Carmel, N. Israel, and from Sierra de Comechingones, Argentina, include spherules, rods and dense branching structures of native vanadium and V-Al alloys. The microstructures suggest that vanadium melts became immiscible with the host Ca-Al-Mg-Si-O melt, which was already depleted in Fe and Si by the immiscible separation of Fe-Si-C silicide melts, and nucleated as droplets on the surfaces of the oxide phases, principally hibonite. Many extended outward into rods or branching structures as the host oxide crystal grew. The stability of V0 implies oxygen fugacities ≥9 log units below the Iron-Wustite buffer, suggesting a hydrogen-dominated atmosphere. This is supported by wt%-levels of hydrogen released by crushing hibonite grains, by Raman spectroscopy on the hibonite, and by the presence of VH2, the first natural hydride, among the vanadium balls. The oxide assemblage formed at 1400-1200 °C; the solution of hydrogen in the metal could lower the melting point of V0 to these temperatures. These assemblages probably resulted from reaction between differentiated mafic melts and mantle-derived CH4+H2 fluids near the crust-mantle boundary, and their fO2 record the most reducing magmatic conditions yet documented on Earth.
|Number of pages||20|
|Journal||Earth and Planetary Science Letters|
|Publication status||Submitted - Sep 2019|