• Cámara, F.
• Bindi, L.
• Shaw, J.
• Saunders, M.
• Gain, Sarah
• Toledo, V.

Abstract

<p>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[sbnd]Al alloys. Microstructures suggest that vanadium melts became immiscible with the host Ca-Al-Mg-Si-O melt, and nucleated as droplets on the surfaces of the oxide phases, principally hibonite. Many extended outward as 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 in gasses released by crushing, by Raman spectroscopy, and by the presence of VH₂ 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 vanadium 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 they record the most reducing magmatic conditions yet documented on Earth.</p>

Topics

• impedance spectroscopy
• microstructure
• surface
• Oxygen
• melt
• Hydrogen
• iron
• Raman spectroscopy
• vanadium