Extreme reduction: Mantle-derived oxide xenoliths from a hydrogen-rich environment

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 VAl 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. •Hibonite-grossite-vanadium xenoliths in basalt record Earth's most reducing conditions.•Vanadium (V0) melts were immiscible with Ca-Al-Mg-Si oxide melts at T > 1200 °C.•Melting T of V0 was lowered by solution of H2; VH2 phase coexists with V0.•V0 “dendrites” contain ca 3% void spaces, developed by expulsion of H2 on cooling.•Raman spectra suggest that some H2 diffused into host hibonite.