TY - JOUR
T1 - Extreme reduction
T2 - mantle-derived oxide xenoliths from a hydrogen-rich environment
AU - Griffin, W.L.
AU - Gain, S. E. M.
AU - Cámara, F.
AU - Bindi, L.
AU - Shaw, J.
AU - Alard, O.
AU - Saunders, M.
AU - Huang, J.-X.
AU - Toledo, V.
AU - O'Reilly, S. Y.
PY - 2020/4
Y1 - 2020/4
N2 - 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 V 0 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 VH 2 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.
AB - 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 V 0 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 VH 2 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.
KW - Native vanadium
KW - Super-reducing conditions
KW - Mantle xenoliths
KW - Immiscible melts
KW - Mantle-derived methane
KW - Mantle-derived hydrogen
UR - http://www.scopus.com/inward/record.url?scp=85079168985&partnerID=8YFLogxK
U2 - 10.1016/j.lithos.2020.105404
DO - 10.1016/j.lithos.2020.105404
M3 - Article
SN - 0024-4937
VL - 358-359
SP - 1
EP - 8
JO - Lithos
JF - Lithos
M1 - 105404
ER -