Extreme reduction

mantle-derived oxide xenoliths from a hydrogen-rich environment

W.L. Griffin, S. E. M. Gain, F. Cámara, L. Bindi, J. Shaw, O. Alard, M. Saunders, J.-X. Huang, V. Toledo, S. Y. O'Reilly

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

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.

Original languageEnglish
Article number105404
Pages (from-to)1-8
Number of pages8
JournalLithos
Volume358-359
DOIs
Publication statusPublished - Apr 2020

Keywords

  • Native vanadium
  • Super-reducing conditions
  • Mantle xenoliths
  • Immiscible melts
  • Mantle-derived methane
  • Mantle-derived hydrogen

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