Ferric iron in Al-bearing akimotoite coexisting with iron-nickel metal in a shock-melt vein in an L-6 chondrite

Nobuyoshi Miyajima 1 , Ahmed El Goresy 1 , Catherine Dupas-Bruzek 2 , Friedrich Seifert 1 , David C. Rubie 1 , Ming Chen 3  and Xiande Xie 3
  • 1 Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
  • 2 Centre d'Etudes et de Recherches Lasers et Applications, Universite des Sciences et Technologies de Lille, F-59655 Villeneuve d'Ascq Cedex, France
  • 3 Guangzhou Institute of Geochemistry, CAS, Wushan, Guangzhou 510640, China

Abstract

We report evidence for high ferric iron to total iron (Fe3+/ΣFe) ratios in Al-bearing akimotoite coexisting with other high-pressure silicates and Fe-Ni metal from shock melt-veins in the Sixiangkou (L-6) chondrite. The measurements were made using electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that akimotoite in shock-melt veins of this meteorite has high proportions of Fe3+, with a Fe3+/ΣFe ratio of 0.67(3). In contrast, the coexisting majoritic garnet and ringwoodite, which are the typical Fe-bearing phases in shock veins in this meteorite, are enriched in Fe2+ rather than Fe3+, with Fe3+/ΣFe ratios of 0.10(5) and 0.15(5), respectively. We conclude that the higher affinity of Fe3+ for akimotoite, rather than for the other dense silicate phases, is related strongly to the substitution mechanism of trivalent cations. This mechanism is described as VI(A)Fe3+ + VI(B)Al3+ = VI(A)Mg2+ + VI(B)Si4+ in the ABO3 structural formula of MgSiO3-ilmenite under high pressures and temperatures, and operates even at a low oxygen fugacity where Fe-Ni metal is stable.

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