The University of Arizona

Olivine zoning and retrograde olivine-orthopyroxene-metal equilibration in H5 and H6 chondrites

R. J. REISENER, J. I. GOLDSTEIN, M. I. PETAEV

Abstract


Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned taenite + kamacite particles (formed after relatively small amounts of taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after ~200 °C of taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of taenite undercooling at temperatures <700 °C. The relevant silicate-metal reactions are modeled using chemical thermodynamics. Systematic olivine Fe-Mg zoning adjacent to metal is an expected consequence of retrograde silicate-metal reactions, and the presence of such zoning provides strong evidence that the silicate and metallic minerals evolved in situ during cooling from the peak metamorphic temperature.

Keywords


Taenite;Metal;Pyroxene;Olivine

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