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Natural occurrence of Fe₂SiO₄-spinel in the shocked Umbarger L6 chondrite

¹ Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404, U.S.A.

Correspondence: ^* E-mails: zhidong.xie{at}asu.edu, nao{at}kobe-u.ac.jp,tom.sharp{at}asu.edu

Here we report the first natural occurrence of Fe₂SiO₄-spinel in a shock-induced melt pocket of the Umbarger L6 chondrite. Optical microscopy, scanning electron microscopy, electron microprobe analysis, and analytical transmission electron microscopy were used to examine the sample. Fe₂SiO₄-spinel was identified by TEM using selected-area electron diffraction and energy-dispersive X-ray spectroscopy. The symmetry of the diffraction patterns, the ratios of d-spacings, and interplanar angles are consistent with the spinel structure. However, the cell parameter of Fe₂SiO₄-spinel (8.52 Å), calculated from d-spacing data, is 3.5% larger than that of synthetic Fe₂SiO₄-spinel (8.235 Å). Chemical analyses of the spinel show olivine stoichiometry with Fe/(Fe + Mg) ratios ranging from 0.62 to 0.99. Fe₂SiO₄-spinel and stishovite occur within FeO-SiO₂-rich zones in the melt pocket, surrounded by SiO₂-rich glass and Fe-rich phyllosilicates. Fe₂SiO₄-spinel plus stishovite also occur with other high-pressure minerals in the melt pocket: ringwoodite, akimotoite, augite, and hollandite-structured plagioclase. We infer that the Fe₂SiO₄-spinel crystallized from a zone of FeO-SiO₂-rich melt within the shock-induced melt pocket. Two models for FeO-SiO₂-rich melt are discussed: it was either a residual melt after crystallization of MgO-rich silicates in a chondritic melt pocket, or it was produced by shock melting of FeO-SiO₂-rich material.