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

Abstract

Here we report the first natural occurrence of Fe 2 SiO 4 -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. Fe 2 SiO 4 - 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 Fe 2 SiO 4 -spinel (8.52 Å), calculated from d-spacing data, is 3.5% larger than that of synthetic Fe 2 SiO 4 -spinel (8.235 Å). Chemical analyses of the spinel show olivine stoichiometry with Fe/(Fe + Mg) ratios ranging from 0.62 to 0.99. Fe 2 SiO 4 -spinel and stishovite occur within FeO-SiO 2 -rich zones in the melt pocket, surrounded by SiO 2 -rich glass and Fe-rich phyllosilicates. Fe 2 SiO 4 -spinel plus stishovite also occur with other high-pressure minerals in the melt pocket: ringwoodite, akimotoite, augite, and hollanditestructured plagioclase. We infer that the Fe 2 SiO 4 -spinel crystallized from a zone of FeO-SiO 2 -rich melt within the shock-induced melt pocket. Two models for FeO-SiO 2 -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-SiO 2 -rich material.