MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement, cooling history, and trace element contents

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites were studied by X-ray single crystal diffraction, SEM, electron microprobe, LA-ICP-MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeOtot zonation. Spinels from NWA 763 show Mg-Fe2+ substitutions. Almost pure MgAl2O4 spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (Tc) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a Tc of about 720 °C. X-ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr-spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO.