Lamellar pyroxene-spinel symplectites in lunar olivine from the Luna 24 regolith

Abstract

Cr-Ca lamellae in a magnesian olivine grain (section 1611) from the Luna 24 regolith were investigated in detail by electron microprobe analysis (EMPA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the lamellae are parallel to the (100) plane of oxygen closest packing in olivine and consist of regular vermicular intergrowths of two phases, diopside (Di) and chromite (Chr), in the volume proportion Di: Chr ≈ 3: 1. The bulk chemical composition of the lamellae is approximated as Ca2Mg2Fe2+(Cr3+)2Si4O16. They are identical in phase composition to type A, F, and E symplectites from Apollo lunar samples [9]. Based on morphology and phase composition, the lamellar aggregates in the olivine grain from the Luna 24 regolith were classified as pyroxene (Px)-spinel (Spl) symplectites of a lamellar type, the formation of which was related to olivine oxidation at IW ≤ logfO2 ≤ QFM. The obtained data indicate a solid-phase mechanism of lamella formation and the existence of a lamellar precursor phase, which transformed subsequently into the Px-Spl symplectite. It was supposed that uvarovite-knorringite garnet produced by the oxidation of olivine at high pressures and t > 800°C could be the transitional phase during symplectite formation. The subsequent conversion of the garnet into the low-pressure assemblage of Px-Spl symplectites could occur via cellular decomposition in accordance with the reaction Ca2MgCr2Si3O12 + (Mg,Fe)2SiO4 = 2CaMgSi2O6 + FeCr2O4. The reported results are the first data of a detailed nanomineralogical investigation of lamellar Px-Spl symplectites in lunar olivine.