Mineralogy, petrology, and shock history of lunar meteorite Sayh al Uhaymir 300: A crystalline impact-melt breccia

Abstract

Sayh al Uhaymir (SaU) 300 comprises a microcrystalline igneous matrix (grain size 10 m), dominated by plagioclase, pyroxene, and olivine. Pyroxene geothermometry indicates that the matrix crystallized at ~1100 degrees C. The matrix encloses mineral and lithic clasts that record the effects of variable levels of shock. Mineral clasts include plagioclase, low- and high-Ca pyroxene, pigeonite, and olivine. Minor amounts of ilmenite, FeNi metal, chromite, and a silica phase are also present. A variety of lithic clast types are observed, including glassy impact melts, impact-melt breccias, and metamorphosed impact melts. One clast of granulitic breccia was also noted. A lunar origin for SaU 300 is supported by the composition of the plagioclase (average An95), the high Cr content in olivine, the lack of hydrous phases, and the Fe/Mn ratio of mafic minerals. Both matrix and clasts have been locally overprinted by shock veins and melt pockets. SaU 300 has previously been described as an anorthositic regolith breccia with basaltic components and a granulitic matrix, but we here interpret it to be a polymict crystalline impact-melt breccia with an olivine-rich anorthositic norite bulk composition. The varying shock states of the mineral and lithic clasts suggest that they were shocked to between 5-28 GPa (shock stages S1-S2) by impact events in target rocks prior to their inclusion in the matrix. Formation of the igneous matrix requires a minimum shock pressure of 60 GPa (shock stage >S4). The association of maskelynite with melt pockets and shock veins indicates a subsequent, local 28-45 GPa (shock stage S2-S3) excursion, which was probably responsible for lofting the sample from the lunar surface. Subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor.