Mineralogy of a basaltic clast in lunar highland regolith breccia 60019

Abstract

The mare basalt component in two lunar meteorites ALHA81005 and Y791197 has been useful in constraining their ejection sites. Although Apollo 16 regolith breccias with glassy matrices have been proposed to be the best analogs for the lunar meteorites, no mare basalt component has been found in such breccias studied so far (e.g., 60019). However, a basaltic clast, Ba‐2, was found on the cut surface of a new slab of 60019, so we have studied Ba‐2 using mineralogical techniques for comparison with the lunar meteorites. Ba‐2 is a coarse‐grained, granular basalt clast, 2.5 × 5 mm. It consists of plagioclase, high‐Ca pyroxene, ilmenite, silica, and mesostasis. The pyroxene and plagioclase crystals show moderate chemical zoning. Most of the pyroxene crystals are augite, with minor iron‐rich rims adjacent to the mesostases. The An content of plagioclase ranges from 95 to 75. Compared to Apollo and Luna mare basalts, the Ba‐2 mineral chemical trends are most like high‐alumina, moderate‐TiO2 basalts found in the Apollo 16 rake samples and to Luna 16 basalt fragments; however, olivine is not present in Ba‐2. The bulk chemical composition of Ba‐2 estimated by modal combination supports the similarity. The highland pyroxene types in the lunar meteorites bear a strong resemblance to one another and to those in 60019, but 60019 appears to have significantly fewer pyroxenes with the extremely low Mg contents of the VLT basalt component in the lunar meteorites. This difference can be explained by the fact that the amount of mare basalt in 60019 is extremely small (less than 0.1%) and the fact that Fe enrichment of pyroxene in the basalt is minor.