Chemical discrimination of petrographically defined clast groups in Apollo 14 and 15 lunar breccias

Abstract

Many studies of the diversity of lunar rock types have concentrated on either petrographic (e.g. Wilshire and Jackson 1972a) or chemical (e.g. Reid et al. 1972) classifications. This study was undertaken to investigate the compatability of these approaches and also the range and relationships of rock types in the Imbrium region of the moon. Clasts in breccias and fines from the Apollo 14 and 15 landing sites have been studied with microscopic, energy dispersive microprobe and statistical techniques. On petrographic criteria, seven clast groups can be distinguished: (1) anorthositic rocks, (2) mare basalts, (3) mesostasis-rich non-mare basalts, (4) clasts of monzonitic to granitic composition, (5) metamorphosed basaltic rocks, (6) recrystallized breccias, and (7) glasses. Multiple discriminant analysis and principle component analysis, applied to chemical analyses of the clasts, showed that the clast groups of primary or probably primary origin (1, 2 and 3) are chemically distinct and well defined. However, compositions of clasts of secondary origin (groups 5, 6 and 7) only partly overlap those of the primary groups, suggesting that they may represent mechanical mixtures. The variation between and within the plagioclase-rich clasts from highland areas can be accounted for by an enrichment in a granitic component like group 4, and an enrichment in calcic plagioclase.