Petrogenesis of Apollo 15 Olivine-Normative and Quartz-Normative Mare Basalts

New data are presented for four Apollo 15 low-Ti mare basalts, two from the olivine-normative suite (15106, and 15555) and two from the quartz-normative suite (15475, and 15499). Previous studies have examined the relationships between these groups on the basis of whole-rock chemistry, many with analyses that may have been of insufficient sample-size, or of limited range of elements analyzed. To determine a relationship between these basalts, these samples have been analyzed for their mineral major-element and trace-element compositions by electron-microprobe and laser-ablation inductively-coupled-plasma mass-spectrometry, respectively.

The trace-element compositions of the main silicate phases, olivine, pyroxene, and plagioclase, are consistent with both groups of basalts having a common parental melt and the groups being related by simple fractional crystallization processes rather than coming from different sources. This observation is supported by similar radiogenic ages for the basalt groups and by petrogenetic modeling. Distribution coefficients have been used to determine the parental and evolved melt compositions for these basalts. Also, an independent set of distribution coefficients has been determined from pyroxene- melt relationships in sample 15499. The calculated parental melts are similar to measured whole-rock compositions. The calculated evolved melts are consistent with fractional crystallization of olivine. From a trace-element perspective, these basalts appear to be from a single parental melt, and related by fractional crystallization of 5-15% olivine.