PETROGENESIS OF APOLLO 15 BASALTS

The Apollo 15 Mission to the Moon collected a large suite of low-Ti mare basalts. The whole-rock chemistries of these rocks led to their division into Olivine-Normative Basalts (ONBs) and Quartz-Normative Basalts (QNBs). The relationship of these two groups has been a subject of debate for decades. Are they directly related or do they represent melting of different sources in the lunar mantle? This study addresses the relationship of Apollo 15 olivine-normative and quartz-normative basalts using an in-situ mineralogic approach. Individual minerals in the ONBs and QNBs (two thick sections of each basalt type) were analyzed for major-element compositions with an electron microprobe and for trace-element compositions by Laser-Ablation Inductively-Coupled Mass-Spectrometry (LA-ICP-MS). The pyroxene data suggest an origin consistent with fractional crystallization from a parental melt. The rare-earth-element (REE) patterns for pyroxenes are very similar for the four samples, and REE abundances increase with the extent of fractionation (i.e., pigeonites correspond well with other pigeonites, augites with augites, etc.). The low abundance of many rare-earth-elements in olivine, plagioclase, and spinel limit their usefulness. The preliminary comparisons made are not specific enough to the processes involved in the formation of basalts; thus, modeling is required to fully understand the complex relationship between these rocks. Distribution coefficients for elements in these rocks have been determined using the composition of a quenched melt in sample 15499. Combination of major- and trace-element data, modal analyses, and calculated distribution coefficients allows modeling of the compositions of the equilibrium melts from which these basalts originated. An intensive look at the high-precision data set obtained sheds light on this decades-old debate and provides important evidence that these basalts had a common mantle source heritage.