Chemical variation of the large Apollo 15 olivine-normative mare basalt rock samples

Most chemical analyses of Apollo 15 olivine-normative mare basalts have been conducted on subsamples of <1 g, and dominantly rather less than 0.5 g, very small by the standards of analyses of terrestrial rocks. Most analyses have been partial (a small number of elements) rather than comprehensive. They show considerable scatter, but the general inference has been that the rock samples represent a single suite dominated by olivine control. However, because of the scatter, some workers have suggested more complex petrogeneses and even that the rock samples do not form a single comagmatic suite. We analyzed the 25 largest rock samples identified as Apollo 15 olivine-normative mare basalts using subsamples that were mainly 5 g (and all but three >4 g) to obtain greater whole-rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X-ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short-range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine-normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine-accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.