The oxygen isotope composition, petrology and geochemistry of mare basalts: Evidence for large-scale compositional variation in the lunar mantle

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To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K – potassium, REE – Rare Earth Element, and P – phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ 18O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ 18O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ 18O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ 18O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ 18O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples. Consistent with the results of previous studies, our data reveal no detectable difference between the Δ 17O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.