The gallium isotopic composition of the Moon

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The delta Ga-71 values of these samples range from -0.10 to +0.66 parts per thousand (where delta Ga-71 is the relative difference between the Ga-71/Ga-69 ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than delta Ga-71 values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk delta Ga-71 value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have delta Ga-71 values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher delta Ga-71 values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution. Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Delta Ga-71(Plagioclase-melt) of -0.3 parts per thousand, (where Delta Ga-71(plagioclase-melt) is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have delta Ga-71 values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP. We estimate that the delta Ga-71 value of the LMO was similar to 0.14 parts per thousand prior to the onset of plagioclase crystallization and extraction. Whether this delta Ga-71 value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 +/- 0.06 parts per thousand, Kato et al., 2017) and volatile loss drove the LMO to higher delta Ga-71 values prior to formation of the lunar crust. (C) 2021 Elsevier B.V. All rights reserved.