The gallium isotopic composition of the Moon

1Josh Wimpenny,1Lars Borg,1,2Corliss Kin I Sio
Earth and Planetary Science Letters 578, 117318 Link to Article [https://doi.org/10.1016/j.epsl.2021.117318]
1Lawrence Livermore National Laboratory, Livermore, CA 94550, United States of America
2Department of Earth Sciences, University of Toronto, Ontario, Canada
Copyright Elsevier

In this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The Ga values of these samples range from -0.10 to +0.66‰ (where Ga is the relative difference between the 71Ga/69Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher Ga 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 Gaplagioclase-melt of -0.3‰, (where Gaplagioclase-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 Ga 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 Ga value of the LMO was ∼0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this Ga 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‰, Kato et al., 2017) and volatile loss drove the LMO to higher Ga values prior to formation of the lunar crust.

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