The gallium isotopic composition of the Moon

Journal Article · Mon Dec 06 00:00:00 EST 2021 · Earth and Planetary Science Letters

DOI:https://doi.org/10.1016/j.epsl.2021.117318· OSTI ID:1888089

Wimpenny, Josh  ^[1]; Borg, Lars  ^[1]; Sio, Corliss Kim I. ^[2]

---

1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); University of Toronto, ON (Canada)

Here, in this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The δ⁷¹Ga values of these samples range from -0.10 to +0.66‰ (where δ⁷¹Ga 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 δ⁷¹Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk δ⁷¹Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have δ⁷¹Ga values that are lower than high-Ti basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher δ⁷¹Ga 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 Δ⁷¹Ga_{plagioclase-melt} of -0.3‰, (where Δ⁷¹Ga_{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 δ⁷¹Ga 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 δ⁷¹Ga value of the LMO was ~0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this δ⁷¹Ga 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 δ⁷¹Ga values prior to formation of the lunar crust.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Laboratory Directed Research and Development (LDRD) Program; National Aeronautics and Space Administration (NASA)
• Grant/Contract Number: AC52-07NA27344; 80HQTR20T0010; 19-LW-027
• OSTI ID: 1888089
• Alternate ID(s): OSTI ID: 1862424
• Report Number(s): LLNL-JRNL-822055; 1034242; TRN: US2309870
• Journal Information: Earth and Planetary Science Letters, Vol. 578, Issue N/A; ISSN 0012-821X
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
58 GEOSCIENCES
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
gallium isotopes
lunar volatiles
mare basalts
KREEP
Ferroan Anorthosites