Gallium isotopic constraints for the origin of the Earth-Moon system

Journal Article · Tue Apr 18 00:00:00 EDT 2023 · Earth and Planetary Science Letters

DOI:https://doi.org/10.1016/j.epsl.2023.118146· OSTI ID:1972423

Render, Jan  ^[1]; Wimpenny, Josh  ^[1]; Borg, Lars ^[1]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States). Nuclear and Chemical Science Division

Comparatively heavy isotopic compositions of moderately volatile elements (MVE) in lunar rocks have been advocated to reflect the loss of light isotopes during devolatilization processes from the Moon. In this study we present new gallium (Ga) isotope data for lunar highland rocks, with a focus on the Ferroan Anorthosite Suite (FAS). These are commonly thought to be direct crystallization products from the late lunar magma ocean (LMO) and should contain the majority of the Ga inventory of the Moon. As such, FAS rocks are crucial for identifying the processes that drove Ga isotope fractionation as well as for inferring the Ga isotopic composition of the bulk Moon. Our data reveal that FAS samples have a range in δ₇₁ Ga from -0.27 to 0.22‰ and are generally isotopically light in Ga compared to other lunar lithologies, but straddle values typical of terrestrial rocks. Although Ga is defined as an MVE, these Ga isotope variations do not correspond with concentrations of more volatile elements, indicating that Ga isotope variations in the FAS are not primarily controlled by devolatilization processes. Instead, the Ga isotopic compositions of bulk FAS rocks broadly correlate with the composition of plagioclase, with the calcium content of plagioclase decreasing as Ga becomes isotopically heavier. This suggests that fractionation of Ga isotopes in FAS rocks was caused by the preferential incorporation of isotopically light Ga into plagioclase during the later solidification stages of the LMO. The progressive crystallization and extraction of plagioclase forces the residual melt towards increasingly heavier Ga isotope ratios, corroborating similar conclusions derived from correlations between δ₇₁ and Eu* in the mare basalt suite. Using Ga isotope partitioning calculations, we demonstrate that an isotope fractionation coefficient between plagioclase and coexisting melt of -0.3 to -0.4‰ could explain the observed range of δ₇₁ values in FAS, mare basalt suite rocks, and KREEP. These calculations allow for a first order estimate of the Ga isotopic composition of the bulk silicate Moon prior to plagioclase fractionation and suggest it was close to the composition of the bulk silicate Earth. This would imply that the Moon did not lose a substantial fraction of its Ga inventory during accretion, consistent with new constraints from Rb-Sr isotope systematics that indicate the Moon's volatile deficit was primarily inherited from Theia. In conjunction with the overlap in non-mass-dependent isotope ratios, these collective observations could be reconciled if Theia and the proto-Earth formed in similar regions of the inner Solar System that were already volatile-depleted.

• 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; NNH18ZDA001N-SSW; 19-LW-027
• OSTI ID: 1972423
• Report Number(s): LLNL-JRNL-837296; 1057087; TRN: US2313797
• Journal Information: Earth and Planetary Science Letters, Vol. 611, 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
moon
ferroan anorthosites
Mg-suite