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.

¹Chi Ma,²Alexander N. Krot,¹Julie Paque,³Oliver Tschauner,¹Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13771]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 96822 USA
³Department of Geoscience, University of Nevada, Las Vegas, Nevada, 89154 USA
Published by arrangement with John Wiley & Sons

Beckettite (Ca2V6Al6O20; IMA 2015-001) is a newly discovered refractory mineral, occurring as micrometer-sized grains intergrown with hibonite and perovskite, and surrounded by secondary grossular, anorthite, coulsonite, hercynite, and corundum. It occurs within highly altered areas in a V-rich, Type A Ca-Al-rich inclusion (CAI), A-WP1, from the Allende CV3 carbonaceous chondrite. The type beckettite has an empirical formula of (Ca1.99Na0.01)(V3+3.47Al1.40Ti4+0.57Mg0.25Sc0.08Fe2+0.04)(Al5.72Si0.28)O20, with a triclinic structure in space group Purn:x-wiley:10869379:media:maps13771:maps13771-math-0001 and cell parameters a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°, V = 739.7 Å3, and Z = 2, which leads to a calculated density of 3.67 g cm−3. Beckettite’s general formula is Ca2(V,Al,Ti,Mg)6Al6O20 and the endmember formula is Ca2V6Al6O20. Beckettite is slightly 16O-depleted (Δ17O = −16 ± 2‰) compared to the coexisting hibonite and spinel −24 ± 2‰. Beckettite is a primary high-temperature mineral resulting from igneous crystallization of an 16O-rich V-rich CAI melt together with V-bearing hibonite, perovskite, burnettite, spinel, and paqueite. Subsequently, beckettite experienced an incomplete isotope exchange with an 16O-poor aqueous fluid (Δ17O = −3 ± 2‰) on the Allende parent asteroid.