¹Yan Hu,¹Frédéric Moynier,^2,3Xin Yang
Earth and Planetary Science Letters 620, 118319 Link to Article [https://doi.org/10.1016/j.epsl.2023.118319]
¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, UMR 7154, Paris 75005, France
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
³Robert A. Pritzker Center for Meteoritics and Polar Studies, Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, IL, USA
Copyright Elsevier

The stable potassium isotopic ratios (41K/39K) of Earth and Mars have been interpreted to reflect either nucleosynthetic isotope anomalies or volatility-driven K depletion. Chondrites comprise primordial materials from which planetary bodies are assembled, and thus are critical samples for this discussion. Here, we present high-precision K isotopic analyses (reported as
K) of 33 chondrites and two achondrites, which reveal unprecedented variation from −1.08 to 4.68‰. In addition, there is considerable overlap in
K values between carbonaceous and non-carbonaceous meteorites despite their contrasting nucleosynthetic isotope anomalies. These findings are inconsistent with the nucleosynthetic origin of 41K variations in meteorites. Instead, the
K values of chondrites correlate positively with the isotopic compositions of other moderately volatile elements (e.g., Rb, Cu, Zn, Sn, Ga, and Te). These correlations suggest that volatility-controlled fractionation is a common mechanism for mass-dependent isotopic variations in the Solar System. In particular, carbonaceous chondrites and the angrite parent body exhibit a trend of concomitant decreases in K and its heavier isotope due to incomplete K condensation. Earth and Mars also follow this trend, suggesting that their K depletion may reflect similar volatile-depleting processes that occurred with their respective precursors. That Mars is isotopically heavier than Earth is consistent with it having less K-depleted precursors, in addition to the previous suggestion of a later-stage K loss from proto-Mars during accretionary collisions.

¹Qin Zhou et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2023.08.017]
¹Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
Copyright Elsevier

The U(Pb)-Pb age of zircon is commonly used to represent the crystallization age for igneous rocks due to its high closure temperature and robust resistance to impact disturbance. However, no zircon crystallization age has yet been reported for Chang’e-5 (CE-5) basalt due to their limited occurrence in mare basalt. In this study, rare zircon grains from CE-5 lunar samples were investigated by the in-situ Pb isotopic analysis, and a precise zircon crystallization age of 2036 ± 19 Ma was determined from Pb-Pb isochron. This is hitherto the youngest reported crystallization age of lunar zircon, similar to the ages of CE-5 lunar basalts obtained by zirconium (Zr)-bearing minerals such as baddeleyite, tranquillityite, and zirconolite. Petrographic evidence and rare-earth element geochemistry indicate that zircon in the CE-5 lunar basalts were formed by the reaction of early-formed baddeleyite with SiO2 melt within the latest residue of extreme fractionation of a non-KREEP (an acronym for potassium, REE, and phosphorus) basaltic magma. In contrast to the prevailing view that lunar zircon have formed in late-stage enriched melts resulting from extensive fractional crystallization of the Lunar Magma Ocean, this study shows that zircon could be derived from extreme fractionation of non-KREEP basaltic magma unrelated to Lunar Magma Ocean.