¹Dustin Trail, ²Mélanie Barboni, ¹Miki Nakajima, ^1,3Kim A. Cone
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.10.043]
¹Department of Earth & Environmental Sciences, University of Rochester, Rochester, NY, USA
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, USA
Copyright Elsevier

The Moon underwent extensive internal and external modification following the crystallization of a global magma ocean. However, the isotopic record from this formative period remains poorly constrained. Here, we present the first comprehensive study of coupled δ18OVSMOW and δ30SiNBS28 compositions in 67 lunar zircons from Apollo 14 samples, spanning crystallization ages from 4.34 to 3.93 Ga, a critical 400-million-year window of early lunar history. The zircons exhibit remarkably uniform isotopic compositions throughout this interval, with δ18O = 5.66 ± 0.23 ‰ (1 s.d.) and δ30Si = –0.30 ± 0.16 ‰ (1 s.d.). These values are consistent with both bulk silicate Moon estimates and whole-rock analyses, suggesting minimal isotopic fractionation between zircon-forming melts and their source reservoirs. Importantly, we find no systematic isotopic variations with age, sample, or crystallization temperature. This isotopic uniformity persisted despite large-scale geological processes, including crustal formation, basin-forming impacts, and possible mantle overturn. This implies that neither primary differentiation processes nor later reworking produced detectable Si or O isotope heterogeneities in the zircon source regions, at least within the nearside Procellarum KREEP Terrane. Taken together, these results are consistent with lunar silicate reservoirs being well mixed and isotopically equilibrated by ∼4.3 Ga within Fra Mauro, and possibly more broadly, setting a stringent constraint for models of lunar differentiation.