The moon's formation time recorded in lunar mare basalts

The Moon's formation time is a key factor for understanding the early evolution of the Earth-Moon system. The lunar magma ocean (LMO) model explains how cumulate mafic materials crystallizing from the LMO form the source of mare basalts (SMB). The SMB with an equilibrated Sm Nd system is considered to share an identical initial Pb isotope signature (Pb SMB ). Because Pb is volatile while U is refractory, Pb SMB can provide constraints for the timing of volatile depletion, most likely dating the time of Moon formation by a giant impact. The Pb SMB is a link between the initial Pb composition of lunar mare basalts and the Moon's early evolution via a two-stage Pb evolution model that provides a simplified but informative framework. Using four mare basalts with well-constrained ages and initial Pb isotopic compositions, we estimate the Moon's formation time at 4516 − 18 + 21 Ma and the SMB formation time at 4377 − 27 + 57 Ma, which we regard as the preferred solution within the statistical framework of the model. Our modelling strategy also facilitates the dating of mare basalt fragments lacking Zr-bearing minerals using the initial Pb isotopic compositions constrained by U-poor minerals. • Moon formation and LMO solidification dominated element fractionation processes. • All sources of mare basalt (SMB) share a uniform initial Pb isotope signature (Pb SMB ). • Pb SMB links mare basalts and the Moon's early evolution via a two-stage Pb model. • Accurate Pb initial composition and ages can constrain Moon and SMB formation time. • Modelling yields Moon formed at 4516 +12 / −18 Ma, SMB solidified at 4377 +57 / −27 Ma.