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). If the magma ocean was well-stirred, cumulates from it should share an identical initial Pb isotope signature (PbSMB). Because Pb is volatile while U is refractory, PbSMB can provide constraints for the timing of volatile depletion, most likely dating the time of Moon formation via a giant impact into the proto-Earth. The PbSMB 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 determine the Moon’s formation time at 4516 (-18/+21) Ma and the SMB solidification time at 4377(-27/+57) Ma. Our modelling strategy facilitates the dating of mare basalt fragments lacking Zr-bearing minerals using the initial Pb isotopic compositions constrained by U-poor minerals. This work sheds light on the early evolution of the Moon and taps the potential capacity in chronological studies of mare basalts.