A young solidification age for the lunar magma ocean

The time at which the lunar magma ocean solidified can be determined from the Lu-Hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean. The final magma ocean crystallization product, termed urKREEP, is enriched in incompatible elements including K, REE and P. We have determined the initial Hf isotopic compositions of four samples, two KREEP basalts and two Mg-suite norites. The incompatible trace element compositions of these samples are controlled by an urKREEP component, and therefore the initial Hf isotopic compositions of these samples represent the Hf isotopic evolution of urKREEP. In order to correct the effects of neutron irradiation on the Hf isotopic compositions of these samples, we have developed a model that uses the stable Hf and Sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the Hf isotopic composition of the sample. We use our corrected results to calculate a 176Lu-176Hf urKREEP model age of 4353 ± 37 Ma and the 176Lu/177Hf of urKREEP to be 0.0153 ± 0.0033. The Lu-Hf model age is concordant with the re-calculated Sm-Nd urKREEP model age of 4389 ± 45 Ma, and we take the average of these ages, 4368 ± 29 Ma, to represent the time at which urKREEP formed. This age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as 142Nd model ages for the formation or re-equilibration of mare basalt sources. Taken together, these ages indicate that the Moon experienced a widespread, large-scale magmatic event around 4370 Ma, most plausibly attributed to solidification of the lunar magma ocean.