TEMPORAL RELATIONSHIPS IN THE LUNAR CRUST

High precision U-Pb ages determined on ferroan anorthosite suite (FAS) sample 60025 (4359 ± 2.4 Ma) and Mg-suite (MGS) norite sample 78236/8 (4332 ± 3.8 Ma) demonstrate that these two rocks were produced within a ~25 Ma time interval early in the history of the Moon. These ages have been confirmed with Sm-Nd and U-Pb ages albeit at substantially lower resolutions, supporting the ~25 Ma age gap. Previous chronologic investigations completed on these lithologies have been unable to unambiguously establish this temporal relationship because Sm-Nd, Rb-Sr, Ar-Ar, and U-Pb ages are often disrupted by impact metamorphism, have not been confirmed by repeated measurements or through application of multiple chronometric systems to the same samples, or yield internal uncertainties that are too large to distinguish events separated by only a few tens of million years. Nevertheless, the Sm-Nd systematics of samples from these two crustal rock suites define whole rock isochron ages of 4360 ± 27 Ma (FAS) and of 4348 ± 25 Ma (MGS) implying that both suites were produced over short durations that were separated in time by no more than ~64 Ma. Such a short time interval is problematic because the FAS is thought to represent flotation cumulates produced during primordial crystallization the lunar magma ocean (LMO), whereas the MGS is thought to be comprised of Mg-rich, plagioclase-rich, and incompatible element-rich cumulates that formed previously during crystallization of the LMO. This implies that some primordial crystallization products were remelted at most only 25 Ma after they first solidified, requiring a process that results in melting of primordial LMO cumulates in a regime that recently been cooling. There are at least three mechanisms that might be responsible for remelting of LMO cumulates soon after they formed including, a very large impact, heat production through decay of radiogenic isotopes concentrated in late stage LMO cumulates, or density driven overturn of LMO cumulates. Clear evidence for a major impact at ~4.34 Ga is absent as is evidence for an extended duration of Mg-suite magmatism that might be expected if melting was induced by radiogenic heating. This suggests that decompression melting associated with an overturn event in the mantle is probably responsible for the observed pulse of the MGS.