RECONSTRUCTING LUNAR METEORITE THERMAL HISTORIES WITH 40AR/39AR THERMOCHRONOLOGY

Much of our knowledge about the evolution of the Moon’s surface comes from geochemical studies of Apollo samples. In particular, the application of the ⁴⁰Ar/³⁹Ar system to Apollo samples has formed the backbone of our understanding of the Moon’s bombardment through time. However, the existing body of lunar ⁴⁰Ar/³⁹Ar datasets are biased, in that the Apollo missions sampled a small fraction of the lunar surface. Moreover, the framework in which many existing lunar ⁴⁰Ar/³⁹Ar datasets have been interpreted is overly simplistic and does not account for the complex thermal histories that many lunar samples demonstrably experienced. To address these issues, we conducted detailed, temperature-controlled ⁴⁰Ar/³⁹Ar measurements on more than a dozen lunar meteorites that, based on a comparison of their bulk elemental composition to remote sensing data, likely sample regions of the Moon’s surface outside of the Apollo mission landing sites. The ⁴⁰Ar/³⁹Ar analyses were conducted on individual lithic or impact melt clasts or impact melt regions identified in each sample from initial characterization using scanning electron microscopy. We will present ⁴⁰Ar/³⁹Ar step-heating datasets from a subset of the meteorites studied, and interpret these datasets using (1) Bayesian, nonparametric multiple diffusion domain models that account for Ar diffusion from multiple K-bearing phases, and (2) forward models that predict the entire ⁴⁰Ar/³⁹Ar age spectrum as a function of a sample’s geologic and subsequent laboratory thermal histories. These new ⁴⁰Ar/³⁹Ar datasets and interpretations will contribute to a more complete representation of the lunar crust’s thermal evolution and therefore inform outstanding debates about the Moon’s early impact history that rely heavily on existing ⁴⁰Ar/³⁹Ar datasets from Apollo samples.