Paleotemperatures at the lunar surfaces from open system behavior of cosmogenic 38Ar and radiogenic 40Ar

Abstract The simultaneous diffusion of both cosmogenic 38 Ar and radiogenic 40 Ar from solid phases is controlled by the thermal conditions of rocks while residing near planetary surfaces. Combined observations of 38 Ar/ 37 Ar and 40 Ar/ 39 Ar ratios during stepwise degassing analyses of neutron-irradiated Apollo samples can distinguish between diffusive loss of Ar due to solar heating of the rocks and that associated with elevated temperatures during or following impact events; the data provide quantitative constraints on the durations and temperatures of each process. From sequentially degassed 38 Ar/ 37 Ar ratios can be calculated a spectrum of apparent 38 Ar exposure ages versus the cumulative release fraction of 37 Ar, which is particularly sensitive to conditions at the lunar surface typically over ∼10 6 –10 8 year timescales. Due to variable proportions of K- and Ca-bearing glass, plagioclase and pyroxene, with variability in the grain sizes of these phases, each sample will have distinct sensitivity to, and therefore different resolving power on, past near-surface thermal conditions. We present the underlying assumptions, and the analytical and numerical methods used to quantify the Ar diffusion kinetics in multi-phase whole-rock analyses that provide these constraints. For Apollo 15 samples 15016, 15556, and 15596 we find apparent 40 Ar/ 39 Ar plateau ages between 3.21 and 3.28 Ga and evidence for diffusive loss of radiogenic 40 Ar primarily from K-bearing glass. From 38 Ar/ 37 Ar spectra normalized to the apparent Ca/K ratios, we also find evidence of diffusive loss of cosmogenic 38 Ar that requires elevated temperatures either during or after surface exposure. Using 39 Ar and 37 Ar, we construct multiple-phase–multiple diffusion domain (MP-MDD) models to quantify the diffusion kinetics of Ar from a range of macroscopic grain sizes of each phase. While diffusive loss of 40 Ar can be explained by brief reheating conditions after crystallization (e.g., during an impact event), we find that both the radiogenic 40 Ar and cosmogenic 38 Ar abundances can be explained by internally consistent thermal conditions expected for solar heating of the rocks at the lunar surface. These conditions correspond to effective diffusion temperatures ( EDT , i.e., the temperature corresponding to the mean diffusivity over temperature variability) between 65 and 81 °C, with an error-weighted mean of 77.0 ± 1.3 °C, despite differences in diffusion kinetics and large differences in surface exposure duration between the three samples (∼56–621 Ma). This EDT corresponds to a maximum daytime temperature of ∼96 °C, which is in excellent agreement direct measurements of temperature at the Apollo 15 landing site. The open system behavior in both a radiogenic and a cosmogenic nuclide provides tests for internal consistency in best-fitting solutions of time-integrated thermal conditions of rocks collected from planetary surfaces. This thermal information is important for the study of habitable conditions at planetary surfaces, and has implications for Ar-based geochronology and paleomagnetic observations applied to planetary materials.