Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry

Mineralogic features attributed to impact-induced shock metamorphism are commonly observed in meteorites and terrestrial impact craters. Partly because the duration of shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We measured (U-Th)/He ages of single grains of merrillite and chlorapatite from the Martian meteorite Los Angeles (LA). Merrillite and chlorapatite ages cluster at 3.28 +/- 0.15 Ma (2sigma) and 2.18 +/- 0.19 (2sigma) Ma, respectively. The mean age of the merrillites, which are larger than chlorapatites, is indistinguishable from cosmic-ray exposure ages (3.1 +/- 0.2 Ma), suggesting that impact-induced shock metamorphism was coeval with ejection of the LA precursor from Mars. To constrain the initial temperature of shock metamorphism in the LA precursor body, we modeled diffusive loss of He from merrillite as a function of diffusion domain size, LA precursor body size, and ablation depth. From these calculations, we suggest that the metamorphic temperature of the shock event was higher than 450 degreesC. These results support the idea that shock pressures of the Martian meteorite Shergotty were higher than 45 GPa, as inferred from the presence of post-stishovite SiO2 pollymorphs. Single-grain (U-Th)/He dating of phosphates may provide unique constraints on the timing and pressure-temperature dynamics of shock metamorphism in a wide variety of extraterrestrial materials.