Helium Diffusion Kinetics in Enstatite, Kamacite, and Albite, With Implications for the Cosmic Ray Exposure Ages of Enstatite (E) Chondrites

Cosmic ray exposure (CRE) ages are used to constrain the orbital and impact history of meteorites and identify their parent body or source region. CRE ages of enstatite (E) chondrites obtained from measurements of ³He are often much younger than ²¹Ne CRE ages measured in the same meteorite, which is often attributed to diffusive loss of ³He via solar heating during orbit. With knowledge of the diffusion kinetics of ³He in the major minerals making up E chondrites, we can leverage this discrepancy in CRE ages to infer a meteorite’s recent thermal history. To this end, we performed stepwise degassing experiments on fragments of albite, enstatite and kamacite, the dominant minerals in E chondrites, that were irradiated with protons to produce ³He. We find albite displays simple, Arrhenius-dependent ³He diffusion behavior, whereas enstatite and kamacite exhibit somewhat more complex diffusion behavior. We find that cosmogenic ³He will be readily lost from albite in the space environment, enstatite can exhibit significant ³He loss if exposed to high temperatures characteristic of low perihelion on million year time scales, and kamacite is highly retentive of ³He and unlikely to experience direct diffusive loss. These diffusion kinetics parameters can also be used to understand the exposure and thermal histories of other meteorite classes, terrestrial cosmogenic ³He applications, and mantle noble gas systematics.