Diffusion kinetics of ³He in pyroxene and plagioclase and applications to cosmogenic exposure dating and paleothermometry in mafic rocks

Abstract. In this study, we investigate the diffusivity of cosmogenic ³He in a variety of plagioclase and pyroxene compositions, and its application to paleothermometry and exposure dating in these minerals, through stepwise degassing experiments. While cosmogenic ³He has been utilized for exposure dating in pyroxene for decades due to its retentivity, plagioclase, often found along with pyroxene in mafic rocks, is generally less retentive of cosmogenic noble gas. However, the diffusivity of ³He in either plagioclase or pyroxene has not yet been measured quantitatively. A challenge in measuring diffusion kinetics by step-degassing experiments in poorly retentive minerals is the fact that significant amounts of He can be lost prior to the experiment. To address this issue, we apply a forward ‘multiple diffusion domain’ (MDD) inversion model that includes model predictions of initial gas loss during irradiation and storage of the samples to account for this observation and add constraints to the diffusion parameters. We find that ³He diffusivity in plagioclase appears to be highly variable. This variability can be explained by the MDD inversion models’ inability to constrain the diffusion parameters when significant gas has been lost during irradiation and/or prolonged storage prior to experiment analysis, resulting in an overestimation of ³He retentivity. Plagioclase samples that were kept frozen after irradiation to limit the initial gas loss yielded the most reliable estimate of diffusion kinetics. We find that ³He in plagioclase is diffusively lost at Earth’s surface temperatures on a timescale of hundred years, and therefore, unsuitable for surface temperature paleothermometry. Contrary, we find cosmogenic ³He in pyroxene to be retentive at Earth’s surface temperatures on a million-year-timescale.