Pyroxene separation by HF leaching and its impact on helium surface-exposure dating

Cosmogenic 3He surface-exposure dating has become an important tool in earth surface sciences. Pyroxene, together with olivine, is the principal mineral phase for this technique, making 3He particularly appropriate for volcanic lithologies. However, two important factors that affect the viability of the method are the extensive, thus expensive, preparation procedure and the often under-constrained purity of pyroxene separates. Here, we present an approach to preparing pyroxenes for 3He dating, adapted from the quartz-separation method for 10Be and 26Al analyses, which utilises hydrofluoric-acid leaching to improve pyroxene purity and streamline the pyroxene separation procedure. In addition to producing abundant sample in relatively little time, the results of two experiments demonstrate that acid leaching (i) yields samples of higher purity than conventional methods, reflected in slightly elevated 3He concentrations, and (ii) reduces 4He concentrations, in turn elevating the 3He/4He ratio. This new protocol for preparing pyroxenes thus has the potential to increase the precision and accuracy of cosmogenic 3He surface-exposure dating.