Cosmogenic 3He in alluvial metal and alloy grains: Assessing the potential for quantifying sediment transport times

We have measured the He isotope composition in detrital native gold, copper, silver, Pt3Fe and OsIr alloy grains from placer deposits in order to assess the extent to which cosmogenic He can be quantified and used to determine the exposure history of grains in sedimentary systems. 3He/4He ratios (0.1–273 Ra) exceed values typical of radiogenic He. 4He is dominantly the result of the decay of U and Th, or 190Pt in the case of Pt-rich alloys. 3He concentrations in grains from alluvial deposits range from 0.05 to 2 × 108 at g−1 and appears to be dominantly cosmogenic in origin. The absence of significant nucleogenic or mantle-derived 3He in metals is tentatively confirmed by the analysis of grains from underground mine workings. The implantation of cosmogenic He is not the main source of 3He in alluvial metals but can be relatively important for small grains. New GEANT4-based calculations predict the production rate of 3He (including in precursor 3H) from Au and Pt to be approximately 25 at g−1 yr. Using this we determine that cosmogenic 3He concentrations in the detrital grains record minimum exposure times of between 0.5 and 7 million years that generally increase with distance from the putative source. Paired fragments from three large grains show that there is considerable heterogeneity in cosmogenic He concentrations that may reflect the complex origin of detrital metal grains. This work suggests that cosmogenic He in detrital metals and minerals may find use for providing the chronology of sedimentary processes.