Cosmogenic ¹⁰Be in Zaire alluvial diamonds: implications for ³He contents of diamonds

Abstract

To determine the amounts of cosmic-ray produced ('cosmogenic') ³He which could be created in diamonds during their post-eruptive residence in near-surface regions, we have measured the concentration of ¹⁰Be (half life, 1.6 Myr) in an industrial diamond sample. The motivation to study cosmogenic ³He in diamonds comes from the recent observations^1,2 of ³He/⁴He ratios in some diamonds that are even higher than meteoritic values (so-called 'planetary' helium with ³He/⁴He = 1.4×10⁻⁴). We measured ¹⁰Be in an alluvial diamond sample, using radiochemical and accelerator mass spectrometry (AMS) techniques. The observed ¹⁰Be concentration, 2.46 ± 0.39×10⁶ atoms g⁻¹, leads to an effective surface exposure of 3.3×10⁵ yr for these diamonds. This corresponds to a substantial cosmogenic production of ³He in the alluvial diamonds, ≥=29% of the total ³He. We conclude that the high ³He/⁴He ratios in at least some diamonds are due to cosmogenic ³He production in near-surface regions during their residence in alluvial or weathering deposits. Thus, in order to establish that such high ³He/⁴He ratios existed in the mantle at one time, it is a priori necessary to study diamonds collected by underground mining methods.