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 observations1,2 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.