Abstract
We have measured 3He and 4He concentrations in a number of diamonds received directly from mines as well as in industrial samples. None of our diamonds recovered from underground mining has a 3He/ 4He ratio, R, exceeding 3 R A ( R A = atmospheric 3 He/ 4 He ratio = 1.40 · 10 −6). One of the five diamonds from the Consolidated Diamond mine (CDM-5; an alluvial deposit) has a ratio of 142 R A . Based on a number of facts: (i) the Zaire alluvial diamonds have> 29% cosmogenic 3He; (ii) some documented high 3He/ 4He ratio diamonds are from alluvial mines, and (iii) the apparent diffusion coefficients of 3He and 4He in diamonds are ten orders of magnitude too large to preserve diamond helium ratios in the mantle, we conclude that Ozima's hypothesis ( Ozima et al., 1983) of high primordial 3He/ 4He ratios in diamonds exceeding MORB ratios is not required. The present data are readily explainable as the result of cosmogenic and nucleogenic production of 3He within the diamonds.
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