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Noble gases in diamonds: Occurrences of solarlike helium and neon

American Geophysical Union
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We have measured noble gases in 17 diamond samples, mostly inclusion free, from diverse, known locations. The ^3He/^4He ratios are characterized by a large spread (10^4), ranging from values below atmospheric to close to the solar ratio. Highest ratios were seen for an Australian colorless diamond composite and an Arkansas diamond. These samples also have imprecise but intriguing neon isotopic ratios, which are close to the solar value. An origin for the solarlike He and Ne in the diamond samples is unlikely to be accounted for by the presence of nucleogenic or spallogenic components. For single diamond stones a positive correlation is found between ^3He/^4He and ^(13)C/^(12)C, possibly indicating that heavy carbon is accompanied by primordial helium. However, the He result for the Australian colorless diamond composite with low δ^(13)C value requires another explanation, possibly sedimentary carbon contaminated with cosmic dust. The wide variation in ^4He/^(40)_*Ar ratios observed from diamond samples suggests a complex history for the source regions and the diamond crystallization processes. Results for two Australian diamond composites (colorless and colored), which came from the same kimberlite pipe, are especially notable: the colorless stones contain no radiogenic components but solarlike He and Ne isotopic ratios, whereas the colored stones are enriched in radiogenic and fissiogenic components. Seemingly the Australian diamonds crystallized in a heterogeneous environment in the mantle source region. A pair of Arkansas diamonds, believed to be from a single pipe, exhibits similar anomalies.

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