Noble-gas enrichment in vapour-growth diamonds and the origin of diamonds in ureilites

Abstract

Ureilites show high carbon contents comparable with those of CM chondrites1,2. One of the major questions about ureilites is why they contain large amounts of noble gases concentrated in carbon-rich veins3,4. Diamond is shown to be one of the noble-gas carriers, while graphite is gas-free5. We synthesized diamonds by chemical vapour deposition (CVD)6−8 from gaseous mixtures of H2 and CH4 including Ar, and examined the Ar trapped in diamond by mass spectrometry employing the stepwise heating technique. The partial pressure of 36Ar in the gaseous mixture during the synthesis of diamond was 5.3×10−6 atm. The content of 36Ar was about as great as 8×10−6 cm3 STP g−1 which was extracted at 2,000°C. Meanwhile, the 36Ar concentration in graphite was much less than 5% of that in diamonds. These results suggest that diamonds in ureilites may have been directly formed from the solar nebula.