Carbon and noble gases in Archean chert

Abstract

In order to study the origin and evolution of volatiles trapped in Archean and Early Proterozoic cherts, we have measured contents and isotopic compositions of carbon and noble gases using step-heating and mass spectrometry. The observed negative correlation between the δ 13C-value and nitrogen/carbon ratio in the samples suggests that trapped carbon in Archean cherts is a mixture of carbonate, sedimentary organic matter and an atmospheric component. Elemental abundance patterns of noble gases show a slight depletion of 20Ne and excess of 132Xe relative to air, which is explained by atmospheric noble gases dissolved in water at relatively low temperature. The 3 He 4 He ratios indicate an apparent crustal value of (1–3)·10 −3, consistent with the in situ radiogenic production ratio. Significant excess 21Ne, 131Xe and 132–136Xe as well as slight enrichment of 38Ar and 86Kr relative to atmosphere are found in the Onverwacht chert at the 800° and 1200°C steps. Similar but small isotope anomalies are detected in the Frere Formation chert at the 1800°C step. Excess 21Ne shows positive correlations with 38Ar, 86Kr and 132Xe- 136Xe enrichments while excess 131Xe has no relation with the other isotope anomalies. The 21Ne anomaly can be attributable to the nuclear reaction 18O(α,n) 21Ne. The enrichments of 86Kr and 132Xe- 136Xe are derived from spontaneous fission of 238U in the cherts. The slight excess 38Ar may be due to the 35Cl(α,p) 38Ar reaction based on the positive correlation between 21 Ne 22 Ne and 38 Ar 36 Ar ratios, but should be verified in future work.