Very high18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

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Abstract
Stable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar‐Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ18O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ18O of 0%0, calculation of temperature based on the maximum δ18O value of 31.4%0yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert‐phosphate pairs. Diagenetically recrystallized microcrystalline chert‐dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.

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