VERY HIGH O-18 ENRICHMENT IN ARCHEAN CHERTS FROM SOUTH-INDIA - IMPLICATIONS FOR ARCHEAN OCEAN TEMPERATURE

Stable carbon and oxygen isotopic compositions of essentially unmetamo rphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar S equence of southern India, from the northernmost part of the Dharwar-S himoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture an d cryptocrystalline silica, show highly enriched deltaO-18 values rang ing from 28 to 31.4parts per thousand relative to SMOW. Such values ar e the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seaw ater deltaO-18 of 0 parts per thousand, calculation of temperature bas ed on the maximum deltaO-18 value of 31.4 parts per thousand yields a value of 40-degrees-C. This is significantly less than 70-80-degrees-C reported for the Archean oceans based on cherts and chert-phosphate p airs. Diagenetically recrystallized microcrystalline chert-dolomite pa irs of Kalche area exhibit a range of oxygen isotopic ratios similar t o those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68- degrees-C.