Significance of Oxygen and Carbon Isotope Variations in Early Precambrian Cherts and Carbonate Rocks of Southern Africa

摘要

Bedded cherts and carbonates from the early Precambrian Bulawayan Group and the Pongola and Swaziland Sequences of southern Africa have O18_SMOW values ranging from 9.8 to 14.5 for calcite, 8.4 to 16, for dolomite and iron carbonate, and 11.0 to 18.7%,, for SiO₂. Values of C13_PDB for calcite and dolomite are similar to those of modern carbonates; they range from –1 to + 1.8. In contrast, carbonates from magnetite-bearing iron formations have (C13_PDB values of –3.5 to –9.6. Well-preserved sedimentary magnesite from Barton's Farm, Rhodesia, has a maximum O18_SMOW value of 28.8. If the highest O18 values of these ancient chert and carbonate rocks are primary, O18 of the ocean probably has increased by about 15 since their deposition. Available data on the oxygen isotope composition of crustal rocks indicates a probable large excess in the crustal abundance of O18 compared to the O18 concentration in rocks from the mantle. These observations suggest that large amounts of water have been recycled through the mantle. Our finding that carbon isotope ratios in early Precambrian carbonate rocks of southern Africa are similar to ratios in modern marine carbonates corroborates other studies of carbonates in Precambrian rocks.