Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa):: implications for the composition of Paleoproterozoic seawater and 'dating' the increase of oxygen in the Precambrian atmosphere

The Mooidraai Dolomite Formation is a unit of marine sedimentary carbonates in the upper Transvaal Supergroup which recorded significant changes in the composition of the Earth's atmosphere and ocean. Previously available data had suggested that this dolomite was about 2.2 Ga old and showed delta(13)C(carb) values around 0.8 parts per thousand PDB, and hence was an exception to the positive excursion of delta(13)C(carb) values observed worldwide in marine carbonates deposited between 2.25 and 2.05 Ga ago. We studied the Pb-O-C isotope systematics of drill core samples from a highly silicified and aluminosilicate-free sub-unit of the Mooidraai Dolomite Formation, that yields well-preserved micritic dolomite grains. Selective leaching of the silicified dolomite revealed a significant Pb isotopic contrast between the carbonate and the quartz fraction. The former shows only a narrow range of Pb-208/Pb-204 (35.35-35.58) which does not correlate with Pb-206/Pb-204, indicating the absence of Th-derived detrital Pb. The carbonate-bound Pb defines an isochron that corresponds to an age of 2394 +/- 26 Ma (2 sigma; MSWD = 1.8, n = 22) which we interpret as the diagenetic age of the Mooidraai dolomite. The delta(13)C(carb) values range from 0.51 to 0.64 parts per thousand PDB and confirm the previous data. The delta(18)O(carb) values range from -2.08 to +0.18 parts per thousand PDB and are amongst the heaviest reported yet from Early Precambrian marine sedimentary carbonates. The quartz fraction is considerably lighter than the carbonate fraction, and shows delta(18)O(qtz) values between -9.22 and -6.73 parts per thousand PDB (+21.40 and +23.97 parts per thousand SMOW). Together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 degrees C, this indicates that the intense silicification was post-depositional. Comparison of delta(18)O(carb) values of silicified and non-silicified Mooidraai samples suggests that the post-depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite. The heavy delta(18)O(carb) values suggest that 2.4 Ga ago the oxygen isotopic composition of seawater differed only little from that of today. The new Pb-Pb carbonate age for the Mooidraai dolomite indicates that the upper Transvaal Supergroup is about 200 my older than previously thought. This explains the 'normal' carbon isotopic composition of the Mooidraai dolomite, and suggests that the pronounced increase of the oxygen content in the Precambrian Earth's atmosphere that is seen in strata underneath the Mooidraai dolomite occurred before 2.4 Ga ago. On this background. it appears unlikely that the positive excursion of the carbon isotopic ratios of 2.25-2.05 Ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen. (C) 1999 Elsevier Science B.V. All rights reserved.