Transvaal Supergroup carbonates: Implications for Paleoproterozoic δ18O and δ13C records

Abstract

The Paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean/atmosphere system. In this study, carbonates from the Transvaal Group, the Silverton, Lucknow, Rooinekke and particularly the Duitschland Formations, were analyzed for isotopes of carbon, oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution. Although the trace element data showed variable alteration of the carbonate samples, selected samples with a lesser degree of alteration yielded interpretable results. The “best” measured 87Sr/ 86Sr isotope value of 0.7031 at 2.35 Ga ago fits well with the published curve for Precambrian seawater confirming that Paleoproterozoic budget of Sr was dominated by input from continental sources. During the Paleoproterozoic times, carbonates with δ 13C values of about +10‰ are known to have been present worldwide and they have been studied also during this research. The observation that in the Duitschland Formation such high isotope values and “normal marine” values were both present within a single formation, indeed within its single beds, suggested that the large spread of δ 13C values reflects local or regional rather than global phenomena. The oxygen isotope data show a large spread of values, from −3‰ to −16‰, while their corresponding δ 13C signals are almost unchanged over this entire range, with the Lucknow and Silverton Formations clustering around 10‰ and the Rooinekke Formation around −2‰. The Duitschland Formation has a bimodal distribution with both δ 13C populations present. The last formation also contains samples with δ 18O as low as −20‰, particularly in the vicinity of the Bushveld intrusive body. These highly 18O-depleted samples show homogenization of δ 13C values towards +5‰. Modelling suggests that the trends could have resulted from post-depositional alteration by basinal fluids, at near-surface to about 50 °C temperatures, in a rock buffered system. The Duitschland Formation samples, particularly at Duitschland Farm, the locality closest to the Bushveld Complex, were then further altered at temperatures up to about 80 °C, in response to this intrusion.