The sulphur and oxygen isotopic composition of Lower Cambrian anhydrites in East Siberia

摘要

Published sulphur and oxygen isotope age curves for the late Neoproterozoic-Early Cambrian time interval have been based on studies of sulphate intervals of the East Siberian salt giant. We report here on sulphur and oxygen measurements for sulphate dispersed in, or forming laminae in, the rock salt deposits in all Lower Cambrian basins of East Siberia. Sulphur isotope data for 26 samples of Lower Cambrian anhydrites from East Siberia range from +22.6 to +34.5 per thousand. No difference was observed between different suites and between samples taken from anhydrite intercalations in rock salt and from water-insoluble residue in rock salt. Oxygen isotope data for 25 anhydrite samples range from +12.4 to +17.8 per thousand, and thus delta~(18)O values have a smaller range of variation (5.5 per thousand) than delta~(34)S (11.8 per thousand) over the entire set of Lower Cambrian anhydrites. The great delta~(34)S variability observed in the Lower Cambrian of Siberia seems to reflect mixing of sulphates coining from the ocean and due to the riverine input. The lowest delta~(18)O values may indicate the input values from both the sources, whilst the highest value may result from isotope exchange between S04 and water. Our results combined with data provided by previous workers could indicate a clear stratigraphic trend in delta~(34)S values, with a remarkable fall of ca. 9 per thousand in delta~(34)S value during the earliest Cambrian and then a slight rise in delta~(34)S values in the younger part of Early Cambrian. However, if only the highest values are taken, the measured values are compatible with seawater delta~(34)S>=30 per thousand during the entire Early Cambrian. Sulphur isotopic composition of sulphate minerals did not become heavier from the sulphate stage toward the chloride stage.