The relationship of brine chemistry of the Pennsylvanian Paradox Evaporite Basin (southwestern USA) to secular variation in seawater chemistry

To establish the brine chemistry associated with the evaporites in the Pennsylvanian Paradox Basin of southeastern Utah and southwestern Colorado (USA), the composition of primary fluid inclusions was determined for sedimentary halite from two drill cores, one near the central part of the basin (Shafer Dome No. 1) and one from a more marginal location of the basin (Gibson Dome No. 1). Chemical analysis of halite fluid inclusions was done on six samples from three different evaporite cycles of the Paradox Formation; cycle 10 in the Shafer Dome core and cycles 6 and 18 from the Gibson Dome core. The inclusions that range in size from 2 to 80 microns across, were analyzed using the Petrychenko method. Large inclusions (40 to 80 microns across) that were used for the chemical analyses contain one fluid phase with a carnallite or sylvite daughter crystal. Also reported in this study are fluid inclusion homogenization temperatures for sylvite or carnallite from primary halite crystals in the Gibson Dome core and in Shafer Dome. The relationship between K+ and Mg2+ in chloride rich inclusions corresponds to their proportion in MgSO4-depleted marine waters concentrated to the stage of carnallite deposition. A correlative relationship was observed between K+2+4-rich to MgSO4-poor compositions that have been proposed by other workers. A transition from MgSO4-rich to MgSO4-poor seawater composition may have occurred between Pennsylvanian and Permian times. This paper presents a possible alternate explanation to those already proposed in the literature, that the Paradox Formation mineralogy resulted from an intermediate seawater composition that records the global transition from MgSO4-rich to MgSO4-poor seawater

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

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‰. 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‰, and thus δ18 O values have a smaller range of variation (5.5‰) than δ34 S (11.8‰) over the entire set of Lower Cambrian anhydrites. The great δ34 S variability observed in the Lower Cambrian of Siberia seems to reflect mixing of sulphates coming from the ocean and due to the riverine input. The lowest δ18 O values may indicate the input values from both the sources, whilst the highest value may result from isotope exchange between SO4 2- and water. Our results combined with data provided by previous workers could indicate a clear stratigraphic trend in δ34 S values, with a remarkable fall of ca. 9‰ in δ34 S value during the earliest Cambrian and then a slight rise in δ34 S values in the younger part of Early Cambrian. However, if only the highest values are taken, the measured values are compatible with seawater δ34 S 3 30‰ during the entire Early Cambrian. Sulphur isotopic composition of sulphate minerals did not be come heavier from the sulphate stage to ward the chloride stage