Geochemical constraints on the origin of post-depositional fluids in sedimentary carbonates of the Ediacaran system in South China

Abstract

In order to evaluate the preservation of primary geochemical signatures in sedimentary carbonates and the effect of post-depositional fluids, a combined study of CO isotopes and trace elements was carried out for different microfacies of the Lantian carbonates in the southern Anhui, which is equivalent to the Doushantuo Formation of the Ediacaran system elsewhere in South China. Stratigraphically the Lantian carbonate can be subdivided into the Upper and Lower Units. For the Upper Unit (UU), petrographic observations indicate that the wallrock carbonates experienced only limited recrystallization. The REE+Y patterns of wallrock carbonates with CaO contents higher than 53% are consistent with those treated by the acetic acid solution, representing the primary records of carbonates deposition. The veins and wallrock carbonates in this unit exhibit consistent REE+Y patterns and similar δ13C and δ18O values, suggesting the same origin of their depositional fluids. In contrast, there are significant differences in the geochemical signatures between veins/cements and wallrock carbonates in the Lower Unit (LU), suggesting that different origins of their depositional fluid.The REE+Y patterns for calcite veins and cements in the both units indicate incorporation of terrigenous materials into post-depositional fluids. With respect to correlations between δ13C and δ18O values, negative one occurs in the UU, whereas positive one occurs in the LU. Unusually low δ18O values of −28.6‰ to −22.0‰ for the veins and cements in the both units indicate that the post-depositional fluids were derived from negative δ18O continental freshwater subsequent to the Gaskiers and Marinoan iceages. The REE+Y patterns and δ18O variations for the veins in UU suggest the parent fluid contained a component of continental deglacial meltwater. The unusually low δ18O values also occur in the veins and cements of LU, indicating a significant contribution from meteoric water. On the other hand, the δ13C values for the veins and cements of the both units suggest that the same carbon sources as the wallrock. The present results provide insights into the preservation of primary geochemical records in sedimentary carbonates and into the effect of post-depositional processes on the Precambrian carbonates. In particular, the origin of post-depositional fluids can be distinguished from the depositional fluids by means of the combined trace element and stable isotope tracing.