Constraints of fluid inclusions and C, O isotopic compositions on the origin of the dolomites in the Xisha Islands, South China Sea

Abstract

The “dolomite problem” is a key issue in research on carbonate diagenesis, and it is still a controversial scientific problem. The origin of the Neogene dolostones in the Xisha Islands has remained disputed until now. However, the geochemical characteristics of dolomite-hosted fluid inclusions can potentially provide a relatively direct record of the origin and evolution of their dolomitizing fluid. In this article, based on the samples selected from the well “Xike-1” (also known as “ZK-1” and “XK-1”) reef core of the Xisha Islands, the hydrogen isotopic compositions and gas and liquid compositions of the fluid inclusions hosted in calcite and dolomite were analyzed. Meanwhile, the isotopic compositions of carbon and oxygen in the calcite and dolomite minerals were also tested. The origin of the dolomitizing fluid was traced in detail, which provided important evidence for establishing the dolomitization model of the primary dolomites in the study area. The main gas compositions of the dolomite-hosted fluid inclusions are CO2, H2O, N2, and O2, which are similar to those of the atmosphere, indicating that the dolomites in this area experienced a partly oxygenated diagenetic environment. From high to low, the cation concentrations of the liquid component in the dolomite-hosted fluid inclusions are Mg2+, Ca2+, Na+, and K+, whereas their main anion compositions are SO42−, Cl−, and F−, which are characterized by high SO42− concentrations and low Cl/Br ratios. The δ13C values (PDB) of the dolomites range from 1.9‰ to 3.0‰, and their δ18O values (PDB) range from 2.1‰ to 4.7‰. The calculated Z values (which can be used to trace the salinity of diagenetic fluid) based on the δ13C and δ18O values of the dolomites range from 133.0 to 135.8, which are higher than that of modern seawater (approximately 120). These data indicate that the dolomitizing fluid of the primary dolomites in the Xisha Islands is the high-salinity concentrated seawater derived from a surface environment. In addition, the model of seepage-reflux dolomitization can better interpret the dolomitizing process of the primary dolomites in the Xisha Islands.