Geochemical characteristics of Lower Permian cold seep carbonate rocks in Xainza, Xizang (Tibet), China

Cold seep carbonates are one of the most important products of cold seep fluid activity. Their geochemical information is of great significance for revealing the evolution of cold seep systems, the source of fluids, and the redox environment when cold seep carbonates were formed. Taking the paleo-cold seep carbonates of the Lower Permian in Xainza, Xizang (Tibet), SW China as the research object, we explored their fluid sources and sedimentary environments in petrology, mineralogy, carbon and oxygen isotopes, and rare earth element geochemistry. Results show that the cold seep carbonates in the region are mainly calcite, whose δ¹³C values ranged from -19.6‰ to -17.5‰, and δ¹⁸O values from -9.2‰ to -7.2‰, indicating that the carbon source of the cold seep carbonates was mainly from thermogenic methane and was also affected by the mixing of a small amount of seawater. The rare earth element distribution pattern shows a “bell-shaped” feature with a slight anomaly of Ce, indicating that the cold seep carbonates were deposited in a weakly oxidizing or reducing environment.