Chemical and sulfur isotopic compositions of anhydrite from the Tangyin hydrothermal field in the Okinawa Trough

Anhydrite is one of the earliest minerals in forming the hydrothermal chimney walls, which is important for understanding the fluid-seawater mixing, and elemental migration and cycling in the seafloor hydrothermal system. Anhydrite minerals samples from the Tangyin hydrothermal field in the southwestern Okinawa Trough, western Pacific were investigated on the in-situ element concentrations and sulfur (S) isotopic compositions. The crystal morphology of anhydrite could be divided into two types. Type I anhydrite formed earlier is subhedral or anhedral and occurred in radial or irregular crystal aggregation, and Type II anhydrite formed later is euhedral and occurred in plate or granular crystal aggregation. When the hydrothermal fluid first met with seawater, Type I anhydrite precipitated rapidly and formed the wall of the hydrothermal chimney. Subsequently, Type II anhydrite experienced relatively longer growth stage. However, the Ba, Al, Sr, Ni, Fe, Mn, and Cr contents of anhydrite are significantly higher than that of seawater, suggesting that those elements are derived mainly from hydrothermal fluid duo to the subseafloor fluid-rock and/or sediment interactions. The Mg content of anhydrite is significantly lower / higher than that of seawater / vent fluids, which was resulted from the fluid-seawater mixing. Most of the Sr contents of Type I anhydrite are significantly lower than that of Type II anhydrite, suggesting that more Sr from fluids were involved into Type II anhydrite and formed euhedral, plate, or granular minerals. The Fe, As, Sr, Ba, and Pb contents of anhydrite are significantly higher than that of vent fluids in the Okinawa Trough, which indicates that these elements enter preferentially anhydrite from the fluids, and resulted in the enrichment of these elements in the anhydrite. REEs of anhydrite and their REE patterns show positive Ce and negative Eu anomalies, which could be resulted from fluids leaching out from local sub-seafloor volcanic rocks and/or sediments and having undergone fluid-seawater mixing. Furthermore, S in the anhydrite was mainly from seawater during fluid-seawater mixing.