Entrainment of sea water into the active deposit has now been recognized as an important control on the structure and composition of the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound. One key manifestation of sea water-fluid interactions is the precipitation of anhydrite from heated mixtures of sea water and hydrothermal fluids. Submersible and drilling recovery of anhydrite from the TAG active hydrothermal mound has generated a wide range of samples from depths of up to 120 m below seafloor (mbsf). Information on trapping temperatures and salinities from analyses of fluid inclusions in anhydrite crystals and the Sr, Ca, Mg and Sr isotopic compositions of anhydrite samples are used here as tracers of entrainment, fluid mixing and evolution during mound circulation. Trapping temperatures in samples from the central and southeast quadrant of the active mound sites are all high, with many higher than the current measured temperature of fluids venting from black and white smokers. There is also a trend of increasing temperature with depth in both of these areas, with cooler temperatures near the mound surface close to the highest measured temperatures of fluids venting from the mound. Salinities of fluids in all inclusions analysed are consistent with phase separation at depth below the seafloor. Mg partitioning into anhydrite is minimal due to mismatch in cationic radius, though high apparent partition coefficients for Mg are observed in surface anhydrite samples which are inferred to be caused by the presence of a fine-grained Mg bearing phase (e.g. talc).