Genesis of anhydrite in hydrothermally altered basalt from the East Pacific Rise near 13°N

This study reports the occurrence of anhydrite in hydrothermally altered pillow basalt (12°50.55'N, 103°57.62'W, water depth 2 480 m), which may have been produced in the basalt during seawater-basalt interaction in the laboratory. The existence of anhydrite in the altered basalt indicates extensive hightemperature hydrothermal alteration at the surface of seafloor pillow basalt. Microprobe analysis shows significant chemical zoning in the hydrothermally altered pillow basalt, in which Ca, Si and Al contents decrease and P, Fe, Mn, Cr and S contents increase from fresh basalt to altered basalt. The negative correlation between Rb-Sr and Li-Sr, and negative correlation between Li-Ca and Rb-Ca in the high-temperature vent fluids show that these fluids underwent anhydrite precipitation before fluid jetting due to mixing with seawater in the sub-seafloor. Based on these observations, we show that not all Ca in the anhydrite comes from basalt in the reaction zone, and that the basalts on the seafloor or in the upflow zone may also provide Ca for anhydrite.