Precipitation of Calcite Veins in Serpentinized Harzburgite at Tianxiu Hydrothermal Field on Carlsberg Ridge (3.67°N), Northwest Indian Ocean: Implications for Fluid Circulation

Serpentinization and calcite precipitation of mantle peridotites exhumed along detachment faults at the slow- to ultraslow-spreading centers can provide important clues to the hydrothermal alteration processes. The Tianxiu hydrothermal field is a new-found active and ultramafichosted hydrothermal vent site along the Carlsberg Ridge, Northwest Indian Ocean. Two types of calcite veins are recognized in serpentinized harzburgite samples collected from the seafloor at the water depth of 3 500 m (3.67°N/63.83°E) and 400 m north of Tianxiu hydrothermal field. Calcite veins I occur in the fractures that cut through mesh texture in the highly serpentinized harzburgite, while calcite veins II precipitate within the mesh texture in the relatively weaker serpentinized harzburgite. Both veins show similar δ 13 C PDB (+0.54‰ and +0.58‰) but different δ 18 O PDB (−16.67‰ and +4.46‰) values, suggesting that they were derived from the same carbon source but precipitated at different temperatures. Taking the deep seawater temperature of 2 °C as the precipitation temperature of the calcite veins I, the equilibrium δ 18 O V-SMOW of calcite-precipitating fluid was calculated to be 1.78‰, which is close to the average δ 18 O V-SMOW value (1.74‰) of vent fluid samples from the ultramafic-hosted hydrothermal systems worldwide. The formation temperature of calcite veins II is inferred to be approximately 134 °C, based on the calculated δ 18 O V-SMOW above. The temperature differences of calcite precipitation probably resulted from the fluid cooling conductively and mixing with seawater along the presumed fractures during slow upflow. The low-temperature calcite postdates the mesh texture, while the high-temperature calcite may precipitate under relatively low water/rock ratios, alkaline and reduced conditions among the mesh texture, which is revealed by the geochemical models. Therefore, it is suggested that they both have been influenced by hydrothermal fluids and the sampling site is near the discharge zone of hydrothermal circulation.