Serpentinization and calcit e precipitation of mantle peridotites exhumed along de-tachment faults at the slow-to ultraslow-spreading centers can provide important clues to the hydro-thermal alteration processes.The Tianxiu hydrothermal field is a new-found active and ultramafic-hosted hydrothermal vents ite along the Carlsberg Ridge,Northwest Indian Ocean.Two types of calcite veins are recognized in serpentinized harzburgite samples collected from the seafloor at t he 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 fracture s that cut through mesh texture in the highly serpentinized harzburgi te,while calcite veins II precipitate within the mesh texture in the relatively weaker serpentiniz ed harzburgite.Both veins show similar δ13CPDB(+0.54‰and+0.58‰)but different δ18OPDB(-16.67 ‰and+4.46‰)values,suggesting that they were derived from the same carbon source but preci pi-tated at different temperatu res.Taking t he deep seawater temperature of 2 ℃ as the precipitation temperature of the calcite veins I,the equilibrium δ18OV-SMOW of calcite-precipitating fluid was c al-culated to be 1.78‰,which is close to the average δ18OV-SMOW value(1.74‰)of vent fluid samples from the ultra mafic-hosted hydrothermal systems worldwide.The formation temperature of calci te veins II is inferred to be app roximately 134 ℃ ,based on the calculated δ18OV-SMOW above.The te m-perature differences of calcite precipitation probably resulted from the fluid cooling conductively and mixing with seawater along the presumed fractures during slow upflow.The low-temperatu re calcite postdates the mesh texture,while the high-temp erature calcite may precipitate under rel a-tively low water/rock ratios,alkaline and reduced conditions among the mesh texture,which is re-vealed by the geochemical models.Therefore,it is suggested that they both have been influenced by hydrothermal fluids and the sampling site is near the dis charge zone of hydrothermal circulation.