ALBERT

Description: The distribution of hydrothermal vents and the biogeography of associated faunal communities in the Indian Ocean are still not well studied. This is especially true for Carlsberg Ridge, the northernmost part of the Indian Ocean spreading system. Here we report geological, morphological, biological, and hydrochemical data for the newly discovered Daxi Vent Field (DVF) on the slow-spreading Carlsberg Ridge at 6°48′N. The DVF is a basalt-hosted hydrothermal field situated atop a rifted volcanic ridge, located in a non-transform offset between two second-order ridge segments. There are three hydrothermal sites, i.e. Central mound, NE mound, and South mound. Eight vigorously venting black smokers were observed in the central hydrothermal mound. The largest sulfide chimney “Baochu Pagoda” is ~24 m tall. Another inactive chimney, which is silica-rich is observed in the NE mound. The sulfide chimneys are dominated by sphalerite and pyrrhotite containing high Sn, Co and Ag. The silica-rich chimney contains high SiO2 and Ba contents. Seven species of megafauna were identified, including alvinellid worms, which were collected in the Indian Ocean for the first time. Rimicaris kairei and actinostolid anemones dominate the community in the central areas and on the periphery of the vent field, respectively. The occurrence of DVF is quite unique as it is located on a non-transform offset and it is mafic-hosted. So far only nine hydrothermal fields with the similar geological setting have ever been reported among nearly 700 hydrothermal sites in the World’s Ocean. Graphical abstract The mafic-hosted Daxi Vent Field with high-temperature hydrothermal mineralization was discovered at a non-transform offset along the slow-spreading Carlsberg Ridge.