ALBERT

Description: Highlights • The hydrothermal fluids were sampled from a neovolcanic ridge within a non-transform offset. • Serpentinization has been involved on the pathway of hydrothermal circulation • The fluids are strongly affected by phase separation with extremely high Cl content in brine phase • A hybrid model of hydrothermal circulation controlled by tectonic and magmatic activities simultaneously was proposed. The Daxi Vent Field (DVF) is located on a neovolcanic ridge within a non-transform offset at water depths of ∼3500 m, on the Carlsberg Ridge, northwest Indian Ocean. In 2017, we investigated this site using the submersible Jiaolong and collected two fluid samples from orifices of chimneys named “Buddha's Hands” and “A1”, about 37 m apart. Their in-situ measured temperatures are 273 °C and 272 °C, respectively. The Buddha's Hands fluid is highly Cl-enriched (928 mM), while the A1 fluid is Cl-depleted (303 mM). This indicates that they have undergone phase separation. The segregated phases must have remixed during the ascent because the vapor and brine phases sampled cannot be produced by the same phase separation history without other processes. Olivine-rich and/or ultramafic mantle rocks must have been involved during the hydrothermal circulation as evidenced by high dissolved H2 (7.07 mM) and methane (0.884 mM) concentrations, a depletion in B relative to seawater, high Ca and low K, and large positive Eu anomalies. The Fe content in Buddha's Hands fluid is extremely high (11,900 μM) as a result of phase separation, while the Cu concentrations in both fluids are relatively low due to entrainment of seawater which results in precipitation of Cu-rich sulfides in the subseafloor. The concentrations of Zn, Ag, Ga, Sn, Sb, and Cd in A1 vent fluid are significantly elevated due to generation of acidity and remobilization of these elements as Cu-rich sulfides are deposited. The subseafloor processes and associated geochemistry of hydrothermal fluids at the DVF are distinct from other mid-ocean ridge hydrothermal systems due to the specific geologic setting. Hence a hybrid model of hydrothermal circulation is proposed. This study broadens our understanding of the hydrothermal processes occurring in areas of NTO setting and provides more information on mass fluxes discharging from hydrothermal systems and the formation of sulfide deposits.