Brothers volcano is a submarine dacitic caldera located on the southern Kermadec arc. It is host to the NW Caldera vent field (Site U1530 and Hole U1530A) that locally discharges more focused, metal-rich fluids, and the Upper Cone hydrothermal vent field (Site U1528 and Hole U1528D) that discharges predominantly diffuse, acidic fluids (pH 1.9). These two active vent sites were drilled in 2018 by the International Ocean Discovery Program (IODP) Expedition 376. Fluid inclusions hosted in anhydrite, quartz, barite, and alunite recovered from drill core samples were studied by microthermometry, Raman spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometery (LA-ICP-MS) to obtain detailed depth profiles of temperature, salinity, and composition of the hydrothermal fluids. These analyses allow for a better understanding of complex hydrothermal processes such as phase separation and an assessment of magmatic-hydrothermal contributions while making reference to the dynamics of the deep hydrothermal fluid that rises beneath the hydrothermal vents at Brothers.

The fluid inclusions have homogenization temperatures (Th) ranging from 149° to 358°C and salinities between 0.7 and 10.0 wt % NaCl equiv at the Upper Cone site and Th of 254° to 394°C and salinities between 0.7 and 9.8 wt % NaCl equiv at the NW Caldera site. Microthermometry of fluid inclusions hosted in sulfate minerals from the NW Caldera site indicates subseafloor mixing between hydrothermal fluids and seawater. The enrichment of vapor-partitioning elements B and As in the fluid inclusions suggests phase separation subseafloor, which may be accompanied by halite dissolution and precipitation. Highly diverging Cl/Br values provide indirect evidence for halite dissolution that occurred via subseafloor convection of seawater. Petrographic observations made of the fluid inclusions, such as the recognition of combined liquid-rich and vapor-rich “boiling assemblages” and the occurrence of CO2 in the inclusions, indicate phase separation of hydrothermal fluids.

The CO2 and the content of trace elements and metals in the fluid inclusions are significantly higher than that reported for Brothers vent fluid values, which reflects a magmatic-hydrothermal contribution. At the NW Caldera site, relatively high-temperature hydrothermal fluids with high Cu (max 560 ppm) and Zn (max 740 ppm) mixed with seawater before discharging at the sea floor. Depth profiles of the fluid inclusion data identify a few specific depths of channelized (focused) hydrothermal fluid flow. We suggest that the hydrothermal fluids are mainly focused along lithological contacts which act as permeable pathways, enhancing subseafloor hydrothermal fluid flow.

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