Fluid inclusion evidence for subsurface phase separation and variable fluid mixing regimes beneath the deep‐sea PACMANUS hydrothermal field, Manus Basin back arc rift, Papua New Guinea

Altered volcanic rocks were cored from over 350 m below the seafloor at the Papua New Guinea‐Australia‐Canada Manus Basin Hydrothermal Field (PACMANUS) deep‐sea hydrothermal field, in the eastern Manus back arc basin. Fluid inclusions in anhydrite veins reveal phase separation and fluid mixing beneath the seafloor. The anhydrite precipitated from high‐temperature fluids (150–385°C). At Roman Ruins, a site of active high‐temperature venting (220–276°C, measured by submersible), the fluid inclusion thermal depth profile is uniform and high temperature (242–368°C). At Snowcap, a site of warm water effusion (6–65°C), the fluid inclusions indicate high temperatures at depth (270–385°C) but both low and high temperatures in the shallower section. This indicates a flow regime dominated by vertical advection and shallow entrainment and mixing with cool seawater. Inclusions at Snowcap exhibit extreme salinity variations due to phase separation at temperatures above 350°C. Fluids contain Na, Cl, Fe, Zn, Mg, and Ba and a minor gas component such as CO 2 or CH 4 . Most inclusions at Roman Ruins exhibit salinities that fall within the range of those observed at modern active vent sites along the mid‐ocean ridge system. Fluid inclusion temperatures support a hypothesis, developed previously from Sr‐isotopic analysis, that the subseafloor at Snowcap is characterized by mixing between deep‐sourced hot hydrothermal fluids and cold seawater‐like fluid. Both heating of seawater and cooling of upwelling hydrothermal fluids can be recognized by combining isotopic and fluid inclusion data. In contrast to Snowcap, the regime at Roman Ruins is less varied, with uniformly high‐temperature upwelling fluids that have hydrothermally dominated Sr‐isotopic ratios.