Parameters of subsurface brines and hydrothermal processes 12–15 months after the 1999 magmatic event at the Main Endeavor Field as inferred from in situ time series measurements of chloride and temperature

Abstract

Phase separation at mid‐ocean ridge hydrothermal systems is a widely distributed frequently occurring process. It is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid. Although vapor phases produced by phase separation are observed in a variety of hydrothermal settings, observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood. Here we use an array of in situ chloride sensors deployed at the Main Endeavor Field on the Juan de Fuca Ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution. Our results provide evidence of near‐critical chloride depleted fluids within the crust up to 486 m beneath the seafloor. We estimate that a brine reservoir stored beneath the Main Endeavor Field is constrained to temperatures between 131°C and 326°C for chloride concentrations ranging from 6 to 25 Wt.% NaCl, respectively. Our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters. Under these circumstances, simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events.