Phase separation in submarine hydrothermal systems: evidence from the Juan de Fuca Ridge

Abstract

In order to understand processes affecting the composition of hydrothermal fluids, a detailed study was made of the chemical variability within two vent fields along the Juan de Fuca Ridge. Results indicate that phase separation is the cause of compositional gradients at both sites. Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area ($\sim$60 m in diameter). Compositions range from a 300$\sp\circ$C, gas-enriched (285 mmol/kg CO$\sb2$), low-chlorinity ($\sim$33% of seawater) fluid to a 328$\sp\circ$C, relatively gas-depleted (50 mmol/kg CO$\sb2$), high-chlorinity ($\sim$116% of seawater) fluid. The range of compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. There is good agreement between the measured fluid compositions and compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low-chlorinity fluids over levels predicted by the model are attributed to fluid-rock interaction in the upflow zone. Iron and calcium have been removed by iron-sulfide and anhydrite precipitation at some time in the history of the low-chlorinity fluids. The shallow depth to the seafloor (1540 m) and the observed chemistry at ASHES are consistent with sub-critical phase separation.Hydrothermal fluids collected at the Endeavour site in 1984, 1987 and 1988 also display major ion trends indicative of phase separation. The low chloride level of all fluids collected, the lack of strong correlation of chloride and gas content, and the repeated measurements of temperatures $>$400$\sp\circ$C are consistent with phase separation in the supercritical region ($>$407 $\sp\circ$C), resulting in removal of a gas-depleted brine.