Effect of boiling on the acidity of hydrothermal solutions

Natural seawater and H2O-NaCl solutions were equilibrated along the two-phase (liquid-vapor) curves between 150 and 390 degrees C to re-examine the effect of liquid-vapor phase separation on element fractionations between saline brines and vapor phases. The experimental setup allows vapor and brine to be sampled separately at in situ P-T conditions. Experimental vapor pressure is constrained by temperature and the electrolyte concentration of the brines. With increasing temperature, dissolved electrolytes react with increasing temperature to hydroxides and HCl, in both seawater and H2O-NaCl solutions. The extent of hydrolysis is more pronounced in seawater than in H2O-NaCl solutions because seawater contains, in addition to NaCl, a range of other electrolytes capable of hydrolysis. Associated HCl has a great affinity to fractionate to the vapor phase when phase separation occurs. At 365 degrees C, halite-saturated vapor phases have a pH(25) (pH after condensation to 25 degrees C) of 1.8 (seawater) and 2.8 (H2O-NaCl brines). Our data suggest that boiling of hydrothermal solutions followed by mixing of vapor condensates with seawater can impose pH(25) values as acidic as the most acidic natural hydrothermal solutions vented on the ocean floor