RATES OF REACTION AND SOLUBILITY CONTROLS ON MASS TRANSFER IN HYDROTHERMAL SYSTEMS

Mass transfer in the crust at low temperatures depends on the rate of mineral-fluid reactions while mineral solubilities control its extent at high temperatures. At low temperatures steady-state far from equilibrium aluminiosilicate dissolution rates increase at both low and high pH relative to near neutral pH as a result of increased Si + Al surface charge. As temperature increases experiments indicate the rates become more pH dependent at both low and high pH with the near neutral pH region of little pH dependence expanding. At supercritical conditions, the dissolution rate mechanism for most fluid-rock reactions, involves the expanded near neutral, low pH dependent region.

At midcrustal conditions mass transfer of alkalis and alkaline earths in aqueous solutions is dominated by chloride complexes. Interactions of Si with chloride rich solutions are best modeled with Setchénow terms involving a neutral Si(OH)₄ complex as quartz solubilities change from salting out to salting in with increasing temperature. Corundum solubility experiments also show salting out at low temperatures but salting in with increasing temperature in chloride rich solutions at 1 and 2 kbar. pH effects are thought to be minimal as the increase in corundum solubility from pure water to dilute HCl solutions is low. Therefore, the dominate Al complex in chloride rich solutions is modeled with changing stability of an uncharged Al(OH)₃ species with a Setchénow term that changes sign.

Increased NaCl and CaCl₂ in fluids increases corundum solubility and therefore Al mass transfer significantly at temperatures above 450 ^oC at 1 and 2 kbar but less so for Si. The presence of significant Al-rich phases in midcrustal quartz veins is interpreted as generally due to elevated chloride concentrations and temperature with both quartz and aluminiosilicate phases precipitating with decreasing pressure and temperature as the fluids are transported upward due to buoyancy forces. The lack of aluminiosilicate phases in quartz veins suggests the fluid has low salt content.