CHEMICAL EQUILIBRIUM CONSTRAINTS ON HYDROTHERMAL FLUID COMPOSITION: REACTION OF SEAWATER AND METEORIC WATER WITH BASALT AND PERIDOTITE

Among the most common crustal hydrothermal fluids are those produced by reaction of the most abundant surface waters, meteoric water and seawater, with common rocks, such as basalt, peridotite and granite. Such reactions dramatically change fluid composition, and illustrate the critical role of rock reaction in fixing fluid composition, most notably pH and redox state. Using computer calculations of multiphase equilibria (program CHILLER), we explore basalt and peridotite reactions from moderate temperature to little-explored temperatures up to 500°C as a basis for understanding hydrothermal fluids from deep ocean hot springs and subaerial volcanic settings where meteoric waters dominate. In seawater-basalt reaction the dominant effect at T≥400°C, as at lower T, is the production of acid (H+) by precipitation of chlorite, driven by reaction of seawater Mg2+ with Al and Si from the basalt, yielding minimum pH's of 3.8 at 400° and 500°C. As at lower T, the acid conditions enable dissolution and transport of Fe, Cu and Zn, but the higher temperatures enable more than an order of magnitude larger concentrations of the metals than at T≤300°. At every temperature, aqueous Cu and Zn concentrations are limited by precipitation of sulfides, which form from sulfide produced by reduction of seawater sulfate as ferric iron minerals precipitate. The larger metal concentrations at T≥300°C reflect the enhanced stability of Cl- complexes at elevated T. In meteoric reaction with basalt at T≥300°C, pH climbs from 5.5 to 7.5 with decreasing water/rock ratio and metal concentrations remain relatively small, but substantial H2 is produced. In contrast to the seawater-basalt reaction, seawater-peridotite reaction yields no acidic pH's at any temperature explored up to 500°C (pH 5.2 to 8.1 at 500°C) because brucite and Mg-silicates buffer pH at neutral values at the elevated Mg2+ concentrations of seawater. Sulfide from reduction of seawater sulfate precipitates heavy metals in sulfide minerals, holding aqueous metal concentrations (e.g. Cu, Co) very small at the alkaline pH's of the system. At every T, reduction of H2O as magnetite precipitates yields abundant H2–two orders of magnitude more than in seawater-basalt reaction at 500°C. Meteoric water reaction with peridotite at 100°C yields hyperalkaline pH's (10-12) and abundant H2.