THE CHEMISTRY OF SUBDUCTION-ZONE FLUIDS

Abstract

Subduction zones generate voluminous magma and mediate global element cycling. Fluids are essential to this activity, yet their behavior is perhaps the most poorly understood aspect of the subduction process. Though many volatile components are subducted, H2O is the most abundant, is preferentially fractionated into the fluid phase, and, among terrestrial volatiles, is by far the most effective solvent. H2O therefore controls the chemical properties of subduction-zone fluids. Rising pressure (P) and temperature (T) along subduction paths yield increased H2O ionization, which enhances dissolved solute concentrations. Under appropriate conditions, silicate solubilities may become so high that there is complete miscibility between hydrous melts and dilute aqueous solutions. Miscible fluids of intermediate composition (e.g., 50% silicate, 50% H2O) are commonly invoked as material-transport agents in subduction zones; however, phase relations pose problems for their existence over significant length scales in the mantle. Nevertheless, this behavior provides a key clue pointing to the importance of polymerization of alkali aluminosilicate components in deep fluids. Aqueous aluminosilicate polymers may enhance solubility of important elements even in H2O-rich fluids. Subduction-zone fluids may be surprisingly dilute, having only two to three times the total dissolved solids (TDS) of seawater. Silica and alkalis are the dominant solutes, with significant Al and Ca and low Mg and Fe, consistent with a role for aqueous aluminosilicate polymers. Trace-element patterns of fluids carrying only dissolved silicate components are similar to those of primitive island-arc basalts, implying that reactive flow of H2O-rich, Cl-poor, alkali-aluminosilicate-bearing fluid is fundamental to element transport in the mantle wedge. Better understanding of the interaction of this fluid with the mantle wedge requires quantitative reaction-flow modeling, but further studies are required to achieve this goal.