Basalt-Fluid Interaction at Supercritical Conditions (400?C, 500 Bar): an Experimental Approach

Abstract

Two experimental simulations of basalt-fluid interaction under supercritical hydrothermal conditions (400?C – 500 bar) have been performed. The goal is to examine the interaction between two fluids (distilled water and geothermal brine) with basalt in terms of secondary mineral compositions with respect to fluid composition and time.
The basalt was reacted using a flow-through high P-T hydrothermal apparatus. Results indicate that high silica concentrations, which reached quartz saturation early in the experiments, are due to the devitrification of volcanic glass. Low pH is attributed to the release of sulfur from the glass.
SEM analysis for the basalt-distilled water experiment shows an assemblage of grossular - wollastonite. Diopside was also identified by XRD. In the brine experiment, SEM analysis shows a secondary assemblage of amphibole – biotite.
Activity-activity diagrams indicate that fluid compositions are reasonably consistent with known mineral equilibria implying that reaction rates are extremely rapid at these conditions.