Hydrothermal interaction of Columbia Plateau basalt from the Umtanum flow (Washington, U.S.A.) with its coexisting groundwater

A series of experiments have been performed in which tholeiitic basalt from the Umtanum flow of the Columbia River plateau was reacted with the synthetic equivalent of groundwater with which it is in contact at a pressure of 300 bar, temperatures of 100°, 200° and 300°C, and water/rock mass ratios of between 5 and 50. Reaction products formed included: silica, illite, scapolite, wairakite, heulandite, K-feldspar and smectite. The alteration mineral assemblage formed during the experiments are similar to those predicted for equilibrium with geothermal waters in a tholeiitic system. Solution pH decreased relative to that of the initial solution. The final calculated high-temperature pH-values were near 7.6 at 300°C, 7.5 at 200°C and 7.2 at 100°C. The pH decrease is much less than that of basalt-seawater experiments. The pH decrease may be due to incorporation of Na into secondary products. Solution composition was modified. Silica, Al, K and sulfide increased, whereas Na decreased. The oxidation state of the solution became reduced during the experiment, approaching the hematite-magnetite-pyrite invariant point at 300°C. The solution composition from the experiments compare favorably with the compositions of some Icelandic geothermal fluids.