Geochemical and hydrologic modeling indicates that geothermal waters in the T > 270??C reservoirs beneath Yellowstone National Park have HCO3 ??? Cl and contrast with waters in reservoirs at lower temperatures which attain HCO3 about equal to Cl. Experiments reacting rhyolite with 0.5 molal solutions of CO2 at 200?? and 350??C were carried out to test the hypothesis of Fournier (1981, 1989) to explain the chemistry of these springs: that CO2 is relatively unreactive with volcanic rocks at temperatures >270??C. The experimental results strongly support this hypothesis. Extent of alteration is twenty-seven times greater at 200??C than at 350??C. The dominant process in the experiments appears to be the alteration of the albitic component of the rhyolite by dissolved CO2 to form a kaolinite-like alteration product plus quartz: 2NaAlSi3O8 + 2CO2 + 3H2O = 2Na+ + 2HCO-3 + Al2Si2O5(OH)4 + 4SiO2, CO2 reacts with water to form H2CO3 which dissociates to H+ and HCO-3, more so at lower temperatures. Kinetic and thermodynamic considerations suggest that the reactivity of H2CO3 with wallrocks is at its maximum between 150?? and 200??C, consuming most of the H+ and liberating equivalent amounts of cations and bicarbonate. Wallrocks in higher temperature reservoirs are relatively unreactive to dissolved CO2 which is eventually lost from the system by boiling. These observations also offer a possible explanation for the change in chemical sediments from chloride-dominated to bicarbonate-dominated salts found in the stratigraphic section at Searles Lake, California, the terminus of the Owens River which derives its dissolved load from hot springs of the Long Valley caldera.
Additional Publication Details
The alteration of rhyolite in CO2 charged water at 200 and 350??C: The unreactivity of CO2 at higher temperature