The alteration of rhyolite in CO2 charged water at 200 and 350 degrees C: The unreactivity of CO2 at higher temperature

Geochemical and hydrologic modeling indicates that geothermal waters in the T > 270 degrees C reservoirs beneath Yellowstone National Park have HCO3 much less than 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 degrees and 350 degrees 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 degrees C. The experimental results strongly support this hypothesis. Extent of alteration is twenty-seven times greater at 200 degrees C than at 350 degrees 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: 2NaAlSi(3)O(8) + 2CO(2) + 3H(2)O = 2Na(+) + 2HCO(3)(-) + Al2Si2O5(OH)(4) + 4SiO(2). CO2 reacts with water to form H2CO3 which dissociates to H+ and HCO3-, more so at lower temperatures. Kinetic and thermodynamic considerations suggest that the reactivity of H2CO3 With wallrocks is at its maximum between 150 degrees and 200 degrees 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.