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 i n the T > 270 degrees C reservoirs beneath Yellowstone National Park h ave HCO3 much less than Cl and contrast with waters in reservoirs at l ower temperatures which attain HCO3 about equal to Cl. Experiments rea cting 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, 1 989) to explain the chemistry of these springs: that CO2 is relatively unreactive with volcanic rocks at temperatures > 270 degrees C. The e xperimental results strongly support this hypothesis. Extent of altera tion is twenty-seven times greater at 200 degrees C than at 350 degree s C. The dominant process in the experiments appears to be the alterat ion 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 rea cts with water to form H2CO3 which dissociates to H+ and HCO3-, more s o at lower temperatures. Kinetic and thermodynamic considerations sugg est that the reactivity of H2CO3 With wallrocks is at its maximum betw een 150 degrees and 200 degrees C, consuming most of the H+ and libera ting equivalent amounts of cations and bicarbonate. Wallrocks in highe r temperature reservoirs are relatively unreactive to dissolved CO2 wh ich 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 s tratigraphic section at Searles Lake, California, the terminus of the Owens River which derives its dissolved load from hot springs of the L ong Valley caldera.