Robert J. Rosenbauer
James L. Bischoff
1996
<p id="">Geochemical and hydrologic modeling indicates that geothermal waters in the <i>T</i> > 270°C reservoirs beneath Yellowstone National Park have HCO<sub>3</sub> ≪ Cl and contrast with waters in reservoirs at lower temperatures which attain HCO<sub>3</sub> about equal to Cl. Experiments reacting rhyolite with 0.5 molal solutions of CO<sub>2</sub> at 200° and 350°C were carried out to test the hypothesis of <a id="bbib3" class="intra_ref" href="http://www.sciencedirect.com/science/article/pii/0016703796002086#bib3">3</a> and <a id="bbib4" class="intra_ref" href="http://www.sciencedirect.com/science/article/pii/0016703796002086#bib4">4</a> to explain the chemistry of these springs: that CO<sub>2</sub> 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 CO<sub>2</sub> to form a kaolinite-like alteration product plus quartz:</p>
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<div class="mathml"><span class="mathmlsrc scrollOn"><span class="mathmlsrc scrollOn"><a class="mathImg" title="View the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-0016703796002086&_mathId=si1.gif&_user=111111111&_pii=0016703796002086&_rdoc=1&_issn=00167037&md5=14cdb41063afc63d904032465ed043d9"><img class="imgLazyJSB inlineImage" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-0016703796002086-si1.gif" alt="View the MathML source" width="1279" height="58" data-loaded="true" data-inlimgeid="1-s2.0-0016703796002086-si1.gif" /></a><span class="mathContainer hidden"><span class="mathCode">2NaAlSi3O8+2CO2+3H2O=2Na++2HCO3-+Al2Si2O5(OH)4+4SiO2.</span></span></span></span>
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<p id="">CO<sub>2</sub> reacts with water to form H<sub>2</sub>CO<sub>3</sub> which dissociates to H<sup>+</sup> and HCO<sub>3</sub><sup>−</sup>, more so at lower temperatures. Kinetic and thermodynamic considerations suggest that the reactivity of H<sub>2</sub>CO<sub>3</sub> with wallrocks is at its maximum between 150° and 200°C, consuming most of the H<sup>+</sup> and liberating equivalent amounts of cations and bicarbonate. Wallrocks in higher temperature reservoirs are relatively unreactive to dissolved CO<sub>2</sub> which is eventually lost from the system by boiling.</p>
<p id="">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.</p>
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10.1016/0016-7037(96)00208-6
en
Elsevier
The alteration of rhyolite in CO<sub>2</sub> charged water at 200 and 350°C: The unreactivity of CO<sub>2</sub> at higher temperature
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