Stable chlorine isotopic composition of geothermal waters from Yellowstone National Park

摘要

Sixteen spring-water samples from four different thermal areas in Yellowstone National Park (YNP) have been analyzed for their stable chlorine isotopic signatures with a standard deviation of 0.1 per thousand. The δ~(37)Cl values of the waters ranged from —0.13 to +0.42 per thousand. The most enriched δ~(37)Cl values were measured in Norris Geyser Basin, with an average value of +0.36 per thousand. Within the Gibbon Geyser Basin, waters with pH values between 2.5 and 5.5 had the most positive δ~(37)Cl signatures (≈0.3 per thousand), while an acid-sulfate water had the most depleted signature (—0.13 per thousand). Examination of the δ~(37)Cl signatures of all the spring waters versus their δ~(18)O signatures shows two possible groups of samples, with those waters that are enriched in δ~(18)O also having more positive chlorine isotopic signatures. Chloride in deep reservoir water(s) at YNP could be derived from mixing of a small amount of magmatic brine, from the leaching of rhyolitic wall rocks, or both. Although the slightly positive signatures measured for most of the hot spring waters are consistent with a magmatic brine source for chloride, the dissolved salts in these waters are believed to be derived from leaching of the rhyolitic wall rocks. In this case, measurements of the wall rock δ~(37)Cl signature could be used to test this hypothesis. More extensive surveys of the δ~(37)Cl signatures of geothermal waters from YNP may expand the range of δ~(37)Cl, and verify the possible relationship between chlorine isotopic composition and other geochemical parameters.