TRACKING INTRUSIONS BENEATH THE YELLOWSTONE CALDERA WITH GAS GEOCHEMISTRY

A six-year program of sampling geothermal gases and associated waters provides insight on intrusions, wallrocks, and degassing processes beneath Yellowstone Caldera. Though all gases are CO₂-dominated (>95% on a water-free basis), other gases display large regional variations yet local uniformity. For example, Norris and the Upper Geyser Basin are notably constant in molar gas ratios such as CH₄/He [67±12 (n=22) and 225±20 (n=11) respectively]. Such uniformity in gas ratios implies thorough homogenization, at least at a local (basin) scale. The regional differences, however, may reflect gas derivation from diverse crustal wallrocks such as rhyolite, Tertiary Absaroka volcanics and Paleozoic metasediments. For example, Washburn Hot Springs and Hot Spring Basin on the eastern caldera boundary have high relative CH₄ concentrations, probably resulting from thermal breakdown of Cretaceous and Tertiary (Absaroka) sediments. Such a crustal signature is nearly absent in a 13-km-long NNW trend in the eastern part of the caldera, including Mud Volcano, and in the 4-km-long, NE trending Gibbon River Canyon in the western part of the caldera, both areas where He isotope (R_C/R_A) values exceed 10. Gas samples from these areas have <30 ppm He (in dry gas), H₂ <2000 ppm, CH₄/He < 50, and δ¹³C CO₂ values between -2 and -4.5. He isotope ratios are positively correlated with δ¹³C of CH₄, but anti-correlated with He concentration. These gases show the least influence of crustally derived He and CH₄, either because they are fed by more recent intrusions (our preferred explanation), or because they reside in areas with infertile (with respect to gas) or previously degassed crust. Either way, these regions represent areas of more direct gas upflow as they lack the crust-rich, low R_C/R_A gas sources seen elsewhere in and near the caldera.