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Intensive sampling of noble gases in fluids at Yellowstone: I. Early overview of the data; regional patterns

Authors:	BM Kennedy MA Lynch JH Reynolds SP Smith

Institution:	Department of Physics, University of California, Berkeley, CA 94720 U.S.A.

Abstract:	The Roving Automated Rare Gas Analysis (RARGA) lab of Berkeley's Physics Department was deployed in Yellowstone National Park for a 19 week period commencing in June, 1983. During this time 66 gas and water samples representing 19 different regions of hydrothermal activity within and around the Yellowstone caldera were analyzed on site. Routinely, the abundances of five stable noble gases and the isotopic compositions of He, Ne, and Ar were determined for each sample. In a few cases the isotopes of Kr and Xe were also determined and found to be of normal atmospheric constitution.Correlated variations in the isotopic compositions of He and Ar can be explained within the precision of the measurements by mixing of only three distinct components. The first component is of magmatic origin and is enriched in the primordial isotope ³He with $^{3} He^{4} He ≥ 16$ times the air value. This component also contains radiogenic ⁴⁰Ar and possible ³⁶Ar with $^{40} Ar^{36} Ar ≥ 500$ , resulting in a $^{3} He^{36} Ar$ ratio ≥ 41,000 times the air value. The second component is assumed to be purely radiogenic ⁴He and ⁴⁰Ar ( $^{41} He^{401} Ar = 4.08 ± .33$ ). This component is the probable carrier of observed excesses of ²¹¹Ne, attributed to the α,n reaction on ¹⁸O. Its radiogenic character implies a crustal origin in U. Th, and Krich aquifer rocks. The third component, except for possible mass fractionation, is isotopically indistinguishable from the noble gases in the atmosphere. This component originates largely from infiltrating run-off water saturated with atmospheric gases.In addition to exhibiting nucleogenic ²¹¹Ne, Ne data show anomalies in the ratio $^{20} Ne^{20} Ne$ , which correlate roughly with the $^{21} Ne^{22} Ne$ anomalies for the most part, but not as would occur from simple mass fractionation. Some exaggerated instances of the $^{20} Ne^{22} Ne$ anomaly occur which could be explained by combined mass fractionation of Ne and Ar isotopes to a severe degree coupled with remixing with normally isotopic gases. Otherwise exotic processes have to be invoked to explain the ²⁰Ne data.Relative abundances of the non-radiogenic and non-nucleogenic noble gases (²²Ne, ³⁶Ar, ⁸⁴Kr, and ¹³²Xe) are highly variable but strongly correlated. High Xe/Ar ratios are always accompanied by low Ne/ Ar ratios and vice versa. Except for water from the few cold (T < 20°C) springs analyzed, none of the samples have relative abundances consistent with air saturated water and the observed variations are not readily explained by the distillation of air saturated water.In characterizing each area of hydrothermal activity by the highest $^{3} He^{4} He$ ratio found for that area, we find that within the caldera this parameter is somewhat uniform at ~7 ± 1 times the air value. There are exceptions, most notably at Mud Volcano, an area located along a crest of recent and rapid uplift. Here the maximum $^{3} He^{4} He$ ratio is ~ 16 times the air value. Also noteworthy is Gibbon Basin which is in the vicinity of the most recent rhyolitic volcanism and exhibits a $^{3} He^{4} He$ ratio ~ 13 times the air value. Immediately outside the caldera the maximum $sol^{3} He^{4} He$ ratio decreases rapidly to values < ~3 times the air value.

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