324 Geochemistry (Posters)

Wednesday, 28 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

EVALUATING MANTLE-CRUSTAL GAS DYNAMICS OF SOUTHWESTERN US CO2 SPRINGS USING HELIUM ISOTOPES AND HEAVY NOBLE GASES

Paper number: 324-17
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Recent work identifies a correlation between low-velocity mantle seismic anomalies and the presence of mantle-derived gases, particularly CO2 and 3He, in geothermal springs across the continental US. Still, there are considerable uncertainties in the mechanisms and timing by which mantle gases reach the crust and the degree to which mantle-lithosphere-crust interactions occur during magmatic gas emplacement and alter composition. These uncertainties limit our ability to determine the implications of low-velocity anomalies or elevated mantle contributions in crustal settings. For example, do elevated mantle contributions reflect an actively degassing mantle in regions with low-velocity anomalies? Or, have the mantle-derived gases been stored in the lithosphere or crust for extended time-periods and are only now migrating to the surface?

Although 3He/4He can provide unambiguous indications of mantle-derived gases in continental settings, helium isotope ratios can result from mixing with radiogenic 4He or variable mantle or lithospheric contributions. We anticipate that heavier noble gases (Ne, Ar, Kr, Xe) and their isotopes (particularly 21Ne, 40Ar, 129Xe, and 136Xe) may provide additional insight on the mantle vs. lithospheric source (e.g., 129Xe is dominantly primordial while other isotopes, including 136Xe, have mantle and radiogenic contributions) and the relative timing of fluid emplacement.

To address these questions, we analyzed a suite of helium and heavy noble gas isotopic compositions in concert with molecular gas composition (CO2, N2, hydrocarbons) and δ13C-CO2 in gas and water samples from geothermal springs from the San Juan mountains (CO), the Valles caldera and Rio Grande rift (NM), and the Colorado Plateau (Grand Canyon, AZ). Site selection was based on the presence of distinct low-velocity mantle anomalies. All samples display resolvable helium isotopic ratios above crustal production values, and the majority of samples also display resolvable enrichments in 129Xe relative to air and variable excesses in 21Ne*, 40Ar*, and 136Xe*. Excess 3He and 129Xe are consistent with mantle contributions, while variable radiogenic gases reflect the relative mixtures of air-saturated water, mantle, lithosphere, and the crust providing insight on their history during crustal emplacement.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287315

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