Continental-scale links between the mantle and groundwater systems of the western United States: Evidence from travertine springs and regional He isotope data

to understand regional mantle degassing, we compiled new and existing helium isotope data measured in hot springs, gas fields, and travertine-depositing cool springs and compared these geochemical data with mantle velocity structure determined from tomographic studies. these data suggest heterogeneous mantle degassing, with regions of highest He/He in groundwaters (hence, highest mantle helium contribution) corresponding to regions of lowest mantle velocity, a reflection of tectonically active and partially molten mantle. new He isotope and water chemistry data from travertinedepositing cool springs of the western United States show marked variability consistent with mixing between surface water recharge and inputs from deep crustal and mantle sources. the deeply sourced end-member fluids of these mixing trends have high He/He, high dissolved cO 2 , and high salinity compared to shallow recharge waters, and commonly have elevated trace element concentrations. consequently, these fluids cause degradation of water quality in western U.S. aquifers. Our conclusions highlight a connection between neotectonics (e.g., mantle degassing) and water quality in the western United States. INTRODUCTION Distributed deformation associated with the western north American plate margin extends >1000 km inboard from the San Andreas fault zone to the rocky Mountain and western Great Plains regions. this region forms an orogenic plateau with high average heat flow and is characterized by relatively low upper mantle P-wave velocities with marked heterogeneity (Godey et al., 2003; Humphreys et al., 2003). Progressive geochemical depletion of the upper mantle during generation of basaltic GSA Today: v. 15, no. 12, doi: 10.1130/1052-5173(2005)015 2.0. cO;2 melt likely occurred in several episodes since the Proterozoic (Karlstrom et al., 2005). the mantle was hydrated by flat-slab subduction during the laramide orogeny (Humphreys et al., 2003) and now is partially molten, leading to small-scale convective exchange between an upwelling asthenosphere (Gao et al., 2004) and compositionally variable lithosphere (Dueker et al., 2001; Karlstrom et al., 2005). the mantle underlying western north America is marked by one of the largest known shear wave velocity contrasts on earth (van der lee and nolet, 1997). At the continental scale, this transition reflects the heterogeneous thinning and warming of north America’s lithospheric keel as the plate moved southwest in absolute plate motion in the cenozoic into a wide zone of warm asthenosphere (cDrOM Working Group, 2002). We hypothesize that cO 2 -rich mineral springs and related travertine deposits in the western United States are a manifestation of this mantle tectonism, and hence the geochemistry of spring waters and gases can be used in conjunction with geophysical data sets to understand mantle heterogeneity and the processes of lithosphere-asthenosphere interaction. We report new water and gas chemistry with associated carbon and helium isotope data in the context of a synthesis of the existing noble gas isotope chemistry database for western north America. Our literature synthesis (table Dr1) builds on previous work in the area, with the regional helium isotope data presented in the context of a tomographic image of today’s mantle. We also show that travertine-depositing cool springs contain mantle-derived volatiles in a variety of locations and tectonic settings throughout the western United States, such that many aquifer systems are influenced by mixing of deeply sourced and circulated waters. HE ISOTOPES—BACKGROUND the isotope geochemistry of noble gases is a sensitive tracer of mantle-derived volatiles even with a large input of volatiles derived from earth’s crust. this is because the mantle has retained a significant fraction of the terrestrial inventory of the primordial isotope He acquired during earth formation (clarke et al., 1969), and it is still leaking to earth’s surface. in contrast, the crust has been extensively reworked over geological time and has retained very little He: its helium inventory is dominated by radiogenic He produced from the decay of Uand th-series nuclides. consequently, helium presently emanating from regions of mantle melting, such as mid-oceanic ridges or helium trapped in glass and phenocrysts in mid-oceanic-ridge basalts (MOrB), is characterized by a relatively high He/He ratio (r) of 8 ± 1 times that of air (r A ), which has a He/He ratio of 1.4 × 10 (Graham, 2002). indeed, values as high as 37 × r A have been observed in some ocean island basalts (Hilton et al., 1999) and are thought to be related to deep plumes tapping less degassed mantle reservoirs. When mantle-derived fluids are injected into the crust, mantle helium becomes progressively diluted by crustal helium characterized by low He/He ratios of ~0.02 r A . therefore, any value higher than 0.1 GSA Data repository item 2005199, a description of sampling and analytical methods and geochemical data tables Dr1–Dr3, is available online at www. geosociety.org/pubs/ft2005.htm or on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, cO 80301-9140, USA, or editing@geosociety.org.