Origins of geothermal gases at Yellowstone

    Origins of geothermal gases at Yellowstone Jacob B. Lowenstern, Deborah Bergfeld, William C. Evans, Andrew G. Hunt PII: DOI: Reference:

S0377-0273(15)00186-9 doi: 10.1016/j.jvolgeores.2015.06.010 VOLGEO 5566

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Accepted date:

18 March 2015 15 June 2015

Please cite this article as: Lowenstern, Jacob B., Bergfeld, Deborah, Evans, William C., Hunt, Andrew G., Origins of geothermal gases at Yellowstone, Journal of Volcanology and Geothermal Research (2015), doi: 10.1016/j.jvolgeores.2015.06.010

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Origins of geothermal gases at Yellowstone

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Jacob B. Lowenstern, Deborah Bergfeld, William C. Evans, Andrew G. Hunt

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U.S. Geological Survey, Menlo Park, CA USA U.S. Geological Survey, Denver, CO USA

ABSTRACT

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Gas emissions at the Yellowstone Plateau Volcanic Field (YPVF) reflect open-system mixing of gas species originating from diverse rock types, magmas, and crustal fluids, all

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combined in varying proportions at different thermal areas. Gases are not necessarily in chemical equilibrium with the waters through which they vent, especially in acid sulfate terrain where bubbles stream through stagnant acid water. Gases in adjacent thermal

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areas often can be differentiated by isotopic and gas ratios, and cannot be tied to one

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another solely by shallow processes such as boiling-induced fractionation of a parent liquid. Instead, they inherit unique gas ratios (e.g., CH4/He) from the dominant rock

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reservoirs where they originate, some of which underlie the Quaternary volcanic rocks. Steam/gas ratios (essentially H2O/CO2) of Yellowstone fumaroles correlate with Ar/He and N2/CO2, strongly suggesting that H2O/CO2 is controlled by addition of steam boiled from water rich in atmospheric gases. Moreover, H2O/CO2 varies systematically with geographic location, such that boiling is more enhanced in some areas than others. The 13C and 3He/CO2 of gases reflect a dominant mantle origin for CO2 in Yellowstone gas. The mantle signature is most evident at Mud Volcano, which hosts gases with the lowest H2O/CO2, lowest CH4 concentrations and highest He isotope ratios (~16Ra), consistent with either a young subsurface intrusion or less input of crustal and meteoric gas than any other location at Yellowstone. Across the YPVF, He isotope ratios (3He/4He) inversely vary with He concentrations, and reflect varied amounts of longstored, radiogenic He added to the magmatic endmember within the crust. Similarly, addition of CH4 from organic-rich sediments is common in the eastern thermal areas at Yellowstone. Overall, Yellowstone gases reflect addition of deep, high-temperature magmatic gas (CO2-rich), lower-temperatures crustal gases (4He- and CH4-bearing), and

ACCEPTED MANUSCRIPT those gases (N2, Ne, Ar) added principally through boiling of the meteoric-water-derived geothermal liquid found in the upper few kilometers. We also briefly explore the

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pathways by which Cl, F, and S, move through the crust.

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1. Introduction

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For over two million years, magmas of the Yellowstone Plateau Volcanic Field (YPVF) have erupted as abundant rhyolitic and basaltic lavas and tuffs, and have contributed heat

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and gas to one of the planet’s most active geothermal systems1 (Allen and Day, 1935;

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Christiansen, 2001; Fournier, 1989; Hurwitz and Lowenstern, 2014). Diverse surface manifestations of the geothermal system include the acid-sulfate barrens of the Central

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Plateau, the travertine terraces of Mammoth Hot Springs, and the silica-sinters of geyser

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basins along the Firehole River. Fluids from each type of thermal area have a distinct

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chemistry, which in turn relates to the dynamics of the subsurface magma-hydrothermal system (Allen and Day, 1935; Fournier, 1989; Lowenstern and Hurwitz, 2008). Most of our understanding of the workings of the Yellowstone geothermal system comes from studies of the waters that issue from geyser basins and from the shallow wells drilled in the 1960s (White et al., 1975; Fournier et al., 1989). For example, we know that the hot waters are rich in solutes derived from high-temperature interaction with buried rhyolites at temperatures typically ranging from 150 to 275°C (Fournier, 1989). We know that the waters are largely meteoric in origin, and have isotopic 1

In this manuscript, we use the term “geothermal” for those waters, gases, and rocks in the upper crust (upper few km) that are at elevated temperatures due to transfer of anomalous subsurface heat. We reserve the term “magma-hydrothermal” for the broader crustal system that transfers heat and mass from the mantle to the surface through advective, convective and conductive processes. At the surface, acid sulfate barrens, geyser basins and other hydrothermally altered terrain are referred to simply as “thermal areas.” 2

ACCEPTED MANUSCRIPT compositions similar to local rainwaters that recharged the groundwater decades to centuries ago (Craig, 1956; Pearson and Truesdell, 1978; Kharaka et al. 2002; Rye and

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Truesdell, 2007): and we know that most acid waters at the surface contain condensed

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steam from boiling of subsurface neutral thermal waters (White et al., 1971; Fournier, 1989; Goff and Janik, 2000; Nordstrom et al., 2009; Lowenstern et al., 2012).

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Less is understood about the crustal and magmatic roots of the system (i.e., the

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magma-hydrothermal system). Studies of gases offer the potential to trace deep input of mass and energy from beneath the shallow, meteoric-dominated, geothermal reservoirs.

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Early studies of noble gases (Craig et al., 1978) identified a strong magmatic signal in the fluids feeding the Mud Volcano area of Yellowstone. More in-depth studies confirmed

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that crustal gases also mix with the magmatic and meteoric endmembers (Kennedy et al., 1985). Chiodini et al. (2012) pointed out that gas chemistry and isotopes could be used to

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identify a high-temperature (340°C) magma-hydrothermal signature, as well as a lowertemperature (~170°C) crustal input. The gases mix and partly equilibrate with the meteoric-derived hot waters that reside in the upper crust and feed the geyser basins, and are ultimately discharged at the surface. Werner and Brantley (2003) demonstrated that the flux of CO2-rich gas at Yellowstone requires considerable input of mantle-derived basalt (see also Christiansen, 2001) that transmits heat and mass into and through the superjacent rhyolitic and geothermal systems. Lowenstern and Hurwitz (2008) further concluded that the gas flux likely induces phase separation (vapor and liquid) in the upper several kilometers of the geothermal system. Therefore, the high efflux of gas seems to require open-system addition of mass and heat through the meteoric-dominated hot water system that resides in the upper crust.

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ACCEPTED MANUSCRIPT Over the past decade, our research has focused on collecting gases from geographically and chemically diverse areas within Yellowstone National Park. In a

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series of papers, we’ve explored specific sites such as Hot Spring Basin (Werner et al.,

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2008), Brimstone Basin (Bergfeld et al., 2012), Heart Lake Geyser Basin (Lowenstern et al., 2011), Mud Volcano (Evans et al., 2010), and more regional topics (Bergfeld et al.,

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2011, 2014; Chiodini et al., 2012; Lowenstern et al., 2014). In this paper, we review

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current knowledge about gas sources within the crust and mantle, the relationship between geographically distinct thermal areas, and the movement of gas relative to

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waters in geothermal systems at Yellowstone. Our goal is to show how the recent data demand a rethinking of controls on gas and steam chemistry from those that were

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proposed decades ago. We highlight the evidence for widespread open-system mixing of gases from disparate sources beneath and surrounding the caldera. We demonstrate that

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closed-system boiling of geothermal waters, though a common process at Yellowstone, cannot explain most of the observed variations of gas ratios and isotopes. We observe that some geothermometers may be affected by open-system addition of gas species at rates too fast for equilibration. And finally, we stress that geothermal gases form through addition of components sourced from magmatic, metamorphic, sedimentary and meteoric sources, where each gas species tells a different story of its journey through the magmahydrothermal system to the surface.

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ACCEPTED MANUSCRIPT 2. Setting 2.1. Geology overview

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The basement beneath the YPVF is dominated by Archean rocks of the Beartooth

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Province, igneous and meta-igneous rocks that were largely formed between 3.5 and 2.5 Ga (Mueller et al., 2008). Evidence for Proterozoic rock is largely missing, but Paleozoic

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and Mesozoic marine sedimentary rocks are found in the Gallatin Range and in

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mountains immediately south of the caldera (Christiansen, 2001; Fig. 1). Thick deposits of shale and sandstone from the Cretaceous inland seaway are found on Mount Everts in

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the north (Ruppel, 1982). In the Eocene Epoch (Tertiary Period), the Absaroka Supergroup, a thick series of trachyandesite lavas, volcaniclastic rocks and sediments

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were deposited along a ~200 km SE-NW trend, overlapping the eastern half of Yellowstone (Smedes and Protska, 1972). Petroleum seeps and hydrocarbon-rich gases

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are well known in the eastern part of Yellowstone and are thought to be sourced from the organic-rich sediments in the Absaroka rocks, as well as the older units that likely underlie them (Love and Good, 1982). Quaternary Yellowstone volcanism began about 2.5 million years ago, with basalts found at the northern edge of the plateau (Christiansen, 2001). The first rhyolitic eruption, the Snake River Butte, closely preceded the cataclysmic eruption of the Huckleberry Ridge Tuff (Christiansen, 2001). Over the past two million years, eruptions of the YPVF were dominated by rhyolite units that were only minimally interbedded with non-volcanic and glacial sediments. The primary units include lavas and pyroclastic rocks during three volcanic cycles, each culminated by a large caldera-forming ignimbrite (the Huckleberry Ridge Tuff at 2.1 Ma, the Mesa Falls Tuff at 1.3 Ma and the Lava Creek Tuff at 0.64 Ma (Christiansen et al., 2007). Post-

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ACCEPTED MANUSCRIPT caldera volcanism products (pink in Fig. 1) are principally composed of the Upper Basin Member rhyolites and the more recent Central Plateau Member that erupted from ~160

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ka to 70 ka (Christiansen et al., 2007). The estimated total volume of erupted magma is

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>6500 km3 (Christiansen, 1984), almost all of which was deposited near present-day Yellowstone. No pre-YPVF rocks crop out within the Yellowstone caldera. Scientific

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drilling as deep as ~300 meters did not encounter older rocks. However, in parts of the

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eastern caldera, especially close to the caldera rim (Fig. 1), map relations indicate that depths to pre-Quaternary rocks are likely <300 m (R.L. Christiansen, personal

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communication, 2014). Resurgent domes within the Yellowstone Caldera (Mallard Lake and Sour Creek in Fig. 1) are regions of active and recurrent uplift and subsidence related

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to magmatic influx and related hydrothermal activity beneath the caldera (Husen et al.,

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2004; Smith et al., 2009; Farrell et al., 2014).

2.2. The distribution of thermal areas and their associated waters About 65 km2 of Yellowstone National Park (~9000 km2), is mapped as thermal ground (Vaughan et al., 2014). Over half of that ground is steam-heated terrain (also known as acid-sulfate) composed dominantly of kaolinite and mixed kaolinite-alunite soil, (Livo et al., 2007). Thermal waters found within acid sulfate terrain represent a mixture of condensed fumarolic steam and perched dilute groundwater (Nordstrom et al., 2009). Only rarely are fumaroles found to be more than a few degrees above the local boiling temperature. The maximum temperature measured by Bergfeld et al. (2011, 2014) is 115°C, though temperatures as high as 135°C were recorded in the 1920s at the Norris Geyser Basin by Allen and Day (1935). When fumarolic steam condenses near the

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ACCEPTED MANUSCRIPT surface, some of the H2S is dissolved and converted to sulfuric acid, strongly aided through biogenic reactions. This results in an acid fluid that reacts with the surface rocks

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to create clays and forms the muddy acid sulfate terrains (Nordstrom et al., 2009). The

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acid-sulfate terrain is typically thought to overlie vapor-dominated (steam) reservoirs in the shallow subsurface, as was confirmed by scientific drilling at Mud Volcano (White et

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al., 1975).

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Neutral to alkaline, Cl-rich waters emerge in low elevation basins at temperatures close to the local boiling temperature. Because these waters become super-saturated with

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respect to opaline silica, terraces of silica sinter are created as the waters flow away from their vents (Jones and Renaut, 2003). Other forms of silica, such as geyserite are created

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where abundant splashing and spraying of silica-saturated water forms knobby-towers and benches (Campbell et al., 2015). Virtually all geysers are associated with neutral to

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alkaline (neutral chloride) water and spout through vents made of silica. Fournier (1979, 1989) focused most of his research on these neutral chloride fluids, finding evidence that most of the waters in and near the Yellowstone caldera could be derived from a single parent fluid at 340°C and containing ~400 mg/L Cl. Variable mixing, cooling and boiling paths would produce the diversity of neutral chloride waters found in the geyser basins. Though dominated by neutral chloride waters, most geyser basins do contain some acid waters, presumably because subsurface boiling may generate local fumarolic zones where sulfur oxidation creates acidic steam condensates and mud pots. Norris Geyser Basin, north of the caldera within the 640-ka Lava Creek Tuff, is well known for its mixture of acid and neutral pools, along with mixed Cl-SO4 waters. Within the caldera, such mixed waters are rare.

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ACCEPTED MANUSCRIPT Morgan and Shanks (2005) noted that within the caldera, the locations of thermal areas often correlate with the edges of lava flows. This leads one to consider whether

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these thermal areas may have been distributed differently prior to eruption of the adjacent

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lavas. Christiansen et al. (2007) estimated that over 600 km3 of observed and buried Central Plateau Member rhyolite lavas were emplaced between ~170ka and 70 ka within

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the caldera. These post-caldera lava flows are generally thick, and far less fractured than

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the underlying, fracture-riddled Lava Creek Tuff (Jaworowski et al., 2006). One explanation for such a distribution is that springs previously emerged along

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fractures within the Lava Creek Tuff (as at Norris) or along fractures in post-caldera sediments and lavas, but later were buried by the lava effusions that make up the various

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plateaus in and around the Yellowstone caldera (Hurwitz and Lowenstern, 2014). The buried (trapped) thermal waters would then flow along the base of lavas like the Elephant

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Back or Nez Perce flows (Fig. 7 of Hurwitz and Lowenstern, 2014) until they encounter the ground surface and emerge at low elevations (e.g., the Lower Geyser Basin). In contrast, steam produced by deep boiling is able to rise along any existing fractures or permeable volcanic vents to create acid sulfate terrains within and on top of the lava plateaus (e.g., Central Plateau, Smokejumper Hot Springs, Phantom Fumarole; all in Fig. 2). Thus, the burial of previous thermal areas beneath the Central Plateau Lavas enhanced the separation of steam and gas from residual thermal waters, aiding the formation of geochemically disparate acid sulfate and neutral chloride thermal areas. In the caldera, travertine-forming waters are found only at the cool periphery of geothermal areas, such as near Firehole Lake, at Hillside Springs, and at Terrace Springs (Fournier, 1989). Outside the caldera, at the northern end of Yellowstone, the Mammoth

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ACCEPTED MANUSCRIPT Hot Springs thermal area is characterized by abundant travertine precipitation forming from bicarbonate-rich waters that equilibrate at relatively low temperature (~73°C in the

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subsurface; White et al., 1975).

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Within the caldera, high volumes of groundwater circulation can be inferred through the annual hydrographs, where the ratio of maximum annual discharge to base

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flow is much lower than outside the caldera where non-volcanic terrain dominates

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(Gardiner et al., 2010). Together with the dearth of cold springs in the caldera, this reflects a high porosity, fractured reservoir where abundant meteoric groundwater

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absorbs heat from the underlying magma-hydrothermal system (Gardiner et al., 2010). Outside the caldera, near the Norris Geyser Basin, Gardiner et al. (2011) used multiple

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isotopic and other tracers to demonstrate that intermediate-temperature springs represented a mixture of young cold recharge water with a premodern (>60 years)

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hydrothermal endmember. The age of the premodern water has been variously estimated to be hundreds to thousands of years old based on D, 18O and 3H data (Rye and Truesdell, 2007; Kharaka et al., 2002).

2.3 Short overview of previous work on Yellowstone gases The CO2-rich nature of Yellowstone gases was recognized by Allen and Day (1935), who provided quality analyses for ~40 gas samples. They also noted that gases bubbling through neutral chloride waters were more N2-rich than gases from acid waters. Though many later researchers studied Yellowstone gases, very few provided full analyses of the entire composition. Sheppard and others (1992) published plots based on gases sampled in pools between 1974 and 1986. They demonstrated that gases obtained

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ACCEPTED MANUSCRIPT from bubbles that issue through large pools were unlikely to reflect the chemistry of deep gas due to differential gas solubility in the near-surface water, convective circulation and

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mixing with air. They urged workers to avoid sampling from pools and instead to focus

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on gas from fumaroles and frying pans (boiling ground with visible bubbling water). They also demonstrated that most Yellowstone gases represent a mixture of gases derived

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from air-saturated meteoric water with a He-rich endmember. Hearn et al. (1990)

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provided a full set of gas and isotope analyses for samples from Shoshone Geyser Basin. Werner and Brantley (2003) did the same for samples at Mud Volcano and a few areas in

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eastern Yellowstone. A full compendium of ~130 gas analyses was published by Bergfeld et al. (2011) and updated to 169 analyses in 2014 (Fig. 2). They also reviewed

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most of the pre-existing literature on gas discharge at Yellowstone. Chiodini et al. (2012) independently analyzed some replicate samples as those published by Bergfeld et al.

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(2011) and also undertook a detailed analysis of noble gas chemistry and geothermometry.

The sources of individual gas components were first explored through stable isotope geochemistry by Craig (1953) and then Craig et al. (1956). Craig (1953) found 13C-CO2 values (-2.8 per mil) similar to those seen today, but interpreted their origin in light of the existing thought that igneous carbon had an isotopic composition <–20 per mil. Craig et al. (1956) showed that water and steam from Yellowstone were almost entirely of meteoric origin. Gas ratios determined by Mazor and Wasserburg (1965) and later Gunter and Musgrave (1966) also pointed to a clear meteoric origin of the waters, as noble gas ratios (other than He) displayed atmospheric values.

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ACCEPTED MANUSCRIPT The presence of high 3He/4He isotope ratios (up to 16 times the air ratio, or RA) were first revealed by Craig et al. (1978), who recognized the mantle hotspot signature at

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Yellowstone. Kennedy et al. (1985) later provided values for He isotopes from the entire

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park, and showed that gases generally represented a three-way mixture of magmatic, crustal and meteoric sources.

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Other workers have focused on the origin of organic gases such as methane (CH4)

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and higher hydrocarbons. Des Marais and others (1981) demonstrated that the decomposition of organic-bearing sediments was responsible for generating methane in

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the petroleum-rich areas in the eastern part of Yellowstone. Clifton and others (1990) studied “hydrothermal” petroleum and inferred Eocene mudstone sources for Rainbow

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Springs, whereas petroleum found at Calcite Spring was derived from the Permian Phosphoria Formation and recent sediments filling the valley of the Yellowstone River.

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Lorenson and Kvenvolden (1993) published hydrocarbon gas and isotopic analyses from these and a variety of other springs and seeps throughout the park. The diffuse flux of CO2 at Yellowstone was first explored by Werner et al. (2000) and Werner and Brantley (2003) who demonstrated that soil emission rates from Yellowstone (45 kt d-1) were among the highest of any volcanic system on Earth. Subsequent studies by Werner et al. (2008), Lowenstern et al. (2011), and Bergfeld et al. (2012) have explored the range of gas discharge at acid sulfate and other thermal areas throughout Yellowstone. Though they largely confirm the high discharge rates from Yellowstone, estimates of the total CO2 discharge are still highly approximate. Hurwitz and Lowenstern (2014) conservatively estimated diffuse CO2 emissions of 10 to 60 kt d-1 from the Yellowstone hydrothermal system.

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3. Key Observations on Gas and Fluid Geochemistry

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Below, we discuss nine topics informed by our ongoing research. Findings from earlier

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work will be clearly cited, but some of the discussion is presented here for the first time,

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at least as applied to Yellowstone.

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3.1.The geothermal system is “open” to gain and loss of vapor (gas and steam) A system can be defined as “open” if mass and energy can freely move in or out

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of its defined boundaries. At Yellowstone, variations in gas flow relative to thermal water argue for such open behavior. A casual observer at Yellowstone may discern that

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the amount of gas and water emerging from thermal features can vary greatly. Allen and Day (1935) first recognized that little water flowed from the acid terrains, whereas

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considerable discharge occurred at neutral chloride features (Fig. 3). Moreover, they found greater gas discharge associated with the acid waters, where they were able to collect one liter of gas in about 6 minutes, whereas it could take over an hour to collect the same volume of gas from neutral chloride waters (Allen and Day, 1935: p. 90). This variation in gas discharge can be explained if some neutral chloride waters have lost gas prior to discharge (see section 2.2), whereas in acid sulfate terrain, perched acid waters are continually flooded with gas from below (potentially originating from boiling of deep neutral chloride fluids). The parkwide variations in gas discharge can be appreciated by quantifying the mass balance of liquid and gas discharge from various thermal areas. For example, about 78 L s-1 of gas-poor neutral chloride water emerges at the Heart Lake Geyser Basin.

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ACCEPTED MANUSCRIPT Through study of the diffuse CO2 flux and gas chemistry, Lowenstern et al. (2012) estimated that the thermal fluid entering the Heart Lake Geyser Basin contained only 21

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mmol CO2, corresponding to 0.1 wt.% dissolved CO2. In contrast, rising gas vigorously churns the acid pool of Churning Cauldron at the Mud Volcano thermal area (Fig. 3).

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Werner et al. (2000) estimated a CO2 emission rate of 0.24 kg s-1 from this large pool.

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The water discharge through the pool outlet is <1 L s-1, implying bulk emissions of >20

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wt.% CO2. A similar calculation can be undertaken for the entire Mud Volcano thermal area, which releases 4.2 kg s-1 CO2 (Werner et al., 2000), compared with an H2O

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discharge of only 1– 2 L s-1 (Allen and Day, 1935). The total fluid emitted from the Mud Volcano thermal area is then calculated to be > 50% CO2, far different than the 0.1% at

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the Heart Lake Geyser Basin. It is clear that the ratio of CO2-rich gas to neutral chloride geothermal water can vary, and that gas is not produced everywhere by near-surface

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boiling of a common geothermal fluid with fixed dissolved CO2. Lowenstern and Hurwitz (2008) performed similar calculations for the overall Yellowstone geothermal system. By comparing total thermal water output estimated from the Cl-flux (e.g., Fournier et al., 1976, Friedman and Norton, 2007) with total CO2 output estimated from diffuse CO2 flux measurements (Werner et al., 2000; Werner and Brantley, 2003), they estimated bulk geothermal emissions for Yellowstone contain 5 mol% CO2 (~11 wt.%). Later in this manuscript, we will review evidence for the sources of the (shallow-derived) H2O and (deep-sourced) CO2, but we stress here solely that the steady state volatile emissions consist of 5 mol% CO2. This value falls between the 0.1 and >50 wt.% estimates for the Heart Lake and Mud Volcano estimates above. Lowenstern and Hurwitz (2008) further demonstrated that any potential thermal fluid

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ACCEPTED MANUSCRIPT with 5 mol% CO2 would separate into a CO2-H2O vapor and an aqueous liquid throughout most of the upper 3–4 km beneath Yellowstone. That is, conditions in the

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upper few km beneath Yellowstone favor saturation with a CO2-steam vapor phase.

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The above discussion demonstrates the following point: gas and liquid water are frequently decoupled at Yellowstone thermal areas. Sampled fluids may have

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components added or removed during subsurface flow. The open-system behavior may

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consist of geothermal gas rising into perched meteoric water at the surface, lateral flow that continuously depletes a geothermal liquid of dissolved gas at 200 m depth, or

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magmatic gas rising into a meteoric-sourced geothermal reservoir at 2 km.

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3.2. Steam/gas ratios at fumaroles reflect addition of steam from boiling groundwater Geothermal researchers often view fumarole emissions as steam and gas produced

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solely by boiling of geothermal liquid, whereas volcano gas chemists may view them solely as products of magma degassing. At Yellowstone, the reality is somewhere in between. The most recent compilation of YPVF gases has 167 separate analyses (Bergfeld et al., 2011: updated 2014), the distribution of which is shown on the map in Fig. 2. A small subset of analyses is compiled in Table 1. After H2O (steam), the primary component of Yellowstone fumaroles is CO2, followed by variable amounts of H2S, N2, H2, CH4, C2H6, Ar, He, O2, NH3 and other trace gases (Fig. 4). The ratio of gas to steam at Yellowstone differs markedly in different locales. The %G (mol% gas in the fumarolic vapor relative to steam + gas) varies from a high of ~15% at Mud Volcano, down to values as low as 0.05% in some geyser basins. The %G does not correlate with fumarole temperature, but does vary in concert with gas chemistry. Fig. 5 is a plot of the log %G

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ACCEPTED MANUSCRIPT versus log N2/CO2 that shows the two variables are anti-correlated. Given that CO2 dominates the gas composition everywhere in Yellowstone, the plot demonstrates that

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samples high in steam (low in gas) have elevated N2. Samples with the highest %G and

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lowest N2/CO2 are found at Mud Volcano, coincident with helium isotope ratios that are the most characteristic of a deep mantle source for all locations at Yellowstone (Craig et

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al., 1978, Kennedy et al., 1985, Bergfeld et al., 2011).

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The Ar/He ratios, as indicated by the size of the symbol on Fig. 5, show a similar trend. He and Ar are not highly fractionated by boiling, especially compared with N2 and

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CO2, yet the Ar/He ratio increases by orders of magnitude from low values in gas-rich samples of Mud Volcano to high values at steam-rich areas like the Upper Geyser Basin.

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The Ar content, as with N2, can be derived from the atmosphere or from meteoric water that is saturated with air, whereas the He concentration in air is very low and must come

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from magmatic or crustal sources. Therefore, a simple explanation for the variations shown in Fig. 5 is that samples with low %G have been diluted by addition of boiled meteoric water, contributing N2, Ar, and steam to an initial gas already rich in CO2 and He from magmatic and deep crustal sources. Traditional models to explain variations in gas chemistry in geothermal systems often start within an initial meteoric-derived fluid, and then calculate the effects on fluid chemistry from boiling. As a Rayleigh or fractional process, boiling should create a series of vapors with progressively lower %G and progressively lower N2 relative to the more soluble CO2 (near vertical trend in Fig. 5). A batch degassing process will create a more horizontal trajectory on Fig. 5, but neither boiling process will generate the sort of trend indicated by the existing Yellowstone data. Though it may be reasonable for progressive

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ACCEPTED MANUSCRIPT boiling to control gas-chemistry variations locally within a thermal area, it clearly cannot explain the regional-scale variations in gas chemistry for Yellowstone fumaroles (Fig. 5).

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Dilution of deep gas with variable amounts of boiled air-saturated meteoric water might

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be more likely in a high-heat flow region like Yellowstone, where conditions beneath

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thermal areas match the boiling-point versus depth curve.

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3.3. Fumarole chemistry: trace gases reflect regional differences and gas mixing Gas ratios vary geographically across Yellowstone, and can be explained by

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different proportions of components added from meteoric, magmatic and crustal sources. The importance of air-saturated meteoric water as a source of some gas components in

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Yellowstone emissions is readily demonstrated in Fig. 6a, a ternary diagram with apices of 10*Ar, 100*He and N2/10. As initially shown by Sheppard et al. (1992) and then

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Bergfeld et al. (2011), nearly all Yellowstone gas samples form a trend from air-saturated meteoric water (N2/Ar < ~40) to a He-rich endmember. Only a minority of samples contain significant amounts of air, as indicated by few N2/Ar ratios shifted toward the atmospheric value of 78. Of those samples, most have O2 below the detection limit, indicating that most oxygen is stripped from groundwater recharge. The gas samples exhibit far more heterogeneity when considering CH4 contents in place of N2. Fig. 6b illustrates that Yellowstone gases have great variation overall in He/CH4, but with local consistency. Some individual thermal areas (for example the Norris Geyser Basin, red dots) trend from the Ar meteoric endmember along a fixed He to CH4 ratio. Some thermal areas are comparatively enriched in CH4, including most of

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ACCEPTED MANUSCRIPT those in Eastern Yellowstone, whereas others, such as Mud Volcano and Roaring Mountain, are comparatively enriched in He.

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Sulfur (as H2S) and helium concentrations are variable at Yellowstone. H2S

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ranges from <0.01 to >30 mol%. Relative Ar concentrations are greater in neutral chloride areas compared with acid sulfate terrain (Fig. 6c and 6d).

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Another means to visualize the systematics of the crustal gases is shown in Fig. 7,

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where log (He/CO2) concentrations are plotted versus log (CH4/CO2). Different thermal areas clearly can be differentiated based on CH4/He ratios, with Shoshone and Heart Lake

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plotting above the Norris Geyser Basin trend, which respectively lies above Gibbon River and Mud Volcano. Samples from individual thermal area form parallel trends where, He

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and CH4 increase sympathetically relative to CO2 Different thermal areas have characteristic He/CH4. Thermal areas outside the caldera tend to have the highest CH4

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and He (normalized to CO2). The trends have relevance for interpretation of CH4-based geothermometers, as will be discussed in section 3.8.

3.4. Radiogenic helium is added to mantle-derived gas throughout the crust Some of the crustal gas emitted at Yellowstone had accumulated over millions of years and is now being released by the magma-hydrothermal system. Lowenstern et al. (2014) used the measured CO2 efflux, He/CO2 ratio, and 3He/4He values to derive the flux of both magmatic and crustal (radiogenic) He from different thermal areas, particularly those found outside the caldera. Radiogenic He is produced by long-term breakdown of radioactive elements like U and Th. The authors calculated that the four studied thermal areas at Hot Spring Basin, Roaring Mountain, Heart Lake Geyser Basin,

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ACCEPTED MANUSCRIPT and Brimstone Basin expelled over twenty times more radiogenic He in a year than could be produced annually within the crust (43-km thick) beneath all of Yellowstone.

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Moreover, the entire Yellowstone geothermal system releases >~500 times that which

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can be supported by present-day radiogenic He production in the crust. Lowenstern et al. (2014) concluded that radiogenic He was stored for hundreds of millions of years within

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the crust in old (mostly Archean) rocks. The stored He is now being released due to

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heating, fracturing and metamorphism associated with the Yellowstone magmahydrothermal system. Locations outside the caldera are currently more prolific sources of

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radiogenic He. Where Archean rocks may underlie the caldera, it is likely that a greater proportion of their radiogenic He was scavenged and outgassed earlier in the two-million-

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year volcanic history of the YPVF.

With respect to gas discharge, crustal systems can therefore be far from steady

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state. Due to low geothermal gradients, tectonic lulls, and low crustal permeabilities, radiogenic gas can build up over eons, especially within inactive cratons. These same geologic terrains can be later tapped for their stored gases, allowing open-system addition of radiogenic material to shallow hydrologic systems. Variations in tectonic and magmatic processes over geologic timescales create radical variations in crustal permeability (Ingebritsen and Manning, 2010) and temperature, resulting in wide temporal fluctuations in gas flux and isotopic ratios. At Yellowstone, melting of the mantle produces juvenile gas that mixes with much older crustal gas, as well as young (hundreds to thousands of years) meteoric recharge. Such mixing of old crustal gas has relevance for studies where isotopes are used to estimate groundwater residence times. For example, Yokochi et al. (2013, 2014)

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ACCEPTED MANUSCRIPT noted that 39Ar/40Ar* ratios implied mean residence times of 17 to 24 kyr for geothermal waters if circulation of the waters were through Quaternary volcanic rocks. If the waters

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had instead circulated through 3.6 Gyr Archean rocks that had retained radiogenic 40Ar*,

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the residence times would decrease to less than 10 years. The reality is likely somewhere in between. Yellowstone thermal waters pass dominantly through the Quaternary

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volcanic rocks that sit within the upper few kilometers, but it is also likely that gas rises

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up and into those waters via degassing of much older source rocks lying far below.

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3.5. Geographic variations in 3He/4He dominantly reflect deep processes The open-system addition of radiogenic He from crustal rocks, invites a new

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perspective on the variable 3He/4He ratios across Yellowstone, which range from values around 16 times that in air at Mud Volcano (i.e., R/RA = 16), to 10-12 in the Gibbon

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River Canyon, to <4 at Roaring Mountain, Heart Lake Geyser Basin and Brimstone Basin (Kennedy et al., 1985; Bergfeld et al., 2011, 2014). Kennedy et al. (1985) pointed out the difficulty in correlating the regional distribution of He isotopes with geophysical parameters such as Bouguer gravity, seismic velocity, and Curie point isotherms. Moreover, He isotope samples from thermal areas near the youngest lavas at Yellowstone (Pitchstone Plateau, Moose Falls flow and the Norris Mammoth Corridor) did not have elevated He isotopic ratios. Presumably for this reason, no one has suggested (in print) the seemingly plausible hypothesis that areas exhibiting high 3He/4He ratios are those with the most recent volcanic (or intrusive) activity. Our recent dataset provides new insight into the regional variations in He isotope values. On a park-wide scale, He concentrations are anti-correlated with 3He/4He (R/RA,

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ACCEPTED MANUSCRIPT Fig. 8a; see also Lowenstern et al., 2014), such that samples with the highest 3He/4He values at Mud Volcano and along the Gibbon River have low He (relative to total gas).

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Samples with the lowest 3He/4He tend to have high He concentrations, and plot toward the left of the diagram. A similar anti-correlation with CH4 abundances (Fig. 8b) hints

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that the source of 4He is non-volcanic in origin and comes from pre-Quaternary crustal

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pans, as well as the fumaroles shown in Fig. 8.

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rocks rather than erupted rhyolite. These trends hold up for gas from pools, and frying

Similarly, Chiodini et al. (2012) found a correlation between He-isotope ratios

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and the calculated equilibration temperature of fumarolic fluids. They inferred mixing between two endmembers: a hotter, 340°C fluid (mantle component) with R/RA value of

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22 and a cooler fluid (crustal component) with R/RA <<1 at a temperature closer to 170°C. The crustal component could be extremely enriched in helium and even in small

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proportions would be capable of substantially reducing 3He/4He ratios in mixed fluids. This end member apparently has access to older, non-volcanic rocks beneath the rhyolites, enriching it in 4He and other gas species such as CH4 (Fig. 8b). We agree with Chiodini et al. (2012) that regional variations in He isotopes relate principally to the amount of crustal (He-rich, low 3He/4He) versus magmatic (He-poor, high 3He/4He) gas feeding the thermal area. Previously, researchers have proposed explanations for local variations in 3He/4He, usually calling upon boiling and mixing processes within the near-surface (i.e., volcanic-hosted) geothermal system (Kennedy et al., 1987; Hearn et al., 1987; Fournier, 1989; Fournier et al., 1994). We find these explanations somewhat satisfying for local variations, but of no use for understanding the broad geographic variations in He-isotope ratios at Yellowstone.

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ACCEPTED MANUSCRIPT Kennedy et al. (1987) proposed a simple model for fluids in the Lower Geyser Basin, whereby boiling of geothermal waters caused loss of CO2 (and ultimately HCO3)

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together with He. Waters undergoing more boiling (and presumably therefore with less remaining He) were more influenced by addition of radiogenic 4He picked up from the

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local volcanic aquifer, causing a correlation of low 3He/4He with low HCO3. Notably,

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Kennedy et al. (1987) did not find a similar trend for HCO3 and 3He/4He in the nearby

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Upper Geyser Basin, where similarly low HCO3 fluids were nonetheless high in 3He/4He. Hearn et al. (1992) applied the Kennedy et al. (1987) model to the hot springs of

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Shoshone Geyser Basin, which show a tenuous correlation between 3He/4He and HCO3. Fournier et al. (1994) adapted the Kennedy et al. (1987) model to explain high 3He/4He

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ratios near 12 RA and high gold concentrations at Beryl Spring and nearby Sylvan

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Springs. In this case the high 3He/4He endmember had low HCO3, in contrast to the initial

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1987 model for the Lower Geyser Basin. The high 3He/4He ratios at low HCO3 concentrations were explained in terms of fluid flow-paths and unusual boiling processes at specific temperatures, conditions that were perhaps unique to these two areas. Fournier (1989) observed that thermal areas with high 3He/4He ratios, like Mud Volcano and Crater Hills, clustered around the outside edge of the 2-Ma caldera, and thus were not underlain by the thick, permeable rhyolite flows that subsequently filled the caldera. Generally lower 3He/4He ratios within the 2-Ma caldera were attributed to enhanced thermal water circulation through these flows, providing a greater opportunity for extraction of 4He. Neither propinquity to magma, nor open-system addition of deep, crustal He were considered as potential contributors to He isotope systematics.

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ACCEPTED MANUSCRIPT In our view, the clear importance of open-system addition of He- and CH4-rich crustal gases obviate the need to explain He isotope variations by variable degassing and

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reaction with Quaternary volcanic rocks. The highest 3He/4He ratios are at Mud Volcano,

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and it seems difficult to conclude anything other than that this area: a) sits above one of the more recent subsurface intrusions or b) lies on a pathway for magma degassing that

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includes the least amount of He-rich crustal gas available for mixing, or c) both.

3.6. CO2/3He ratios and 13C-CO2 values reflect the signature of Yellowstone mantle.

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With the exception of H2O, Yellowstone gases are dominated by CO2, which usually constitutes well over 90% of the non-condensable gas. The CO2/3He values fall

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within the range expected for mantle gases, with 50% containing gas ratios between 2.2

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and 4.1 x 109 (Fig. 9). Higher values occur in a few samples from Washburn Hot Springs,

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Terrace Springs and Devils Den, areas outside the caldera where additional crustal gas may be available. There appears to be a slight trend toward lighter (more sedimentary) carbon at high CO2/3He. Nevertheless, the relatively tight grouping of CO2/3He is consistent with much of the CO2 originating from a mantle source, or multiple mantle sources. One can estimate the amount of non-mantle CO2 by using the method of Sano and Marty (1995) to model the CO2/3He and 13C-CO2 contents of the Yellowstone gas. We choose a Yellowstone mantle endmember of 1.5 x 109 for CO2/3He and a 13C-CO2 of -3.4±0.5‰, which is near the end of the trend of Yellowstone samples and is similar to relevant mantle endmembers (Fig. 9). Using the sediment and limestone end members from Sano and Marty (1995), the sediment component is always less than 20%, with limestone only rarely making up more than 50% of the CO2 in Yellowstone gas.

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ACCEPTED MANUSCRIPT Werner and Brantley (2003), using a smaller dataset, provided a detailed discussion of the potential role of crustal limestone in Yellowstone geothermal gases, and

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similarly concluded that up to 50% crustal carbon could be present, with the remainder originating from the mantle. The potential role of crustal limestone was emphasized

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partly due to the relatively heavy 13C-CO2 values for Yellowstone samples (Fig. 9) compared with many estimates of mantle values. However, they also acknowledged that

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the range of hotspot values for 13C-CO2 and CO2/3He permitted a much higher proportion of mantle CO2 in Yellowstone gases. In fact, many studies of CO2-rich

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volcanic systems have estimated mantle CO2 sources with 13C of -3 to -4 per mil (Fig. 7;

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Gerlach and Taylor, 1990; Allard et al., 1997. Tedesco et al. 2010; Darrah et al., 2013).

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Thus, it is quite possible that the mantle beneath Yellowstone contributes CO2 with a 13C composition similar to the observed value of -3.4±1.2 per mil.

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Werner and Brantley (2003) also calculated that at current degassing rates and a maximum limestone thickness of 2.5 km beneath the Yellowstone caldera, that the available crustal carbonate would rapidly have been exhausted early in the 2 million year lifetime of the caldera. Therefore, it seems unlikely that current Yellowstone CO2 emissions are sustained by crustal limestone. Based on our substantial 13C-CO2 and CO2/3He dataset, we agree that at least 50% of the CO2 emitted from Yellowstone originates from the mantle.

3.7. CO2/3He, subsurface carbon sinks, and CO2 geothermometry The high rate of CO2 discharge and homogenous, mantle-like CO2/3He has additional relevance to understanding subsurface mineral alteration and to interpretation

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ACCEPTED MANUSCRIPT of some gas geothermometers. The fugacity of CO2 (fCO2) in geothermal systems is generally controlled by fluid-rock equilibrium involving conversion of Ca-silicates to

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calcite. Under full equilibrium conditions between these minerals and CO2, fCO2 in any

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fluid would decrease by 4 orders of magnitude over a temperature drop from 350 to 100°C (Eq. 35 in Giggenbach, 1988; see also Arnorsson and Gunnlaugsson, 1985). For

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Yellowstone, where fluid from a deep reservoir near 350°C feeds into shallow reservoirs

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at cooler temperatures, mineral reactions could represent a significant loss of CO2 to calcite. Such a process could cause the overall low emission of CO2 from the neutral

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chloride areas (e.g., geyser basins). Mineral equilibrium in the neutral chloride waters certainly occurs, as shown by the successful application (and even development) of many

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widely accepted chemical geothermometers for these waters (e.g., Fournier, 1989). The likelihood that calcite is a major carbon sink for CO2 at Yellowstone can be

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examined by comparing CO2 abundance in gas emissions to an inert component originating only in the mantle (3He). CO2 loss to calcite would leave the 3He unaffected, and control of CO2 solely by calcite formation in Yellowstone’s geothermal reservoirs that range in temperature from ~350 to ~100°C would lead to a variation in CO2/3He of ~4 orders of magnitude. Instead, CO2/3He varies by less than a factor of ten throughout Yellowstone, and increases (rather than decreases) in peripheral areas outside the caldera that appear to be low temperature systems (e.g., Terrace Springs and Devils Den). In fact, CO2/3He ratios in gas at Mud Volcano, thought to overlie one of the hottest geothermal reservoirs at T ≥300°C (Chiodini et al., 2012), are virtually identical to CO2/3He ratios at Brimstone Basin where the estimated reservoir temperature is near 100°C (Bergfeld et al., 2012). Substantial formation of calcite over a large temperature range should also result

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ACCEPTED MANUSCRIPT in substantial variability in carbon isotope ratios of the residual CO2 gas in surface emissions, which is not found at Yellowstone.

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As discussed by Chiodini and Marini (1998), fCO2 could be an externally fixed

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parameter, due to a continuous flux of CO2 through the hydrothermal system from mantle-derived basaltic magma (e.g., Werner and Brantley, 2003). Within the

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hydrothermal system, processes like boiling, mixing, and conductive cooling exert the

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main control on fCO2 and create conditions far from the mineral equilibration line, as shown in figure 8 of Giggenbach (1984). Reservoir fCO2 values, derived for example

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from CO2/Ar or CO2/steam in surface emissions, should not be used in equations that are based on calcite solubility to calculate temperatures within Yellowstone’s geothermal

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systems. This finding does not rule out equilibration among reactive gas species and the use of gas ratios to derive reservoir temperatures. However, the abundant evidence for

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mixing of gases from different sources suggests that the utility of such geothermometers should also be evaluated carefully.

3.8. CH4 abundances and isotopes reflect both temperature and source Chiodini (2009) pointed out that magmatic vapors are CH4-depleted compared with crustal gases, and CO2/CH4 ratios correlate well with input of new magmatic fluids into the geothermal systems at Campi Flegrei (Italy), Mammoth Mountain (California), Panarea (Italy) and Nisyros (Greece). The CH4 abundance in these systems can be explained on thermodynamic grounds, as CH4 is generated abiogenically through CH4CO2-H2O-H2 equilibria. As temperature increases, CH4 will break down to form CO2, H2O, and H2.

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ACCEPTED MANUSCRIPT Chiodini et al. (2012) evaluated Yellowstone fumarole chemistry through a thermodynamic and isotope analysis of gas compositions and showed that the combined

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use of CH4/CO2 and H2/H2O ratios was effective for estimating temperatures. Notably,

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the calculated temperature correlated with 3He/4He, such that gases with high R/RA (Mud Volcano area) yielded temperatures ~300°C, much hotter than the fumaroles from the

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Heart Lake Geyser Basin (150 to 180°C). The authors concluded that Yellowstone gases

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were a mixture of high-temperature components with a strong mantle signature, and a low-temperature crustal component. The gases are all swept up in meteoric-derived

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geothermal waters that circulate beneath the caldera.

The model of Chiodini et al. (2012) is consistent with the summary presented

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within this review. We see abundant evidence for mixing of magmatic, crustal and meteoric gases. Fig. 10 presents an update of figure 10a of Chiodini et al. (2012) by

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including samples collected at Yellowstone in 2010 and 2012. Yellowstone gas can be modeled as mixtures of deep vapor and gas equilibrating at temperatures from ~300°C (Mud Volcano) to <175°C (Heart Lake Geyser Basin). The isotopic exchange between CO2 and CH4 can also be used to estimate temperatures. The Horita (2001) carbon isotope exchange geothermometer, independently indicates equilibrium temperatures across Yellowstone that vary between ~100°C to 400°C. Because CO2 is isotopically homogeneous (Fig. 9) and is much more abundant than CH4 at Yellowstone, the calculated temperatures are entirely dependent on 13C-CH4. The relationship between 13C-CH4 and the calculated isotope exchange temperature is displayed in Fig. 11. As in Fig. 10, Mud Volcano (pink filled plus symbols) samples yield some of the highest temperatures and Heart Lake GB (blue X)

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ACCEPTED MANUSCRIPT samples yield some of the lowest. Such qualitative agreement between temperatures indicated by both Figs. 10 and 11 provides evidence that isotopic equilibrium between

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CH4 and CO2 is approached. However, we note that the sample sets from individual thermal areas yield a larger range of temperatures using carbon isotope equilibria (Fig.

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11), suggesting that some factor other than temperature exerts some control on 13C-CH4. In particular, those samples with low apparent temperatures (high CH4/CO2 and

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more negative 13C-CH4) could be affected by introduction of thermogenic CH4 through breakdown of sedimentary organic carbon in metamorphosed crustal rocks. This would

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reinforce the observed temperature distribution of cooler crustal gases mixing with a warmer CO2-rich endmember, but could cause some samples to appear cooler than their

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actual temperatures. Many authors have argued that samples from the eastern regions of Yellowstone show clear evidence that CH4 is derived from sedimentary organic matter, as

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evidenced by high C2H6/CH4 and low 13C CH4 values (DesMarais, 1981; Lorensen and Kvenvolden, 1993; Bergfeld et al., 2011, 2012, and see Fig. 10 of Bergfeld et al., 2012). Because chemical equilibrium can be sluggish (Giggenbach, 1987), it is possible that CO2/CH4 ratios may reflect source lithologies (in some cases) more than geothermal equilibrium. If so, samples with low 13C-CH4 and high C2H6/CH4, such as Hot Spring Basin may have thermogenic CH4 that has failed to re-equilibrate to the otherwise calculated high reservoir temperature (Werner et al., 2008). Addition of various endmembers of crustal gas is evident in Fig. 6b. The correlation of CH4 and He, even within individual thermal areas (Fig. 7, 11), hints that any within-basin temperature variations evident in Fig. 10 or Fig. 11, could be partly due to variable addition of thermogenic CH4 to high-temperature abiogenic gas.

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3.9. Estimating the mantle contributions of H2O, S, Cl, and F

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A rough estimate can be obtained for the mass of mantle-derived H2O released

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from the Yellowstone magma-hydrothermal system. Assuming that 50% of the Yellowstone CO2 flux is derived from the mantle, as discussed above (and in Werner and

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Brantley, 2003), and taking typical ratios for H2O/CO2 (by mass) of 0.5 to 1.2 for mantle-

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derived basalt (Hirschmann and Dasgupta, 2009), the predicted mantle H2O flux would be 1.1 to 2.7x104 t d-1. The total surface H2O discharge from Yellowstone, including

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geothermal and recent meteoric sources, was 1.1 x107 t d-1 in 2004 (Hurwitz et al. 2007). About 3% of that water is estimated as geothermal: given the Cl discharge from

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Yellowstone that year (4.9 x104 t) and the estimated Cl concentration of the parent

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geothermal water (400 mg kg-1; Fournier et al., 1989), 3.4x105 t d-1 of parent geothermal

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water was discharged from Yellowstone. Data for other years are within 10% of these values (Hurwitz and Lowenstern, 2014). The calculations imply that only 3 to 7% of the released parent geothermal water could have a mantle origin, with the rest coming mostly from meteoric water that had previously undergone long-term water-rock reaction within the geothermal system. This conclusion is consistent with the stable isotope values of thermal waters, which have D and 18O values very similar to that of local meteoric waters and are far removed from magmatic H2O (Kharaka et al., 2002, Rye and Truesdell, 2007; Bergfeld, et al., 2011, 2014). The low proportion of calculated mantle H2O, especially compared with CO2, stems from the huge mass of H2O contributed from recharging groundwater.

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ACCEPTED MANUSCRIPT It is yet more difficult to estimate mass balance for S species. Using the 2004 data from Hurwitz et al. (2007), 52 t d-1 of S discharges as dissolved SO4 in Yellowstone

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river waters. The average molar CO2/H2S ratio in Yellowstone gas is 44 (Bergfeld, et al., 2014), corresponding to a mass ratio of 57. Therefore, a rough estimate of the S output in

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gas as H2S is 790 t d-1. The combined dissolved and gaseous S components equal 842 t

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d-1 of S released from Yellowstone.

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This mass of S readily can be accounted for by input of mantle basalt. Using the 0.3 km3/a intrusion rate for basaltic magma estimated by Lowenstern and Hurwitz (2008),

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the observed S efflux from Yellowstone can be accounted for by ~450 ppm S in the deep basalt, or about 20-40% of values typically observed in mantle-derived basalts such as

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MORB and OIB, and much less relative to arc basalts (Wallace and Edmonds, 2011). That is, even relatively S-poor basalts can provide sufficient S to account for what is

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observed at the surface. Moreover, the low S output from Yellowstone could also be due to sequestering of S at a variety of levels as sulfides, sulfates and native S (e.g., Cinder Pool, White et al., 1988) in the magma-hydrothermal system (Werner et al., 2008). The S-isotope composition is also consistent with derivation through magma degassing. Sulfur is normally issued as H2S in geothermal systems, due to the very high solubility of SO2 in water and H2S/SO2 equilibria at temperatures <300°C and crustal oxidation states (Symonds and Reed, 1993; Fournier, 2007). At Yellowstone, 34S of H2S is typically around 0 per mil (Truesdell et al., 1978, Kamyshny et al., 1978, GemeryHill et al., 2007). Sulfate alteration minerals at Yellowstone have 34S values near 0 per mil, consistent with oxidation of H2S.. The modest amounts of sulfate dissolved in neutral chloride waters are heavier, with 34S values ranging from +10 to +20 per mil

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ACCEPTED MANUSCRIPT (Truesdell et al., 1978), consistent with equilibration at geothermal temperatures (> 250°C) with an H2S-bearing vapor. Overall, the dominant S isotope composition of 0 per

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mil for Yellowstone S is similar to values expected in the mantle (Marini et al., 2011). Emitted Cl also can be sourced from subsurface magma and/or buried subsurface

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rhyolitic rocks. About 130 t d-1 of dissolved Cl discharges through Yellowstone rivers

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(Friedman and Norton, 2007; Hurwitz et al., 2007). If the Cl source were ultimately from

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basalt and 22,500 t d-1 of CO2 is emitted from basalt containing 1% CO2 (i.e., 50% of measured flux from crustal carbonate), this would imply 60 ppm Cl in the deep basalt,

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comparable to that expected for Cl-poor MORB and Kilauean basalts (Metrich and Wallace, 2008). Therefore, an ultimate basaltic source for the Cl seems tenable. Some of

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the Cl would be passed along by fractionation of the basaltic magma to rhyolite. Cl would ultimately be released from the silicic magma either through formation of Cl-rich brines

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exsolving from rhyolitic magma (Fournier, 1989, 2007) or by later scavenging of Cl from subsurface rhyolitic rocks (i.e, tuffs, lavas, intrusions), which typically contain 1000 ppm by weight (Christiansen, 2001). In fact, current geothermal Cl discharge can be accounted for by leaching ~0.02 km3 of subsolidus rhyolite (with 1000 ppm Cl) per year. Transport of Cl as HCl is unlikely anywhere at Yellowstone, given that magmatic degassing occurs at pressures favoring NaCl, and geothermal degassing happens at temperatures where Cl partitions entirely as NaCl into the geothermal liquid (Shinohara, 2009). HCl is absent in all fumarolic samples collected at Yellowstone (Bergfeld et al., 2011, 2014). Hurwitz and Lowenstern (2014) discussed the mass balance of F at the YPVF, recognizing that the Cl/F ratio in hot-spring waters was much higher than in the erupted

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ACCEPTED MANUSCRIPT rhyolites or their melt inclusions. Their preferred explanation was that F is precipitated within the geothermal system as fluorite, qualitatively consistent with the mineralogy of

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shallow drillholes (White et al., 1975). Given that Cl abundances in geothermal fluid can

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be attributed to a magmatic origin, the lower-than-expected values for F are also easily

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accounted for by a magmatic source.

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4. Synthesis: Summary observations of gas flow and chemistry The above discussion leads us toward the following conceptual model for gas

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chemistry and discharge beneath the YPVF. 1) Gases and other volatiles represent a mixture of components derived from magmatic, diverse crustal, and meteoric sources

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(see also Kennedy et al, 1985). 2) Each individual species can be tracked independently and its origin cannot necessarily be recognized through isotopic or chemical analysis of

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other species (Fig. 12). The gases Ar, N2 and Ne are derived mostly from meteoric water, whereas He is derived from a mixture of crustal and mantle sources. CO2 is largely mantle-derived, but may also be contributed from crustal sources that have similar 13Cvalues as mantle carbon (Fig. 12). 3) The gas/steam ratio in fumarole emissions largely reflects the degree to which boiled meteoric-derived geothermal water is added to deep gas as it rises through the crust (Fig. 5, 13). Such a situation is possible in high-heat-flow terrain where subsurface conditions lie along the boiling-point curve with depth, such that boiling and generation of steam accompany any gravitational ascent of buoyant geothermal fluid. 4) There are multiple crustal gas sources that contribute varying amounts of 4He, CH4, C2H6 and 40Ar (the latter discussed in depth by Kennedy et al., 1985). Though Brimstone Basin and Hot Spring Basin both show clear evidence for high

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ACCEPTED MANUSCRIPT concentration of crustal He, their CH4/He ratios are considerably different, hinting at different lithologies that contribute the crustal components. 5) Most of the Yellowstone

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magma-hydrothermal system exhibits open-system behavior and only rarely can parts be

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considered closed with respect to input of new gas. Gas is produced and transported at a variety of locations within the mantle and crust, and various gas species may alternatively

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dissolve within, or bubble through, shallow waters. For the most part, one cannot assume

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that bubbling gases have fully equilibrated with the waters through which they rise. Similarly, boiling is a common process at Yellowstone, especially beneath the active

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thermal areas. Boiled waters may migrate laterally to emerge at topographic lows, and may encounter (and dissolve) new gases along the flow path. In spite of this, some

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thermodynamic approaches to geothermometry may still be effective, largely because

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some gas species can equilibrate rapidly.

5. Future directions for gas studies at Yellowstone Isotopic studies indicate that geothermal waters have relatively long residence times, emerging at the surface hundreds, or possibly thousands of years after initial infiltration as meteoric waters (Pearson and Truesdell, 1978; Rye and Truesdell, 2007; Gardner et al., 2011). Gases may flow by different pathways on different timescales as they move from their sources toward the surface. The abundant CO2 emissions at Yellowstone require that parts of the geothermal system are vapor-saturated, such that vapor can rise through the geothermal system (Lowenstern and Hurwitz, 2008). For this general reason, Hurwitz et al. (2007) pointed out studies of gas flux and chemistry might be an effective means for tracking volcanic unrest and monitoring changes to the deep magma system.

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ACCEPTED MANUSCRIPT The gas emerging at acid sulfate areas might travel faster than waters that slowly flow through geothermal reservoirs. We concur with this viewpoint, but also recognize that

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few data exist on the transit time of CO2 from deep basaltic intrusions to the surface, or

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the rate at which CH4 might be generated and expelled from organic-rich sedimentary rocks. As with the disparate sources of gases themselves, there may be a range of

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timescales for gas transport that range from minutes to millennia. Yokochi et al. (2012,

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2014) estimated gas residence times of tens of thousands of years for 39Ar in Yellowstone gas, under the assumption that the 39Ar was acquired during interaction with Quaternary

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volcanic rocks. The ages reduce to tens to hundreds of years if gas is transferred from Archean rocks that contain stored radiogenic gas. The importance of Archean rocks as a

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He source at Yellowstone, recently demonstrated by Lowenstern et al. (2014), favors these younger ages. A key question, however, for future researchers at Yellowstone and

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elsewhere will be to estimate the timescales for gas migration through the crust. One barrier to understanding gas transport is the current lack of temporal monitoring of gas flux and chemistry (i.e., time series). Our work thus far has concentrated on understanding the geographic variations in isotopes and gas chemistry, and to a lesser extent on coarse time variations (year-to-year). The lack of adequate timeseries measurements stems from a combination of factors including: 1) the grand spatial scale of Yellowstone, 2) the harsh winter conditions that hinder deployment of continuous instrumentation, and 3) the still emerging ability of instrumentation to track chemical and mass flux changes without the need for human intervention to deal with instrument drift and calibration.

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ACCEPTED MANUSCRIPT If gas chemistry and flux could be linked adequately to geophysical phenomena such as earthquakes, ground deformation, strain, and hydrothermal explosions, it seems

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likely that our understanding of the timescales for gas movement and our models for gas

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storage and movement would improve significantly. Evans et al. (2010) presented 14C evidence for an episode of rapid gas upflow at Mud Volcano during a seismic swarm in

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1978, but no chemical analyses are available from that time to examine for source

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variations. Large-scale storage of magmatic volatiles beneath the NW edge of the caldera has been inferred through seismic tomography by Husen et al. (2004), but any correlation

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among volatiles, deformation and seismic anomalies has not yet been clarified (Husen et al., 2004; Waite et al., 2002, Dzurisin et al., 2012). Better tracking of gas chemistry over

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time, and especially during incidents of increased seismic unrest or deformation could be

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crucial to establishing the links between the processes.

Acknowledgements

We appreciate detailed reviews by JVGR referees G. Chiodini and anonymous. In addition, J. Rytuba, K. Sims and P. Cervelli provided helpful input on the manuscript. Over the years, we have greatly benefitted from discussions with Giovanni Chiodini, Bob Christiansen, Bob Fournier, Henry Heasler, Shaul Hurwitz, Patrick Muffler, Kirk Nordstrom, Pat Shanks, and Ken Sims. Mark Huebner helps analyze the gases in Menlo Park. Our work is funded by the Volcano Hazards Program of the U.S. Geological Survey.

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ACCEPTED MANUSCRIPT Figure Captions

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Fig. 1. Geologic map of Yellowstone National Park and parts of its vicinity based from

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Plate 1 of Christiansen (2001). Pre-Caldera (volcanic) units refer to those erupted prior to the 640 ka Yellowstone Caldera. Location of the park within nearby states is shown in

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upper left inset. Simplified rock units are shown in legend. Thin black lines are roads.

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The outer rim of the Yellowstone Caldera includes the outer slumped zone (e.g., Fig. 29

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of Christiansen, 2001).

Fig. 2. Shaded relief map of Yellowstone National Park with sample locations (some

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with more than one analysis). Sampled place names are given in the inset along with symbols used in Figs. 6b, 7, 8 and 10. Abbreviated place names (small black circles)

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were not sampled, but are mentioned in the text: HS (Hillside Springs), PF (Phantom Fumarole), CC (Calcite Springs), RS (Rainbow Springs), and FL (Firehole Lake). Sample locations and chemistry in Bergfeld et al., (2011, 2014). Yellowstone Caldera border drawn at the base of the outer scarp (Christiansen et al., 2007), inboard of the caldera rim shown in Fig. 1.

Fig. 3. Photo showing the wide range of gas discharge from thermal pools at Yellowstone. (a) Columbia Spring at Heart Lake Geyser Basin has negligible gas flux but and moderate discharge of near-boiling-temperature H2O. (b) Churning Cauldron at Mud Volcano is a pool of sub-boiling acid water with little outflow but a gas discharge of ~0.24 kg/s CO2. Churning Cauldron photo from Wikipedia.

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Fig. 4. Ternary diagram showing results from fumarole gas samples that includes H2O vs.

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10 x CO2 vs. 10 x all other measured gas species. CO2 and H2O dominate, with a

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maximum of ~15% gas (mostly CO2). Data from Bergfeld et al., (2011, 2014).

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Fig. 5. Bubble plot of results from gas collected from Yellowstone fumaroles with log (% Gas) vs. log (N2/CO2) and bubble size scaled to log (Ar/He). Steam-rich samples tend to

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have higher concentrations of atmospheric gases (N2 and Ar) relative to gases with deeper origins that contain more CO2 and He. Fields for magmatic and meteoric fluids

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are interpretive, and are represented by Mud Volcano and Heart Lake, respectively. The overall trend of Yellowstone fumarole chemistry is inconsistent with the generation of

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gases by progressive boiling of a parent thermal water (batch and Rayleigh degassing processes shown). Data from Bergfeld et al., (2011, 2014).

Fig. 6. Ternary plots showing gas chemistry from Yellowstone thermal features. (a) 1000xHe vs. N2 vs. 100xAr for fumaroles, frying pans and bubbling springs from throughout Yellowstone. Gases display a trend from air-saturated meteoric water (ASMW) toward a He-rich, deeper endmember. (b) 100xHe vs. CH4 vs. 10xAr for fumaroles, frying pans and bubbling springs. The CH4/He concentrations in deep-derived gases show constant ratios within separate geographic areas. (c) H2S vs. CO2/100 vs. 10xAr for fumaroles and frying pans (no bubbling springs or pools). (d) 500xHe vs. CO2/100 vs. 10xAr for fumaroles and frying pans (no bubbling springs or pools). Ar

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ACCEPTED MANUSCRIPT derived from ASMW is more prevalent in samples from geyser basins (open triangles) compared with those from acid-sulfate and other thermal areas (filled triangles).

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Locations of these thermal areas are shown in Fig. 2. Data from Bergfeld et al., (2011,

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2014). HS= Hot Springs. GB = Geyser Basin.

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Fig. 7. Scatter plot showing log (He/CO2) vs. log (CH4/CO2) values for Yellowstone

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fumaroles. Sample symbols as in Fig. 6b. He and CH4 concentrations increase relative to CO2 for gases from individual thermal areas. He/CH4 ratios in gas from different thermal

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areas are variable and therefore plot independently.

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Fig. 8. Scatter plots Yellowstone fumaroles (except the low-temperature, air-rich samples from Devils Den) demonstrate controls on He-isotope ratios. (a) log 1/He vs. Rc/RA.

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Samples with high He concentrations tend to have low Rc/RA. values (b) log (CO2/CH4) vs. Rc/RA. Addition of CH4-bearing crustal gases reduces Rc/RA. Symbols the same as in Fig. 6b. Data from Bergfeld et al., (2011, 2014).

Fig. 9. Plot of 13C-CO2 vs. CO2/3He for Yellowstone and other gases, after Sano and Marty (1995). (a). Yellowstone fumaroles (light green elipse) form a trend with relatively homogenous 13C-CO2 values and variable CO2/3He concentrations. Gas from notable mafic volcanoes (including hotspot systems) have similar 13C-CO2 values as Yellowstone, but somewhat higher CO2/3He concentrations. The carbon isotope composition for the modeled Yellowstone endmember (–3.4 per mil) is slightly heavier than the MORB field for 13C-CO2 and within the range of mantle diamond

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Taylor, 1992), Nyiragongo (Tedesco et al., 2010) and Dallol (Darrah et al., 2012). (b) Enlargement of gray section of (a). Green dots denote Yellowstone gases. The highest

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CO2/3He are from the CH4- and NH3-rich gases of Washburn Hot Springs. Thin red lines

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are isopleths for sediment (S). Thin black lines are isopleths for limestone (L). Using

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endmembers for limestone and sediment from Sano and Marty (1995) and our preferred Yellowstone mantle end member (red square), two-thirds of Yellowstone samples contain

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>50% mantle endmember for CO2. Yellowstone data from Bergfeld et al., (2011, 2014).

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Fig. 10. Plot showing log (H2/H2O) versus log (CH4/CO2) values for Yellowstone gases (fumaroles only). The diagram updates figure 10a of Chiodini et al. (2012). The data are

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consistent with different thermal areas having characteristic temperatures, and a range of H2/H2O values that reflects mixing of reservoir vapor mixed with steam produced by direct boiling of reservoir liquid. Symbols as in Fig. 6b. Data from Bergfeld et al., (2011, 2014).

Fig. 11. 13C-CH4 vs. calculated temperature for Yellowtone gas samples using the carbon isotope geothermometer of Horita (2001). Because 13C-CO2 varies little, the calculated temperature is due almost entirely to variations in 13C-CH4. The sample for Hot Spring Basin (HSB) is from Lorenson and Kvenvolden (1993). All other data from Bergfeld et al., (2011, 2014).

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ACCEPTED MANUSCRIPT Fig. 12. Schematic ternary diagram showing sources of gas species found in Yellowstone fumaroles and bubbling pools. The size of the fields represents the approximate relative

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amounts of crustal, meteoric, and magmatic sources for each species. Representative

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samples are presented at different parts of the fields for He and Ar. The maximum amounts of crustal and magmatic N2 and Ar is difficult to constrain with our current data

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set.

Fig. 13. Schematic diagram of gas origins beneath the Yellowstone Plateau. In regions

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where deep gas flux is absent (left), all gas that issues at the surface is derived from boiling of the local geothermal reservoir containing mostly air-sourced gases including

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N2 and Ar. Where deep-gas flux is high (right), or near-surface geothermal reservoirs are scarce, the deep gas overwhelms any gas derived by boiling of shallow reservoirs. The

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color of the emitted steam and gas reflects the relative amount of deep gas and boiled reservoir water (legend). Fumaroles on the right side of the diagram would be high in %G, CO2/Ar and He/Ar.

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Figure 9

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Table 1.Isotope ratios and bulk gas chemistry of fumarole gas from varied locations around Yellowstone YL03Sample # 02A YL09-08 YL09-13 YL11-16 YL03-20 YL09-16 Lower Heart Lower Shoshone Smokejump Turbid Location GB Lake GB GB GB er Lake -----Neutral Cl----Temp °C 111.1 93.0 93.2 91.6 92.1 91.1 %Gas molar 0.48 0.06 0.06 0.20 0.36 5.8 CO2 mol% 94.3 83.1 95.6 96.8 89.0 97.8 H 2S mol% 0.45 0.66 0.6 0.03 4.09 1.08 NH3 mol% 0.02 0.366 <0.0002 0.093 0.008 0.083 He mol% 0.0051 0.0667 0.0047 0.002 0.0058 0.0041 H2 mol% 0.008 0.023 0.015 0.005 2.63 0.209 Ar mol% 0.1014 0.3101 0.0951 0.0773 0.0846 0.0035 O2 mol% 0.28 0.039 0.013 0.20 0.013 <0.0001 N2 mol% 4.0 10.5 3.0 2.5 3.6 0.13 CH4 mol% 0.8543 4.53 0.4057 0.2160 0.5705 0.6226 C2H6 mol% 0.00103 0.01532 0.00102 0.00051 0.00143 0.01453 CO mol% n.d. n.d. n.d. 0.0013 n.d. n.d. N2/Ar 39.3 33.7 32.0 32.5 42.0 37.1 He/Ne (of sample divided by air) 84 78 5 1008 121 RC/RA 6.13 1.09 8.82 8.60 4.02 335

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n.d.

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YL11-03 Mud Volcano 113.4 12.0 99.5 0.15 0.002 0.002 0.084 0.0035 <0.0001 0.23 0.0453 0.00003 0.0001 64.9 5146 15.97

YL12YL1207 14 YL12-25 Gibbon Highlan Sulfur R. d Hills -----Acid sulfate----114.2 92.3 114.3 4.3 0.30 0.17 99.2 74.3 77.9 0.49 19.64 10.95 0.003 <0.0002 0.058 0.002 0.005 0.007 0.131 0.156 8.67 0.0028 0.0768 0.0511 0.0007 0.023 0.083 0.13 5.6 1.6 0.0196 0.2154 0.6426 0.00010 0.00004 0.00783 0.0001 0.0004 0.0023 45.5 72.5 30.3 5441 41 739 12.31 8.22 6.98

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-4.9

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-32.4

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 C-CH4

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-43.2

n.d.

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2.2E+09 -175

8.2E+08

2.0E+09 -163

4.0E+09 -177

4.3E+09 -158

2.2E+09 -161

2.9E+09 -161

1.3E+09 -177

1.1E+09 -151

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Deep gas mixes with steam and gases of surface origin boiled off geothermal waters Open system processes add gas at different levels beneath Yellowstone

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Different crustal gases differentiated by CH4/He

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Closed-system boiling fails to account for regional trends in gas ratios and isotopes