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Dynamics of the Yellowstone hydrothermal system

2014; Wiley; Volume: 52; Issue: 3 Linguagem: Inglês

10.1002/2014rg000452

8755-1209

Shaul Hurwitz, J. B. Lowenstern,

Geological and Geochemical Analysis

Reviews of GeophysicsVolume 52, Issue 3 p. 375-411 Review ArticleFree Access Dynamics of the Yellowstone hydrothermal system Shaul Hurwitz, Corresponding Author Shaul Hurwitz U.S. Geological Survey, Menlo Park, California, USA Correspondence to: S. Hurwitz, [email protected]Search for more papers by this authorJacob B. Lowenstern, Jacob B. Lowenstern U.S. Geological Survey, Menlo Park, California, USASearch for more papers by this author Shaul Hurwitz, Corresponding Author Shaul Hurwitz U.S. Geological Survey, Menlo Park, California, USA Correspondence to: S. Hurwitz, [email protected]Search for more papers by this authorJacob B. Lowenstern, Jacob B. Lowenstern U.S. Geological Survey, Menlo Park, California, USASearch for more papers by this author First published: 09 June 2014 https://doi.org/10.1002/2014RG000452Citations: 104AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract The Yellowstone Plateau Volcanic Field is characterized by extensive seismicity, episodes of uplift and subsidence, and a hydrothermal system that comprises more than 10,000 thermal features, including geysers, fumaroles, mud pots, thermal springs, and hydrothermal explosion craters. The diverse chemical and isotopic compositions of waters and gases derive from mantle, crustal, and meteoric sources and extensive water-gas-rock interaction at variable pressures and temperatures. The thermal features are host to all domains of life that utilize diverse inorganic sources of energy for metabolism. The unique and exceptional features of the hydrothermal system have attracted numerous researchers to Yellowstone beginning with the Washburn and Hayden expeditions in the 1870s. Since a seminal review published a quarter of a century ago, research in many fields has greatly advanced our understanding of the many coupled processes operating in and on the hydrothermal system. Specific advances include more refined geophysical images of the magmatic system, better constraints on the time scale of magmatic processes, characterization of fluid sources and water-rock interactions, quantitative estimates of heat and magmatic volatile fluxes, discovering and quantifying the role of thermophile microorganisms in the geochemical cycle, defining the chronology of hydrothermal explosions and their relation to glacial cycles, defining possible links between hydrothermal activity, deformation, and seismicity; quantifying geyser dynamics; and the discovery of extensive hydrothermal activity in Yellowstone Lake. Discussion of these many advances forms the basis of this review. Key Points Assess the state of knowledge on Yellowstone's magma-hydrothermal system Emphasize the substantial advances that emerged in the past 25 years Most advances stem from the application of modern technologies "This whole region was, in comparatively modern geologic times, the scene of the most wonderful volcanic activity of any portion of our country. The hot springs and the geysers represent the late stages–the vents or escape pipes-of these remarkable volcanic manifestations of the internal forces. All these springs are adorned with decorations more beautiful than human art ever conceived, and which have required thousands of years for the cunning hand of nature to form" [Hayden, 1883, p. 70]. Ferdinand V. Hayden led the 1871 geological survey of northwestern Wyoming, leading to the establishment of Yellowstone as the first U.S. National Park in 1872. 1 Introduction The Yellowstone Plateau Volcanic Field (YPVF) is responsible for three cataclysmic volcanic eruptions over the past 2.1 million years [Christiansen, 2001] as the most recent manifestation of a mantle hot spot that over the past 17 million years has produced a string of large calderas along the Snake River Plain [Pierce and Morgan, 1992; Smith and Braile, 1994; Pierce and Morgan, 2009]. The youngest cataclysmic eruption at 0.64 Ma resulted in the formation of the Yellowstone Caldera (Figure 1) and was followed by several eruptive episodes of large-volume rhyolite flows. Although the most recent eruption was ~70,000 years ago, the Yellowstone Caldera is one of Earth's most "restless" calderas [Newhall and Dzurisin, 1988; Lowenstern et al., 2006; Smith et al., 2009] characterized by extensive seismicity, episodes of uplift and subsidence, and a very large and dynamic hydrothermal system. Figure 1Open in figure viewerPowerPoint Shaded-relief image of Yellowstone National Park showing acid sulfate (black) and neutral to alkaline (green) thermal features, Yellowstone Caldera boundaries, and the Mallard Lake (ML) and Sour Creek (SC) resurgent domes [Christiansen et al., 2007]. Labeled thermal areas are MHS – Mammoth Hot Springs, NGB – Norris Geyser Basin, LGB – Lower Geyser Basin, MGB – Midway Geyser Basin, UGB – Upper Geyser Basin, SGB – Shoshone Geyser Basin, HLB – Heart Lake Geyser Basin, M – Monument Geyser Basin, BB – Brimstone Basin, MB – Mary Bay explosion crater, LHR – LeHardy Rapids, MV – Mud Volcano, GC – Grand Canyon of the Yellowstone River, WHS – Washburn Hot Springs, HSB – Hot Spring Basin, SJ – Smoke Jumper Hot Springs, HH – Highland Hot Springs, and SP – Solfatara Plateau. The blue stars show the locations of the three large earthquake swarms shown in Figure 2. The diversity of documented phenomena in Yellowstone's hydrothermal system is unparalleled. The system comprises the largest concentration of geysers and hydrothermal explosion craters on Earth, more than 10,000 thermal features, including fumaroles, mud pots, and frying pans, and varicolored thermal pools. Thermal waters have a pH range from 1.5 to 10.0, diverse chemistry, and contain gases from mantle, crustal, and meteoric sources. The fluids discharged at the surface deposit silica sinter, travertine, native sulfur, and other minerals. The springs host biota from all three domains of life (Bacteria, Archaea, and Eukaryote), which use diverse inorganic sources of energy for metabolism. Hydrothermal activity is modulated and perturbed by processes that operate over time scales ranging from seconds (e.g., earthquakes), to days (e.g., air pressure and temperature variations), to seasonal (e.g., precipitation, snow melt, lake level), to decadal, centennial, millennial (e.g., caldera inflation and deflation and ice sheet advance and retreat), and even longer (volcanic cycles). Much of the research on hydrothermal activity in the YPVF is motivated by the need to assess hazards, to preserve unique natural features, and to understand the origin and propagation of life in extreme environments. In contrast to other large Quaternary silicic calderas (e.g., Taupo in New Zealand and Long Valley in Eastern California), Yellowstone National Park is protected from geothermal energy or mineral exploration and development and therefore provides a unique opportunity to conduct research on an undisturbed hydrothermal system and to characterize natural causes of variability that operate at widely varying spatial and temporal scales. However, characterizing and quantifying myriad processes in the hydrothermal system is daunting because of the large extent and the tremendous number and diversity of thermal features and because access is limited when the region is snow and/or ice covered throughout much of the year. Some of the major questions that have guided research in recent decades follow: How does the composition and emission rate of magmatic volatiles and heat measured at the ground surface inform us about the state of magma at depth? How do earthquake swarms and episodic inflation and deflation cycles of the Yellowstone Caldera relate to hydrothermal versus magmatic activity? How do periodic forces that operate at scales ranging from seconds to millennia affect thermal activity? What are the major controls on the functional relationship among geochemical processes and the microbial inhabitants of thermal springs? What are the forces controlling eruptive and explosive behavior? Research aimed at addressing these questions resulted in many new discoveries and provided an improved understanding of many controlling processes. In this review we assess the state of knowledge about Yellowstone's magmatic-hydrothermal system and emphasize the substantial advances that have emerged in the past quarter of a century since a similar review by Fournier [1989]. These recent advances mainly stem from the development of modern technologies, densification of monitoring networks, accumulation of large databases, discovery of hydrothermal vents in Yellowstone Lake, evidence relating thermophile microorganisms to the geochemical cycle, and additional research intensity due to establishment of the Yellowstone Center for Resources in 1993 and the Yellowstone Volcano Observatory in 2001. This progress has built upon more than 140 years of research since the 1870 Washburn expedition, the 1871 Hayden expedition, and the establishment of Yellowstone National Park in 1872. We hope that this review will help establish a basis for future research directions. 2 Volcanic History and State of the Magmatic System The YPVF was formed during three eruptive cycles over the past 2.1 million years [Christiansen, 2001]. The volcanic system emerged from a landscape of Precambrian, Paleozoic, and Mesozoic rocks, overlain in parts by Tertiary volcanic and sedimentary rocks of the Absaroka Ranges. The Pleistocene eruptive rocks consist dominantly of rhyolites and include numerous lava flows and voluminous units of ash flow tuffs from the climactic eruptions. The first eruption at 2.059 ± 0.004 Ma [Lanphere et al., 2002] produced the >2450 km3 Huckleberry Ridge Tuff. The second eruption at 1.285 ± 0.004 Ma [Lanphere et al., 2002], produced the >280 km3 Mesa Falls Tuff and the resulting Henrys Fork caldera, and the last eruption 0.639 ± 0.002 Ma [Lanphere et al., 2002] produced the 1000 km3 Lava Creek Tuff and the associated Yellowstone Caldera [Christiansen, 2001]. During each of these cycles, basaltic lavas erupted around the margins of the active rhyolitic source area but not within it. The Yellowstone Caldera is an elliptical depression that covers an area of ~80 × 50 km (Figure 1) and is filled with 600–1000 km3 of post-0.64 Ma rhyolitic lava [Christiansen et al., 2007]. Following caldera formation, uplift within the caldera formed the Mallard Lake and Sour Creek resurgent domes [Christiansen, 2001]. Subsequent post-caldera eruptions occurred during two major episodes and collectively formed the Plateau Rhyolite. The first episode commenced either at 600 ± 20 ka [Morgan and Shanks, 2005] or 516 ± 7 ka [Gansecki et al., 1996] and continued to 257 ± 13 ka [Christiansen et al., 2007] to form the Upper Basin Member. The second episode formed the Central Plateau Member and started at about 170 ka with effusive eruptions that produced >600 cubic kilometers of rhyolitic lava and ended 72 ± 4 ka [Christiansen et al., 2007]. These rhyolite flows nearly filled the Yellowstone Caldera from eruptive vents along two linear northwest trending zones. At least two significant explosive eruptions are concurrent with the effusive eruptions of the Central Plateau Member, one depositing the tuff of Bluff Point during the formation of the ~10 km diameter West Thumb caldera in the western part of Yellowstone Lake, and the other depositing the tuff of Cold Mountain Creek [Christiansen, 2001; Christiansen et al., 2007]. Basaltic lavas erupted intermittently following the eruption of the Lava Creek Tuff on the northeast, north, west, and south margins of the plateau. Geophysical studies have proposed that an upper crustal magma reservoir with a partially crystallized melt of silicic composition, often described as a "crystal mush," underlies the Yellowstone Caldera. This inference is based on three-dimensional tomographic inversion of local earthquakes [Miller and Smith, 1999; Husen et al., 2004; Chu et al., 2010; Farrell et al., 2014] or ambient noise [Lü et al., 2013], the depth of the seismogenic zone beneath the caldera [Smith and Braile, 1994; Waite and Smith, 2004; Husen et al., 2004; Farrell et al., 2009, 2014], an electrically conductive layer beneath the caldera [Kelbert et al., 2012], inversion of ground-surface displacement measurements [Pelton and Smith, 1979; Dzurisin and Yamashita, 1987; Dzurisin et al., 1990; Wicks et al., 1998; Dzurisin et al., 1999; Wicks et al., 2006; Chang et al., 2007, 2010; Dzurisin et al., 2012], elevated heat flux [Morgan et al., 1977; Hurwitz et al., 2012a], and models of stress transfer within the caldera [Luttrell et al., 2013]. The huge flux of magma-derived volatiles suggests that the upper crustal silicic magma is underlain by a basaltic system [Lowenstern and Hurwitz, 2008], and seismic images obtained from data acquired by the USArray seismic network (http://www.usarray.org/) show substantial low-velocity anomalies in the upper mantle and lower crust beneath the Yellowstone Caldera [Obrebski et al., 2011; Wagner et al., 2012]. All of these results contrast with interpretations of electromagnetic data suggesting little or no melt in the lower crust and upper mantle directly beneath the caldera [Kelbert et al., 2012]. 3 Seismicity and Deformation The YPVF is characterized by thousands of earthquakes annually; they are typically small (M < 3), and within or immediately north and northwest of the Yellowstone Caldera. In contrast, hydrothermal activity in the Mammoth Hot Springs area is not associated with abundant seismicity. Within the caldera, most earthquakes are shallower than ~6 km, but deeper events occur between Norris Geyser Basin and the epicenter of the M −7.3 Hebgen Lake, Montana earthquake [Waite and Smith, 2004; Farrell et al., 2009]. A majority of the small earthquakes occur in swarms, meaning that they are clustered in space and time [Waite and Smith, 2002; Farrell et al., 2009, 2010; Shelly et al., 2013]. These swarms can be characterized by systematic temporal migration of seismicity (Figure 2). For example, during the 2008–2009 earthquake swarm beneath northern Yellowstone Lake, epicenters migrated northward at a rate of ~1 km · d−1, and the maximum focal depths decreased from 12 to 2 km [Farrell et al., 2010]. Nearly all large swarms appear coincident with transition from inflation to deflation (or vice versa) of the Yellowstone Caldera [Smith et al., 2009] (Figure 3). Figure 2Open in figure viewerPowerPoint Space-time progression of three large earthquake swarms labeled in Figure 3: 1 – the October 1985 northwest caldera swarm [Waite and Smith, 2002], 2 – the 2008–2009 Yellowstone Lake swarm [Farrell et al., 2010], and 3 – the 2010 Madison Plateau swarm [Shelly et al., 2013]. The color scale represents (a) 34 days, (b) 12 days, and (c) 12 days, respectively. Figure 3Open in figure viewerPowerPoint Time series of Yellowstone Caldera uplift and subsidence patterns along with quarterly catalog earthquake counts. The numbers in the circles relate to seismic swarms shown in Figure 2. The 1985 and 2010 swarms were associated with the transition from uplift to subsidence of the caldera. Modified from Smith et al. [2009] and Shelly et al. [2013]. Ground-surface elevations have been measured since 1923, initially with leveling techniques and since the 1990s with Global Positioning System (GPS) methods [Meertens and Smith, 1991; Puskas et al., 2007; Chang et al., 2007] and Interferometric Synthetic Aperture Radar (InSAR) [Wicks et al., 1998, 2006; Aly and Cochran, 2011]. These measurements have revealed that the Yellowstone Caldera has experienced multiple episodes of caldera-wide deformation, with uplift and subsidence rates averaging 1–2 cm · yr−1 centered on the Mallard Lake and Sour Creek resurgent domes and the northern boundary of the caldera (Figure 1) [Pelton and Smith, 1979; Dzurisin et al., 1990; Wicks et al., 1998; Dzurisin et al., 1990, 1999; Wicks et al., 2006; Chang et al., 2007, 2010]. InSAR and GPS data have revealed heterogeneous deformation within the Yellowstone Caldera and large vertical displacements, relative to the caldera, of a region just north of the caldera rim near Norris Geyser Basin, the "north rim uplift anomaly" [Wicks et al., 2006; Chang et al., 2007]. Within the caldera, differential uplift and subsidence of the Sour Creek and the Mallard Lake domes has been documented [Wicks et al., 2006]. Models for these deformation episodes typically invoke volume change of a discrete source, or multiple sources in a homogeneous, isotropic, and elastic half-space at depths of 6 to 10 km assumed to represent a magma chamber [Dzurisin et al., 1990; Wicks et al., 2006; Chang et al., 2007; Vasco et al., 2007]. Although these models have provided mathematical solutions relating surface uplift to volume change of the source at depth, they cannot differentiate between the possible deforming fluids (magma or hydrothermal fluid), and explanations or quantitative solutions for episodes of subsidence are somewhat less convincing. In addition to the geodetic measurements of caldera inflation and deflation that are available only for approximately 90 years, tilt in dated Yellowstone Lake terraces provides evidence that large amplitude (meters) and long-wavelength cycles of caldera inflation and deflation have been persistent for the past 14,000 years (Figure 4) [Locke and Meyer, 1994; Pierce et al., 2002]. Figure 4Open in figure viewerPowerPoint Yellowstone lake levels during the past 15 ka (blue dashed curve) reconstructed from shoreline terraces, relative to a gage on the northern shore of the lake. Datum is at LeHardy Rapids, the erosional base level on the Yellowstone River, 4.5 km north of Yellowstone Lake outlet (Figure 1). The brown rectangles represent some of the hydrothermal explosion craters in and around Yellowstone Lake. The width of the rectangle represents calibrated radiocarbon age + 2σ and the height is relatively scaled by the area of the crater. The decline in elevation from 14.4 to 3–4 ka mainly represents erosion of the lake outlet base level. The shorter lake level increases and decreases represent uplift and subsidence of the Yellowstone Caldera. The magenta curve between ~6.5 ka and the present is an alternative lake level record that is allowed by the data. Modified from Pierce et al. [2002] and Christiansen et al. [2007]. 4 Episodic Heat and Mass Transport From Magma to the Hydrothermal System Nearly all the heat and much of the noncondensable gas discharged at Yellowstone are derived from the underlying magmatic system and transported through the hydrothermal system (Figure 5). In the absence of deep drill holes, information on heat and mass transfer into and through the hydrothermal system is available only indirectly from geophysical and geochemical observations, data from deep drilling elsewhere, exhumed ore deposits, and laboratory experiments. In several geothermal exploration wells where temperatures exceed 370°C (e.g., in Lardarello in Italy, Kakkonda in Japan, and Krafla in Iceland), a narrow zone of low-permeability rocks forms by deposition of silicate and other minerals [Fournier, 1991; Ikeuchi et al., 1998; Schiffman et al., 2012]. Sharp fluid-pressure gradients have been measured across this zone: above are brittle rocks where meteoric-derived hydrothermal fluids circulate at hydrostatic to sublithostatic pressures; below is a hotter region with lithostatic fluid pressures in ductile (plastic) rocks [Fournier, 1991, 1999; Cox, 2005]. Hypersaline brine and gas exsolved from magma accumulate at lithostatic pressure in ductile rocks at near-magmatic temperatures in relatively thin, horizontal lenses (Figure 5) [Bailey, 1990; Fournier, 1999]. The temperatures and pressures at which the ductile/brittle transition takes place in these wells are similar to temperatures and pressures where mineral assemblages in silicic rocks undergo a transition from ductile to brittle deformation in tectonically active regions with high-temperature gradients [Evans et al., 1990; Hirth and Tullis, 1994; Dingwell, 1997; Simpson, 2001] and strain rates in the range 10−12 to l0−13 s−1 [Fournier, 1991]. In this temperature range, the solubility of silicate minerals decreases markedly as pressure decreases, enhancing mineral precipitation in fractures [Fournier and Potter, 1982; Fournier, 1985]. Figure 5Open in figure viewerPowerPoint A schematic cross section of the magmatic and hydrothermal systems underlying the Yellowstone Caldera, showing magmatic volatiles exsolved during crystallization of the silicic magma and/or from basalt intrusions or convection and the hypothesized relationship with earthquake swarms in the caldera margins. The exsolved aqueous fluids accumulate at lithostatic pressures in the ductile regime and under high strain rates are episodically injected into the brittle regime, where fluid pressures are generally much lower. The transient increase in fluid pressure in the brittle regime then triggers earthquake swarms, while the movement of fluids facilitates caldera subsidence. Figure modified from Lowenstern and Hurwitz [2008] and Shelly et al. [2013]. The low-permeability zone is episodically breached in response to a temporary and local strain-rate increase [Fournier, 1991, 1999; Cox, 2005]. At high strain rates, the ductile material undergoes shear (brittle) failure in response to small stress perturbations [e.g., Dingwell, 1997; Mader et al., 2013], and hypersaline brine and gas of likely magmatic origin are expelled into the brittle crust. The resulting increase in fluid pressure and temperature within the brittle crust induces faulting and brecciation that allow for enhanced transport of magmatic volatiles and heat [Fournier, 1999]. Within the hydrothermal system, however, elevated temperatures and enhanced reactivity of aqueous fluids tend to decrease permeability. To maintain permeabilities sufficient for advective transport of heat and volatiles, ongoing seismicity and deformation are required. In the past quarter of a century, several studies have documented processes that provide circumstantial support for models that link (1) episodic fluid migration across the brittle-ductile transition zone, (2) heat and volatile transport into and across the hydrothermal system, (3) seismic swarms, and (4) inflation and deflation of the Yellowstone Caldera. Dzurisin and Yamashita [1987] first proposed that magmatic fluids might be controlling deformation of the Yellowstone Caldera "Uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir" and "Subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids." Later, Dzurisin et al. [1990] proposed that "Subsidence occurs during episodic hydrofracturing and injection of pore fluid from the deep lithostatic-pressure zone into a shallow hydrostatic-pressure zone." Thus, hypotheses linking episodes of caldera inflation and deflation to episodic fluid migration were proposed shortly after an inflation-deflation transition was observed. The feasibility of these hypotheses was later verified by numerical models linking hydrothermal injection into the base of the brittle crust with hydrothermal flow, poroelastic deformation, and caldera inflation and deflation [Chiodini et al., 2003; Hurwitz et al., 2007a; Todesco, 2009; Hutnak et al., 2009; Rinaldi et al., 2010]. Nevertheless, most recent deformation models invoke volume changes of a discrete source or sources as the impetus for caldera inflation and/or deflation, but these models cannot distinguish between a hydrothermal or magmatic fluid causing the volumetric change [Wicks et al., 2006; Chang et al., 2007; Vasco et al., 2007; Aly and Cochran, 2011]. Three large earthquake swarms have occurred in proximity to the Yellowstone Caldera boundary since 1985 (Figure 3). The transition from inflation to deflation following two of the earthquake swarms suggests that in addition to episodic injection of magmatic volatiles into the hydrothermal system, which could result in overall pressure increase and inflation, episodic breaching of the deep (and brittle) hydrothermal system might also be the mechanism for pressure release. The focal mechanisms of earthquakes in these swarms, their temporal migration pattern and their depths, were interpreted to result from hydrothermal fluid migration and pressure release from the caldera [Waite and Smith, 2002; Farrell et al., 2009, 2010; Shelly et al., 2013]. The largest recorded earthquake swarm lasted for more than 3 months beginning in October 1985 and included more than 3000 earthquakes with M < 5 [Waite and Smith, 2002; Farrell et al., 2009]. During the first month the swarm front migrated laterally away from the caldera at an average rate of 150 m d−1 (Figure 2a). Coincident with the onset of the swarm, the caldera transitioned from inflation to deflation (Figure 3). The temporal pattern of earthquake swarms and the change in caldera deformation pattern were explained by migration of hydrothermal fluids radially outward from the Yellowstone Caldera following rupture of a sealed hydrothermal system within the caldera [Waite and Smith, 2002]. The December–January 2008–2009 earthquake swarm consisted of 811 earthquakes with M < 4.1. The swarm front migrated laterally along a N-S vertical plane of hypocenters at a rate of 1 km d−1 and vertically from maximum focal depths of 12 km to 2 km (Figure 2b) beneath the northern part of Yellowstone Lake [Farrell et al., 2010]. It was proposed that the swarm was induced by magmatic fluid migration or propagation of a poroelastic stress pulse along a preexisting fracture zone [Farrell et al., 2010]. The January 2010 Madison Plateau swarm (Figure 2c) near the northwest boundary of the Yellowstone Caldera was thought to be triggered by the rupture of a zone of confined high-pressure aqueous fluids into a preexisting crustal fault system, prompting release of accumulated stress [Shelly et al., 2013]. The earthquake centroids migrated with time outward from the initial source. The injection of fluid from high- to low-pressure domains may have been accommodated by hybrid shear and dilatational failure, as is commonly observed in exhumed hydrothermally affected fault zones [e.g., Hill, 1977; Sibson, 1987, 1996]. Shelly et al. [2013] showed that the spatial-temporal migration of the earthquake activity front in the three major swarms can be well fit by a diffusion equation , where r is distance from the initial source, D is hydraulic diffusivity, and t is time. Radiocarbon concentrations in individual growth rings in a tree core from Mud Volcano (Figure 1) showed a sharp ~25% drop in 14C, interpreted as due to a fivefold increase in CO2 emission during the year after a seismic swarm in 1978 [Evans et al., 2010]. These authors concluded that a large pulse of CO2 traversed Yellowstone's hydrothermal system in a relatively short time (<1 year) and ultimately triggered the swarm seismicity [Evans et al., 2010]. Examples linking temporal variations of magmatic volatile discharge with episodes of caldera inflation and deflation were also documented at the Campi Flegrei Caldera in southern Italy. Since measurements began in 1983, increases in some fumarole gas ratios (CO2/H2O, CO2/CH4, and CO2/H2S) were coincident with caldera inflation and earthquake swarms resulting from repeat injections of magmatic gases into the hydrothermal system [Chiodini et al., 2003, 2012a; Caliro et al., 2014]. Chloride discharge measurements through the major rivers of the YPVF have been carried out since 1983 (Figure 6). One of the goals of this effort was to establish a temporal correlation between chloride discharge and inflation and deflation cycles of the Yellowstone Caldera that could indicate possible leakages of magmatic chloride into the hydrothermal system [Dzurisin et al., 1990; Fournier, 2004]. Fournier [1989] estimated that addition of about 0.2–0.4% magmatic brine could account for the concentration of chloride in the deepest circulating thermal water in the YPVF, and therefore, a small increase in magmatic brine input to the hydrothermal system would likely cause a significant change in the chloride discharge. However, no correlation has been found to date in the temporal trends of chloride discharge and deformation (Figures 3 and 6) [Fournier, 2004; Hurwitz et al., 2007b]. Based on the heterogeneous distribution of chloride among the drainage basins in the YPVF and the assumption of a single, uniform parent thermal fluid, Hurwitz et al. [2007b] proposed that large-scale (tens of kilometers) lateral redistribution of Cl− might take decades or longer, as thermal water flows toward its discharge sites (e.g., the geyser basins along the drainages of the Firehole and Gibbon Rivers). With this extended time lag, correlations on annual or decadal time scales should not be expected. Further, chloride discharge variations are mainly dominated by the annual hydrologic cycle [Hurwitz et al., 2007b, 2010], and any variation due to magmatic Cl injection rate would likely be small in comparison. Figure 6Open in figure viewerPowerPoint Chloride discharge from the Yellowstone, Madison, Snake, and Fall Rivers between 1990 and 2010. Details on the calculations and methods are in Friedman and Norton [2007], and Hurwitz et al. [2007b, 2007c]. The error bars represent an estimated 5% uncerta