Yellowstone Plateau Volcanic Field Research Articles

Radiogenic Strontium‐ and Uranium‐Isotope Tracers of Water‐Rock Interactions and Hydrothermal Flow in the Upper Geyser Basin, Yellowstone Plateau Volcanic Field, USA

AbstractNatural radiogenic isotopes (primarily 87Sr/86Sr) from hot springs in the Upper Geyser Basin of the Yellowstone Plateau volcanic field and associated rocks were used to evaluate groundwater flow patterns, water‐rock reactions, and the extent of mixing between various groundwater sources. Thermal waters have very low uranium concentrations and 234U/238U activity ratios near 1.0, which limit their utility as tracers in this reducing setting. Thermal waters have higher Sr concentrations (<22 ng/g) and a wide range of 87Sr/86Sr values that vary both temporally at individual discharge sites and between adjacent springs, indicating that conduits tap different subsurface reservoirs to varying degrees. Sr from local rhyolites have 87Sr/86Sr compositions that bound the range of values observed in groundwater throughout the basin. Non‐boiling springs on the west flank of the basin discharge water with low 87Sr/86Sr consistent with flow through young volcanic rocks exposed at the surface. Boiling springs in the central basin have higher 87Sr/86Sr values reflecting interactions with older, more radiogenic volcanic rocks. Variability in upwelling thermal waters requires mixing with a low 87Sr/86Sr component derived from young lava or glacial sediments, or more likely, from deeper sources of hot groundwater circulating through buried Lava Creek Tuff having intermediate 87Sr/86Sr. Isotope data constrain basin‐wide output of thermal water to 110–140 kg·s−1. Results underscore the utility of radiogenic Sr isotopes as valuable tracers of hydrothermal flow patterns and improve the understanding of temperature‐dependent water‐rock reactions in one of the largest continental hydrothermal systems on Earth.

Rehydrated glass embayments record the cooling of a Yellowstone ignimbrite

Hydration fronts penetrate 50−135 μm into glassy rhyolite embayments hosted in quartz crystals from the Mesa Falls Tuff in the Yellowstone Plateau volcanic field. The hydration fronts occur as steep enrichments that reach 2.4 ± 0.6 wt% H2O at the embayment opening, representing much higher values than interior concentrations of 0.9 ± 0.2 wt% H2O. Molecular water accounts for most of the water enrichment. Water speciation indicates the hydration fronts comprise absorbed meteoric water that modified the original magmatic composition of the rhyolitic glass. We used finite difference diffusion models to demonstrate that glass rehydration was likely produced over a few decades as the ignimbrite cooled. Such temperatures and time scales are consistent with rare firsthand observations of decadal hydrothermal systems associated with cooling ignimbrites at Mount Pinatubo (Philippines) and the Valley of Ten Thousand Smokes (Alaska).

Travertine records climate-induced transformations of the Yellowstone hydrothermal system from the late Pleistocene to the present

Chemical changes in hot springs, as recorded by thermal waters and their deposits, provide a window into the evolution of the postglacial hydrothermal system of the Yellowstone Plateau Volcanic Field. Today, most hydrothermal travertine forms to the north and south of the ca. 631 ka Yellowstone caldera where groundwater flow through subsurface sedimentary rocks leads to calcite saturation at hot springs. In contrast, low-Ca rhyolites dominate the subsurface within the Yellowstone caldera, resulting in thermal waters that rarely deposit travertine. We investigated the timing and origin of five small travertine deposits in the Upper and Lower Geyser Basins to understand the conditions that allowed for travertine deposition. New 230Th-U dating, oxygen (δ18O), carbon (δ13C), and strontium (87Sr/86Sr) isotopic ratios, and elemental concentrations indicate that travertine deposits within the Yellowstone caldera formed during three main episodes that correspond broadly with known periods of wet climate: 13.9−13.6 ka, 12.2−9.5 ka, and 5.2−2.9 ka. Travertine deposition occurred in response to the influx of large volumes of cold meteoric water, which increased the rate of chemical weathering of surficial sediments and recharge into the hydrothermal system. The small volume of intracaldera travertine does not support a massive postglacial surge of CO2 within the Yellowstone caldera, nor was magmatic CO2 the catalyst for postglacial travertine deposition.

Effects of hydrogeochemistry on the microbial ecology of terrestrial hot springs.

Temperature, pH, and hydrochemistry of terrestrial hot springs play a critical role in shaping thermal microbial communities. However, the interactions of biotic and abiotic factors at this terrestrial-aquatic interface are still not well understood on a global scale, and the question of how underground events influence microbial communities remains open. To answer this, 11 new samples obtained from the El Tatio geothermal field were analyzed by 16S rRNA amplicon sequencing (V4 region), along with 191 samples from previous publications obtained from the Taupo Volcanic Zone, the Yellowstone Plateau Volcanic Field, and the Eastern Tibetan Plateau, with their temperature, pH, and major ion concentration. Microbial alpha diversity was lower in acid-sulfate waters, and no significant correlations were found with temperature. However, moderate correlations were observed between chemical parameters such as pH (mostly constrained to temperatures below 70°C), SO4 2- and abundances of members of the phyla Armatimonadota, Deinococcota, Chloroflexota, Campilobacterota, and Thermoplasmatota. pH and SO4 2- gradients were explained by phase separation of sulfur-rich hydrothermal fluids and oxidation of reduced sulfur in the steam phase, which were identified as key processes shaping these communities. Ordination and permutational analysis of variance showed that temperature, pH, and major element hydrochemistry explain only 24% of the microbial community structure. Therefore, most of the variance remained unexplained, suggesting that other environmental or biotic factors are also involved and highlighting the environmental complexity of the ecosystem and its great potential to test niche theory ecological associated questions. IMPORTANCE This is the first approach to investigate whether geothermal processes could have an influence on the ecology of thermal microbial communities on a global scale. In addition to temperature and pH, microbial communities are structured by sulfate concentrations, which depends on the tectono-magmatic settings (such as the depth of magmatic chambers) and the local settings (such as the availability of a confining layer separating NaCl waters from steam after phase separation) and the possibility of mixing with more diluted fluids. Comparison of microbial communities from different geothermal areas by homogeneous sequence processing showed that no significant geographic distance decay was detected on the microbial communities according to Bray-Curtis, Jaccard, unweighted, and weighted Unifrac similarity/dissimilarity indices. Instead, an ancient potential divergence in the same taxonomic groups is suggested between globally distant thermal zones.

Quantifying Non‐Thermal Silicate Weathering Using Ge/Si and Si Isotopes in Rivers Draining the Yellowstone Plateau Volcanic Field, USA

AbstractIn active volcanic regions, high‐temperature chemical reactions in the hydrothermal system consume CO2 sourced from magma or from the deep crust, whereas reactions with silicates at shallow depths mainly consume atmospheric CO2. Numerous studies have quantified the load of dissolved solids in rivers that drain volcanic regions to determine chemical weathering rates and atmospheric CO2 consumption rates. However, the balance between thermal and non‐thermal components to riverine fluxes in these areas remains poorly constrained, hindering accurate estimates of atmospheric CO2 consumption rates. Here we use the Ge/Si ratio and the stable silicon isotopes (δ30Si) as tracers for quantifying non‐thermal silicon contributions in rivers draining the Yellowstone Plateau Volcanic Field, USA. The Ge/Si ratio (µmol.mol−1) was determined for seven thermal water samples (183 ± 22), eight rivers (35 ± 23) and six creeks flowing into Yellowstone Lake (5 ± 3) during base flow and during peak water discharge following snowmelt. The δ30Si value (‰) was determined for thermal waters (−0.09 ± 0.04), Yellowstone River at Yellowstone Lake outlet (1.91 ± 0.23) and creek samples (0.82 ± 0.29). The calculated atmospheric CO2 consumption associated with non‐thermal waters flowing through Yellowstone's rivers during peak discharge is ∼3.03 ton.km−2.yr−1, which is ∼2% of the annual mean atmospheric CO2 consumption in other volcanic regions. This study highlights the significance of quantifying seasonal variations in chemical weathering rates for improving estimates of atmospheric CO2 consumption rates in active volcanic regions.

The Systematics of Chlorine, Lithium, and Boron andδ37Cl,δ7Li, and δ11B in the Hydrothermal System of the Yellowstone Plateau Volcanic Field

AbstractChlorine, lithium, and boron are trace elements in rhyolite but are enriched in groundwater flowing through rhyolite because they tend to partition into the fluid phase during high‐temperature fluid‐rock reactions. We present a large data set of major element and δ37Cl, δ7Li, and δ11B compositions of thermal water and rhyolite from Yellowstone Plateau Volcanic Field (YPVF). The Cl/B, Cl/Li, δ37Cl (−0.2‰ to +0.7‰), and δ11B (−6.2‰ to −5.9‰) values of alkaline‐chloride thermal waters reflect high‐temperature leaching of chlorine, lithium, and boron from rhyolite that has δ37Cl and δ11B values of +0.1‰ to +0.9‰ and −6.3‰ to −6.2‰, respectively. Chlorine and boron are not reactive, but lithium incorporation into hydrothermal alteration minerals result​s in a large range of Cl/Li, B/Li, and δ7Li (−1.2‰ to +3.8‰) values in thermal waters. The relatively large range in δ7Li values of thermal waters reflects a large range of values in rhyolite. Large volumes of rhyolite must be leached to account for the chloride, lithium and boron fluxes, implying deep groundwater flow through rhyolite flows and tuffs representing Yellowstone's three eruptive cycles (∼2.1 Ma). Lower Cl/B values in acid‐sulfate waters result from preferential partitioning of boron into the vapor phase and enrichment in the near‐surface water condensate. The Cl/B, Cl/Li, δ7Li (−0.3‰ to +2.1‰), and δ11B (−8.0‰ to −8.1‰) values of travertine depositing calcium‐carbonate thermal waters which discharge in the northern and southern YPVF suggest that chlorine, lithium, and boron are derived from Mesozoic siliciclastic sediments which contain detrital material from the underlying metamorphic basement.

Sources, fate, and flux of riverine solutes in the Southwest Yellowstone Plateau Volcanic Field, USA

Since the 1970s, temporal variations of hydrothermal discharge and thermal output from the numerous hydrothermal features in the Yellowstone Plateau Volcanic Field (YPVF) have been studied by measuring the chloride flux in the major rivers. In this study, the sources, fate, and flux of solutes in the Fall River and its major tributaries, in southwest Yellowstone National Park, were determined. The considerable precipitation in southwest YPVF and high groundwater flow through Quaternary rhyolites results in river solute fluxes that originate from shallow non-thermal groundwater and deep-thermal water. Specific conductance serves as a surrogate measure for thirteen riverine solute concentrations. Combining continuous 15-minute specific conductance and discharge data, the annual chloride, arsenic, fluoride, and silica fluxes from the Fall River were determined to be 11%, 5%, 25%, and 19% of the total flux exiting YPVF. Approximately 11% of the Fall River chloride flux is from non-thermal waters, which is larger than the previous estimate of 4 to 6%. Furthermore, a large proportion of fluoride and silica in the Fall River are derived from water-rock interaction in the shallow non-thermal groundwater system and the non-thermal weathering rate (30 ± 2 t/yr·km2) is higher than other rivers draining the Yellowstone caldera. Consequently, 73 ± 3% of the annual total dissolved solid flux in the Fall River is from thermal sources. Synoptic sampling of river water and discharge measurements was performed during low-flow conditions that allowed for the determination of solute sources and their downstream fate. It was determined that chloride, sodium, arsenic, rubidium, lithium, and boron are primarily (>89%) associated with thermal waters and the Bechler River is the primary source of most hydrothermal solutes in the Fall River, but the major source of arsenic is Boundary Creek. Using the chloride inventory method, the thermal water discharge from several thermal areas was also determined.

Hydrothermal Activity in the Southwest Yellowstone Plateau Volcanic Field

AbstractIn the past two decades, the U.S. Geological Survey and the National Park Service have studied hydrothermal activity across the Yellowstone Plateau Volcanic Field (YPVF) to improve the understanding of the magmatic‐hydrothermal system and to provide a baseline for detecting future anomalous activity. In 2017 and 2018 we sampled water and gas over a large area in the southwest YPVF and used Landsat 8 thermal infrared data to estimate radiative heat flow. Most of the thermal activity in this region is in close proximity to the Yellowstone Caldera boundary. Springs and fumaroles discharge from a variety of lithologies, including some of the youngest rhyolites in the YPVF. Gas compositions and helium isotope ratios of most samples resemble those in other parts of the YPVF. The waters have meteoric origins, and tritium was detected in several samples. Thermal waters from some areas have compositions that plot along a line connecting thermal and nonthermal water endmember compositions. The thermal water endmember equilibrated at 160°C–170°C, lower than waters in Yellowstone's geyser basins. Heat discharged by springs and fumaroles originates from within the Yellowstone Caldera and is transported laterally by advection, mainly along the base of rhyolite flows that cover the inferred caldera boundaries.

The timing and origin of pre- and post-caldera volcanism associated with the Mesa Falls Tuff, Yellowstone Plateau volcanic field

We present new sanidine 40Ar/39Ar ages and paleomagnetic data for pre- and post-caldera rhyolites from the second volcanic cycle of the Yellowstone Plateau volcanic field, which culminated in the caldera-forming eruption of the Mesa Falls Tuff at ca. 1.3Ma. These data allow for a detailed reconstruction of the eruptive history of the second volcanic cycle and provide new insights into the petrogenesis of rhyolite domes and flows erupted during this time period. 40Ar/39Ar age data for the biotite-bearing Bishop Mountain flow demonstrate that it erupted approximately 150kyr prior to the Mesa Falls Tuff. Integrating 40Ar/39Ar ages and paleomagnetic data for the post-caldera Island Park rhyolite domes suggests that these five crystal-rich rhyolites erupted over a centuries-long time interval at 1.2905±0.0020Ma (2σ). The biotite-bearing Moonshine Mountain rhyolite dome was originally thought to be the downfaulted vent dome for the pre-caldera Bishop Mountain flow due to their similar petrographic and oxygen isotope characteristics, but new 40Ar/39Ar dating suggest that it erupted near contemporaneously with the Island Park rhyolite domes at 1.2931±0.0018Ma (2σ) and is a post-caldera eruption. Despite their similar eruption ages, the Island Park rhyolite domes and the Moonshine Mountain dome are chemically and petrographically distinct and are not derived from the same source. Integrating these new data with field relations and existing geochemical data, we present a petrogenetic model for the formation of the post-Mesa Falls Tuff rhyolites. Renewed influx of basaltic and/or silicic recharge magma into the crust at 1.2905±0.0020Ma led to [1] the formation of the Island Park rhyolite domes from the source region that earlier produced the Mesa Falls Tuff and [2] the formation of Moonshine Mountain dome from the source region that earlier produced the biotite-bearing Bishop Mountain flow. These magmas were stored in the crust for less than a few thousand years before being erupted contemporaneously along a 30km long, structurally controlled vent zone related to extracaldera Basin and Range faults. These data highlight the rapidity with which magma can be generated and erupted over large distances at Yellowstone.

Geophysical imaging of shallow degassing in a Yellowstone hydrothermal system

AbstractThe Yellowstone Plateau Volcanic Field, which hosts over 10,000 thermal features, is the world's largest active continental hydrothermal system, yet very little is known about the shallow “plumbing” system connecting hydrothermal reservoirs to surface features. Here we present the results of geophysical investigations of shallow hydrothermal degassing in Yellowstone. We measured electrical resistivity, compressional‐wave velocity from refraction data, and shear wave velocity from surface‐wave analysis to image shallow hydrothermal degassing to depths of 15–30 m. We find that resistivity helps identify fluid pathways and that Poisson's ratio shows good sensitivity to saturation variations, highlighting gas‐saturated areas and the local water table. Porosity and saturation predicted from rock physics modeling provide critical insight to estimate the fluid phase separation depth and understand the structure of hydrothermal systems. Finally, our results show that Poisson's ratio can effectively discriminate gas‐ from water‐saturated zones in hydrothermal systems.

The Grizzly Lake complex (Yellowstone Volcano, USA): Mixing between basalt and rhyolite unraveled by microanalysis and X-ray microtomography

Magma mixing is a widespread petrogenetic process. It has long been suspected to operate in concert with fractional crystallization and assimilation to produce chemical and temperature gradients in magmas. In particular, the injection of mafic magmas into felsic magma chambers is widely regarded as a key driver in the sudden triggering of what often become highly explosive volcanic eruptions. Understanding the mechanistic event chain leading to such hazardous events is a scientific goal of high priority. Here we investigate a mingling event via the evidence preserved in mingled lavas using a combination of X-ray computed microtomographic and electron microprobe analyses, to unravel the complex textures and attendant chemical heterogeneities of the mixed basaltic and rhyolitic eruption of Grizzly Lake in the Norris-Mammoth corridor of the Yellowstone Plateau volcanic field (YVF). We observe evidence that both magmatic viscous inter-fingering of magmas and disequilibrium crystallization/dissolution processes occur. Furthermore, these processes constrain the timescale of interaction between the two magmatic components prior to their eruption. X-ray microtomography images show variegated textural features, involving vesicle and crystal distributions, filament morphology, the distribution of enclaves, and further textural features otherwise obscured in conventional 2D observations and analyses. Although our central effort was applied to the determination of mixing end members, analysis of the hybrid portion has led to the discovery that mixing in the Grizzly Lake system was also characterized by the disintegration and dissolution of mafic crystals in the rhyolitic magma. The presence of mineral phases in both end member, for example, forsteritic olivine, sanidine, and quartz and their transport throughout the magmatic mass, by a combination of both mixing dynamics and flow imposed by ascent of the magmatic mass and its eruption, might have acted as a “geometric perturbation” of flow fields further fuelling mass exchange between magmas in terms of both chemical diffusion and crystal transfer. These results illuminate the complexity of mixing in natural magmatic systems, identifying several reaction-related textural factors that must be understood more deeply in order to advance our understanding of this igneous process.

Dynamics of the Yellowstone hydrothermal system

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.

Heat flow in vapor dominated areas of the Yellowstone Plateau Volcanic Field: Implications for the thermal budget of the Yellowstone Caldera

Characterizing the vigor of magmatic activity in Yellowstone requires knowledge of the mechanisms and rates of heat transport between magma and the ground surface. We present results from a heat flow study in two vapor dominated, acid‐sulfate thermal areas in the Yellowstone Caldera, the 0.11 km2 Obsidian Pool Thermal Area (OPTA) and the 0.25 km2 Solfatara Plateau Thermal Area (SPTA). Conductive heat flux through a low permeability layer capping large vapor reservoirs is calculated from soil temperature measurements at >600 locations and from laboratory measurements of soil properties. The conductive heat output is 3.6 ± 0.4 MW and 7.5 ± 0.4 MW from the OPTA and the SPTA, respectively. The advective heat output from soils is 1.3 ± 0.3 MW and 1.2 ± 0.3 MW from the OPTA and the SPTA, respectively and the heat output from thermal pools in the OPTA is 6.8 ± 1.4 MW. These estimates result in a total heat output of 11.8 ± 1.4 MW and 8.8 ± 0.4 MW from OPTA and SPTA, respectively. Focused zones of high heat flux in both thermal areas are roughly aligned with regional faults suggesting that faults in both areas serve as conduits for the rising acid vapor. Extrapolation of the average heat flux from the OPTA (103 ± 2 W·m−2) and SPTA (35 ± 3 W·m−2) to the ∼35 km2 of vapor dominated areas in Yellowstone yields 3.6 and 1.2 GW, respectively, which is less than the total heat output transported by steam from the Yellowstone Caldera as estimated by the chloride inventory method (4.0 to 8.0 GW).

Lead isotope ratios of volcanic glass by laser ablation inductively-coupled plasma mass spectrometry: Application to Miocene tephra beds in Montana, USA and adjacent areas

Recent improvements in spatial resolution and sensitivity of laser ablation inductively-coupled plasma mass spectrometry allow exploration of the tephrochronologic potential of Pb isotope ratios of volcanic glass, in particular, their use in characterization and provenance detection of distal tephra beds. Measurements have good accuracy and precision, as determined by comparison with reference materials. A test suite of samples was taken from Miocene tephra beds in Montana and SW Saskatchewan, all of which come from the Yellowstone hotspot track (YHST), but only those tephra beds of known age were used because of the need to track any temporal changes in the Pb isotope ratios. Samples of a few Quaternary tephra beds from the Yellowstone Plateau volcanic field were also included. The Pb isotope ratio analyses form three distinct clusters on a 207Pb/ 208Pb – 206Pb/ 208Pb plot with ages of 0.6–2.0 Ma, 5.5–7.8 Ma, and 7.4–11.1 Ma. The same Pb isotope–age clusters were reproduced when published Pb isotope information on dated rocks from the YHST is used. Rocks from the Heise volcanic field and from a core recovered between the Heise and Picabo volcanic fields all fall within the younger Miocene group, whereas rocks from the Bruneau-Jarbidge volcanic field plot in the older Miocene group. Therefore, Pb isotope ratios of glass shards do help in provenance detection of distal tephra beds – at least in the tectono-volcanic setting of the YHST – and will be a useful addition to the criteria necessary to establish a reliable Miocene tephrostratigraphic framework for Montana and adjacent areas.

Large-volume Rhyolite Genesis in Caldera Complexes of the Snake River Plain: Insights from the Kilgore Tuff of the Heise Volcanic Field, Idaho, with Comparison to Yellowstone and Bruneau–Jarbidge Rhyolites

Generation of large-volume rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous rhyolites from the 6·6–4·5 Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along the Yellowstone hotspot track and it culminated with the eruption of the most voluminous low-δ 18 O rhyolite known on Earth: the 1800 km 3 Kilgore Tuff (δ 18 O = 3·4‰). Such low-δ 18 O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low-δ 18 O magma value of 3·4 ± 0·1‰ (2 standard error) for Kilgore Tuff samples erupted >100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with δ 18 O values that range from –1·3‰ to 6·1‰. U–Pb zircon ages, mineral chemistry, whole-rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the Kilgore Tuff. Normal-δ 18 O Heise tuff units that preceded the Kilgore Tuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low-δ 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal-δ 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low-δ 18 O Kilgore Tuff from remelting of hydrothermally altered ( 18 O depleted) intracaldera and subvolcanic portions of the Blacktail Creek Tuff. Single pockets of melt with variable low-δ 18 O values were assembled and homogenized on a caldera-wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the δ 18 O diversity in Kilgore Tuff zircons. Temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic δ 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in δ 18 O by ∼1‰ for every ∼1000 km 3 of erupted rhyolite. The Kilgore Tuff of the Heise volcanic field has the same timing, magnitude of δ 18 O depletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the Kilgore Tuff may serve as a useful analog for these and perhaps other large-volume low-δ 18 O rhyolites on Earth.

River solute fluxes reflecting active hydrothermal chemical weathering of the Yellowstone Plateau Volcanic Field, USA

In the past few decades numerous studies have quantified the load of dissolved solids in large rivers to determine chemical weathering rates in orogenic belts and volcanic areas, mainly motivated by the notion that over timescales greater than ~ 100 kyr, silicate hydrolysis may be the dominant sink for atmospheric CO 2, thus creating a feedback between climate and weathering. Here, we report the results of a detailed study during water year 2007 (October 1, 2006 to September 30, 2007) in the major rivers of the Yellowstone Plateau Volcanic Field (YPVF) which hosts Earth's largest “restless” caldera and over 10,000 thermal features. The chemical compositions of rivers that drain thermal areas in the YPVF differ significantly from the compositions of rivers that drain non-thermal areas. There are large seasonal variations in river chemistry and solute flux, which increases with increasing water discharge. The river chemistry and discharge data collected periodically over an entire year allow us to constrain the annual solute fluxes and to distinguish between low-temperature weathering and hydrothermal flux components. The TDS flux from Yellowstone Caldera in water year 2007 was 93 t/km 2/year. Extensive magma degassing and hydrothermal interaction with rocks accounts for at least 82% of this TDS flux, 83% of the cation flux and 72% of the HCO 3 − flux. The low-temperature chemical weathering rate (17 t/km 2/year), calculated on the assumption that all the Cl − is of thermal origin, could include a component from low-temperature hydrolysis reactions induced by CO 2 ascending from depth rather than by atmospheric CO 2. Although this uncertainty remains, the calculated low-temperature weathering rate of the young rhyolitic rocks in the Yellowstone Caldera is comparable to the world average of large watersheds that drain also more soluble carbonates and evaporates but is slightly lower than calculated rates in other, less-silicic volcanic regions. Long-term average fluxes at Yellowstone are likely ~ 20% higher than those in the abnormally dry water year 2007, but the protocol used in this study can be easily adaptable to track future changes in low-temperature weathering and hydrothermal flux components, which could provide better monitoring of magmatic unrest.

Density and lithospheric strength models of the Yellowstone–Snake River Plain volcanic system from gravity and heat flow data

The structure and composition of the Yellowstone–Snake River Plain (YSRP) volcanic system were analyzed using gravity data taken at over 30,000 stations in the YSRP and surrounding region. Additional constraints were provided by tomographic seismic velocity models, new heat flow and temperature information, GPS-derived strain rates, earthquake locations, and chemical analyses of volcanic rocks. P-wave velocity models and velocity–density data based on petrologic information were used to constrain three-dimensional density models. Rheology and strength properties were calculated at selected locations and compared to earthquake focal depths. Results of this study suggest that the lower crust of the Snake River Plain (SRP) has been thickened by the addition of an underplated layer composed primarily of clinopyroxene, having a density of 3200 kg/m 3. A mid-crustal high-density sill is composed of a series of gabbroic lenses inter-fingering with the granitic upper crust. This geometry yields a bulk composition comparable to diorite and a density of 2900 kg/m 3. The mid-crustal sill varies from 4 to 11 km in thickness, resulting in a series of SE-NW-trending gravity highs observed along the axis of the SRP. The mid-crustal sill extends up to 20 km southeast of the SRP volcanic field and causes asymmetry of the gravity field. The Yellowstone Plateau volcanic field density model reveals low-density partial melt 10 km beneath the caldera that shallows under the northeastern caldera, and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by ~ 20%. The caldera melt body has a density of 2520 kg/m 3 and a significantly lower value of 2470 kg/m 3 for the northeastern caldera melt body. Southwest of Yellowstone, the crustal section occupied by the mid-crustal sill in the SRP and the partial melt in Yellowstone constitute a transition area between the active Yellowstone magma system and the now volcanically quiescent SRP, with a density of 2820 kg/m 3. Strength models show that the crust of the YSRP becomes progressively stronger and cooler with increasing distance from Yellowstone, and tectonic earthquakes within the Yellowstone caldera are unlikely to nucleate below 4–6 km depth, thus limiting the maximum magnitude of earthquakes to M ≤ 6.5.

Mercury isotopic composition of hydrothermal systems in the Yellowstone Plateau volcanic field and Guaymas Basin sea-floor rift

To characterize mercury (Hg) isotopes and isotopic fractionation in hydrothermal systems we analyzed fluid and precipitate samples from hot springs in the Yellowstone Plateau volcanic field and vent chimney samples from the Guaymas Basin sea-floor rift. These samples provide an initial indication of the variability in Hg isotopic composition among marine and continental hydrothermal systems that are controlled predominantly by mantle-derived magmas. Fluid samples from Ojo Caliente hot spring in Yellowstone range in δ 202Hg from − 1.02‰ to 0.58‰ (± 0.11‰, 2SD) and solid precipitate samples from Guaymas Basin range in δ 202Hg from − 0.37‰ to − 0.01‰ (± 0.14‰, 2SD). Fluid samples from Ojo Caliente display mass-dependent fractionation (MDF) of Hg from the vent (δ 202Hg = 0.10‰ ± 0.11‰, 2SD) to the end of the outflow channel (δ 202Hg = 0.58‰ ± 0.11‰, 2SD) in conjunction with a decrease in Hg concentration from 46.6 pg/g to 20.0 pg/g. Although a small amount of Hg is lost from the fluids due to co-precipitation with siliceous sinter, we infer that the majority of the observed MDF and Hg loss from waters in Ojo Caliente is due to volatilization of Hg 0 (aq) to Hg 0 (g) and the preferential loss of Hg with a lower δ 202Hg value to the atmosphere. A small amount of mass-independent fractionation (MIF) was observed in all samples from Ojo Caliente (Δ 199Hg = 0.13‰ ±1 0.06‰, 2SD) but no significant MIF was measured in the sea-floor rift samples from Guaymas Basin. This study demonstrates that several different hydrothermal processes fractionate Hg isotopes and that Hg isotopes may be used to better understand these processes.

Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western USA

The Snake River Plain (SRP) developed over the last 16 Ma as a bimodal volcanic province in response to the southwest movement of the North American plate over a fixed melting anomaly. Volcanism along the SRP is dominatedby eruptions of explosive high-silica rhyolites and represents some of the largest eruptions known. Basaltic eruptions represent the final stages of volcanism, forming a thin cap above voluminous rhyolitic deposits. Volcanism progressed, generally from west to east, along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera-forming eruptions within a particular field separated by ca. 0.5-1 Ma, similar to, and in continuation with, the present-day Yellowstone Plateau volcanic field. Passage of the North American plate over the melting anomaly at a particular point in time and space was accompanied by uplift, regional tectonism, massive explosive eruptions, and caldera subsidence, and followed by basaltic volcanism and general subsidence. The Heise volcanic field in the eastern SRP, Idaho, represents an adjacent and slightly older field immediately to the southwest of the Yellowstone Plateau volcanic field. Five large-volume (>0.5 km 3 ) rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism for the study region. Field relations and high-precision 4 0 Ar/ 3 9 Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic field and form the framework of the Heise Group. These are the Blacktail Creek Tuff (6.62 ′ 0.03 Ma), Walcott Tuff (6.27 ′ 0.04 Ma), Conant Creek Tuff (5.51 ′ 0.13 Ma), and Kilgore Tuff (4.45 ′ 0.05 Ma; all errors reported at ′ 2a). The fifth widespread ignimbrite in the region is the Arbon Valley Tuff Member of the Star-light Formation (10.21 ′ 0.03 Ma), which erupted from a caldera source outside of the Heise volcanic field. These results establish the Conant Creek Tuff as a distinct and widespread ignimbrite in the Heise volcanic field, eliminating former confusion resulting from previous discordant K/Ar and fission-track dates. New 4 0 Ar/ 3 9 Ar determinations, when combined with geochemical, lithologic, geophysical, and field data, define the volcanic and tectonic history of the Heise volcanic field and surrounding areas. Volcanic units erupted from the Heise volcanic field also provide temporal control for tectonic events associated with late Cenozoic extension in the Snake Range and with uplift of the Teton Range, Wyoming. In the Snake Range, movement of large (≥0.10 km 3 ) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise field occurred between 6.3 and 5.5 Ma and may have been catastrophically triggered by the caldera eruption of the 5.51 ′ 0.13-Ma Conant Creek Tuff. This slide block movement of ∼300 vertical meters indicates that the Snake Range had significant relief by at least 5.5 Ma. In Jackson Hole, the distribution of outflow facies of the 4.45 ′ 0.05-Ma Kilgore Tuff related to eruption from the Kilgore caldera in the Heise volcanic field on the eastern SRP indicates that the northern Teton Range was not a significant topographic feature at this time.

Revised ages for tuffs of the Yellowstone Plateau volcanic field: Assignment of the Huckleberry Ridge Tuff to a new geomagnetic polarity event

40 Ar/ 39 Ar ages were determined on the three major ash-flow tuffs of the Yellowstone Plateau volcanic field in the region of Yellowstone National Park in order to improve the precision of previously determined ages. Total-fusion and incremental- heating ages of sanidine yielded the following mean ages: Huckleberry Ridge Tuff—2.059 ± 0.004 Ma; Mesa Falls Tuff— 1.285 ± 0.004 Ma; and Lava Creek Tuff— 0.639 ± 0.002 Ma. The Huckleberry Ridge Tuff has a transitional magnetic direction and has previously been related to the Reunion Normal- Polarity Subchron. Dating of the Reunion event has been reviewed and its ages have been normalized to a common value for mineral standards. The age of the Huckleberry Ridge Tuff is significantly younger than lava flows of the Reunion event on Re union Island, supporting other evidence for a normal-polarity event younger than the Reunion event.