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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Mixing effects on geothermometric calculations of the Newdale geothermal area in the Eastern Snake River Plain, Idaho

The Newdale geothermal area in Madison and Fremont Counties in Idaho is a known geothermal resource area whose thermal anomaly is expressed by high thermal gradients and numerous wells producing warm water (up to 51 °C). Geologically, the Newdale geothermal area is located within the Eastern Snake River Plain (ESRP) that has a time-transgressive history of sustained volcanic activities associated with the passage of Yellowstone Hotspot from the southwestern part of Idaho to its current position underneath Yellowstone National Park in Wyoming. Locally, the Newdale geothermal area is located within an area that was subjected to several overlapping and nested caldera complexes. The Tertiary caldera forming volcanic activities and associated rocks have been buried underneath Quaternary flood basalts and felsic volcanic rocks. Two southeast dipping young faults (Teton dam fault and an unnamed fault) in the area provide the structural control for this localized thermal anomaly zone. Geochemically, water samples from numerous wells in the area can be divided into two broad groups – Na-HCO3 and Ca-(Mg)-HCO3 type waters and are considered to be the product of water-rhyolite and water-basalt interactions, respectively. Each type of water can further be subdivided into two groups depending on their degree of mixing with other water types or interaction with other rocks. For example, some bivariate plots indicate that some Ca-(Mg)-HCO3 water samples have interacted only with basalts whereas some samples of this water type also show limited interaction with rhyolite or mixing with Na-HCO3 type water. Traditional geothermometers [e.g., silica variants, Na-K-Ca (Mg-corrected)] indicate lower temperatures for this area; however, a traditional silica-enthalpy mixing model results in higher reservoir temperatures. We applied a new multicomponent equilibrium geothermometry tool (e.g., Reservoir Temperature Estimator, RTEst) that is based on inverse geochemical modeling which explicitly accounts for boiling, mixing, and CO2 degassing. RTEst modeling results indicate that the well water samples are mixed with up to 75% of the near surface groundwater. Relatively, Ca-(Mg)-HCO3 type water samples are more diluted than the Na-HCO3 type water samples. However, both water types result in similar reservoir temperatures, up to 150 °C. Samples in the vicinity of faults produced higher reservoir temperatures than samples away from the faults. Although both the silica-enthalpy mixing and RTEst models indicated promising geothermal reservoir temperatures, evaluation of the subsurface permeability and extent of the thermal anomaly is needed to define the hydrothermal potential of the Newdale geothermal resource.

Eastern Snake River Plain, Newdale, RTEst, geother↗
DOE OSTI2020

Hawaii Play Fairway Analysis: Noble Gas Raw Data for Hawaii, Maui, Oahu, Kauai, and Lanai islands

Noble gas raw data for the Hawaiian islands of Big Island, Maui, Oahu, Lanai, and Kauai. Based on results from prior phases of the Hawaii Play Fairway Analysis, this project targeted 66 wells on the islands of Hawaii, Maui, Lanai, Oahu, and Kauai for sampling of dissolved noble gases, trace metals, common ions, and the stable isotopes 2H and 18O. Ultimately, 23 of the 66 well targets were sampled. Noble gas data from this study is supplemented with data shared by the United States Geologic Survey for the summit of Kilauea, and by the geothermal energy company Ormat Technologies Inc. for their geothermal power plant Puna Geothermal Venture on the Lower East Rift of Kilauea, and for their exploration of Kona and Hualalai on Hawaii, as well as the Southwest Rift of Haleakala on Maui. The noble gas helium is used as an indicator of geothermal heat when excess 3He and/or 4He is present when compared to the atmospheric ratio of those isotopes (R/Ra). R/Ra is minimally affected by dilution and transport, allowing even those wells not perfectly situated over a geothermal system to indicate a geothermal anomaly. R/Ra anomalies are present on every island in this study. There is a strong correlation between R/Ra anomalies and proximity to rift zones and calderas. The Hawaii Play Fairway project was funded by the U.S. Department of Energy Geothermal Technologies Office (award DE-EE0006729). For more information, see Colin Ferguson's Master of Science thesis "Exploration for Blind Geothermal Resources in the State of Hawaii Utilizing Dissolved Noble Gasses in Well Waters."

15 GEOTHERMAL ENERGY↗
DOE OSTI2017

Three-Dimensional Electrical Resistivity Tomography of the Solfatara Crater (Italy): Implication for the Multiphase Flow Structure of the Shallow Hydrothermal System: Resistivity Model of Solfatara Crater

The Solfatara volcano is the main degassing area of the Campi Flegrei caldera, characterized by 30 of unrest. Assessing and understanding such renewal activity is a challenging task because hydrothermal interactions with magmatic gases remain poorly understood. In this study, we decipher the complex structure of the shallow Solfatara hydrothermal system by performing the first 3-D, high-resolution, Electrical Resistivity Tomography (ERT) of the volcano. The 3-D resistivity model was obtained from the inversion of 43,432 resistance measurements performed on an area of 0.68 km 2 . The proposed interpretation of the multiphase hydrothermal structures is based on the resistivity model, a high-resolution infrared surface temperature image, and 1,136 soil CO 2 flux measurements. In addition, we realized 27 soil Cation Exchange Capacity (CEC) and pH measurements demonstrating a negligible contribution of surface conductivity to the shallow bulk electrical conductivity. Hence, we demonstrate that the resistivity changes are mainly controlled by fluid content and temperature. The high-resolution tomograms identify for the first time the structure of the gas-dominated reservoir at 50 m depth that feeds the Bocca Grande fumarole through a ~10-m-thick channel. In addition, the resistivity model reveals a channel-like conductive structure where the liquid produced by steam condensation around the main fumaroles flows down to the Fangaia area within a buried fault. The model delineates the emplacement of the main geological structures: Mt Olibano, Solfatara crypto-dome, and tephra deposits. It also reveals the anatomy of the hydrothermal system, especially two liquid-dominated plumes, the Fangaia mud pool and the Pisciarelli fumarole, respectively.

58 GEOSCIENCES↗
DOE OSTI2013

Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body

Supereruptions violently transfer huge amounts (100 s-1000 s km{sup 3}) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted {approx}760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500-3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies.

58 GEOSCIENCES↗
DOE OSTI2015

The Syrtis Major volcano, Mars: A multidisciplinary approach to interpreting its magmatic evolution and structural development: EVOLUTION OF THE SYRTIS MAJOR VOLCANO

Very weak crustal magnetic fields over the Syrtis Major volcanic complex imply almost total thermal demagnetization via magmatic intrusions over a large area less than ~4Ga. We fit a model of these intrusions and the resulting thermal demagnetization to maps of crustal magnetic field strength at 185km altitude. The best fits are most consistent with a "dog bone"-shaped region of intrusive material, elongated approximately north-south, with an area of ~350,000km 2 and an inferred volume of ~4-19×10 6 km 3 . Such a large volume is best explained by a long-lived mantle plume beneath the Syrtis edifice. A free-air gravity anomaly high over the Syrtis Major caldera is consistent with dense mafic residue remaining at depth following crystal fractionation that produced the silicic magmas seen at the surface. The elongation of this region is consistent with ascent and north-south emplacement of magma enabled by structures parallel to and associated with the preexisting Isidis impact basin.

58 GEOSCIENCES↗
DOE OSTI2015

Numerical modeling of cold magmatic CO 2 flux measurements for the exploration of hidden geothermal systems

The most accepted conceptual model to explain surface degassing of cold magmatic CO 2 in volcanic-geothermal systems involves the presence of a gas reservoir. Here, numerical simulations using the TOUGH2-ECO2N V2.0 package are performed to get quantitative insights into how cold CO 2 soil flux measurements are related to reservoir and fluid properties. While the modeling is based on flux data measured at a specific geothermal site, the Acoculco caldera (Mexico), some general insights have been gained. Both the CO 2 fluxes at the surface and the depth at which CO 2 exsolves are highly sensitive to the dissolved CO 2 content of the deep fluid. If CO 2 mainly exsolves above the reservoir within a fracture zone, the surface CO 2 fluxes are not sensitive to the reservoir size but depend on the CO 2 dissolved content and the rock permeability. For gas exsolution below the top of the reservoir, surface CO 2 fluxes also depend on the gas saturation of the deep fluid as well as the reservoir size. The absence of thermal anomalies at the surface is mainly a consequence of the low enthalpy of CO 2 . The heat carried by CO 2 is efficiently cooled down by heat conduction and to a certain extent by isoenthalpic volume expansion depending on the temperature gradient. Thermal anomalies occur at higher CO 2 fluxes (>37,000 g m -2 d -1 ) when the heat flux of the rising CO 2 is not balanced anymore. Lastly, specific results are obtained for the Acoculco area (reservoir depth, CO 2 dissolved content, and gas saturation state).

58 GEOSCIENCES↗
DOE OSTI2015

Ascent and emplacement dynamics of obsidian lavas inferred from microlite textures

Here, to assess the eruption and emplacement of volumetrically diverse rhyolite lavas, we measured microlite number densities and orientations from samples collected from nine lavas in Yellowstone Caldera and two from Mono Craters, USA. Microlite populations are composed of Fe-Ti oxides ± alkali feldspar ± clinopyroxene. Number densities range from 10 8.11 ± 0.03 to 10 9.45 ± 0.15 cm -3 and do not correlate with distance from the vent across individual flows and are remarkably similar between large- and small-volume lavas. Together, those observations suggest that number densities are unmodified during emplacement and that ascent rates in the conduit are similar between small domes and large lava flows. Microtextures produced by continuous decompression experiments best replicate natural textures at decompression rates of 1–2 MPa hr -1 . Acicular microlites have a preferred orientation in all natural samples. Because the standard deviation of microlite orientation does not become better aligned with distance travelled, we conclude that microlites exit the conduit aligned and that strain during subaerial flow was insufficient to further align microlites. The orientations of microlite trend and plunge in near-vent samples indicate that pure shear was the dominant style of deformation in the conduit. In conclusion, we speculate that collapsing permeable foam(s) provides a mechanism to concurrently allow microlite formation and alignment in response to the combination of degassing and flattening by pure shear.

58 GEOSCIENCES↗
DOE OSTI2016

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Furthermore open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

58 GEOSCIENCES↗
DOE OSTI2021

Understanding long-term groundwater flow at Pahute Mesa and vicinity, Nevada National Security Site, USA, from naturally occurring geochemical and isotopic tracers

Abstract Recently collected naturally occurring geochemical and isotopic groundwater tracers were combined with historic data from the Pahute Mesa area of the Nevada National Security Site (NNSS), Nevada, USA, to provide insights into long-term regional groundwater flow patterns, mixing and recharge. Pahute Mesa was the site of 85 nuclear detonations between 1965 and 1992, many of them deeply buried devices that introduced radionuclides directly into groundwater. The dataset examined included major ions and field measurements, stable isotopes of hydrogen (δ 2 H), oxygen (δ 18 O), carbon (δ 13 C) and sulfur (δ 34 S), and radioisotopes of carbon ( 14 C) and chloride ( 36 Cl). Analysis of the patterns of groundwater 14 C data and the δ 2 H and δ 18 O signatures indicates that groundwater recharge is predominantly of Pleistocene age, except for a few localized areas near major ephemeral drainages. Steep gradients in sulfate (SO 4 ) and chloride (Cl) define a region near the western edge of the NNSS where high-concentration groundwater flowing south from north of the NNSS merges with dilute groundwater flowing west from eastern Pahute Mesa in a mixing zone that coincides with a groundwater trough associated with major faults. The 36 Cl/Cl and δ 34 S data suggest that the source of the high Cl and SO 4 in the groundwater was a now-dry, pluvial-age playa lake north of the NNSS. Patterns of groundwater flow indicated by the combined data sets show that groundwater is flowing around the northwest margin of the now extinct Timber Mountain Caldera Complex toward regional discharge areas in Oasis Valley.

58 GEOSCIENCES↗
DOE OSTI2022

Using groundwater noble gas measurements to confirm paleorecharge hypotheses at Pahute Mesa, Nevada National Security Site, Nevada, USA

Pahute Mesa (Nevada, USA) was the site of 85 underground nuclear tests between 1965 and 1992 whose residual radio-chemical inventory poses a contaminant threat to local groundwater resources. To assess the fate and transport of these radionuclides in groundwater, several geochemical studies, including this one, have been conducted to understand long-term groundwater flow patterns at, and downgradient of, Pahute Mesa. Groundwater samples from the Pahute Mesa area at the Nevada National Security Site were analyzed for noble gases ( 3 He/ 4 He isotopic ratio and He, Ne, Kr, Ar and Xe concentrations) to infer groundwater recharge temperatures and provide additional evidence in support of groundwater flow patterns and the presence of late-pluvial Pleistocene groundwater. These data confirm recent findings (based on groundwater geochemistry and isotope compositions) that most recharge in the area occurred under temperature conditions much cooler than are found today, except near major drainages where ephemeral run of continues to recharge shallow groundwater. These noble gas data indicate that noble gas recharge temperatures and terrigenic He components have been reset where groundwater has flowed upward over barriers formed by caldera margins, or near the regional discharge area (Oasis Valley) where shallow groundwater has interacted with shallow soil gas. This evidence from groundwater noble gas measurements, in combination with other isotopic and geochemical studies, supports the current understanding of the unusual hydrogeology and recharge history of Pahute Mesa.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2015

Snowpack-climate manipulation using infrared heaters in subalpine forests of the Southern Rocky Mountains, USA

Effects of infrared heaters on snow accumulation, snowmelt, and snow–atmosphere energy exchange were examined at Niwot Ridge, Colorado (CO) and compared to a naturally warmer, but otherwise similar subalpine site in the Valles Caldera National Preserve, New Mexico (NM). Observed snow accumulation was 30% lower on average and snow melted out 16 days earlier in the heated plots compared to the controls. Soil temperature during snowmelt was 3 °C greater on average and soil moisture was 4% lower on average in heated plots compared to controls. In NM, snow accumulation was 23% lower, snow melted 23 days earlier, soil temperature was 0.6 °C greater, and soil moisture was 13% lower on average relative to CO controls. In order to estimate differences in energy and mass balance fluxes at the snow–atmosphere interface in control versus warmer plots, the 1-D, physically based snowmelt model, SNOWPACK, was used. Model results indicated that heaters alter radiative, turbulent and mass fluxes by amounts comparable to the differences between CO and NM fluxes. The proportion of the energy flux associated with latent heat exchange during snowmelt was 9–27% of the total energy flux in heated models and 19–22% of NM models compared to 3–7% in control models. Thus, sublimation loss to the atmosphere was greater in both experimentally and naturally warmer cases relative to the control case. We conclude that IR heaters can provide alterations to the timing and magnitude of snow accumulation and snowmelt consistent with conditions observed at a warmer analog site and with climate and hydrology model projections. Finally, impacts of IR heating on energy partitioning and sublimation should be considered when designing manipulations of the snowpack, as reductions in snowmelt water may alter biological or ecological processes.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

High pressure experimental investigation of clinopyroxene dissolution in a K-basaltic melt

Dissolution of clinopyroxene (cpx) in a K-basaltic melt from the Campi Flegrei Volcanic District (Italy) has been investigated through dissolution and dissolution-crystallization experiments at pressure of 0.8 GPa, superliquidus temperature of 1350 °C, and dwell times between 0.5 and 1 h. The obtained dissolution rates range from 7.9·10 –6 cm s –1 to 6.1·10 –6 cm s –1 as a function of dwell time. In the dissolution-crystallization experiment (1300 °C; 0.8 GPa; 2 h), the formation of overgrowth rims accompanied by new cpx crystals suggests that the injection of recharge magmas in basaltic reservoir may lead to inverse or oscillatory zonation. The interaction between cpx and K-basaltic melt at ~1570 °C was studied by in situ radiography using synchrotron radiation combined with the Paris-Edinburgh press. Here, this resulted in cpx resorption to occur depending on the temperature conditions with respect to the liquidus temperature of the cpx (T cpxL ). The calculated cpx dissolution rates are ~5·10 –3 cm s –1 at T ≤ T cpxL and ~ 3·10 –2 cm s –1 at T ≥ T cpxL . The role of crystal dissolution in the estimation of magma residence times has been also tested for a natural magmatic system by interpolating the dissolution rates (~10 –5 –10 –6 cm s –1 ) with the textural data of cpx phenocrysts from the Agnano-Monte Spina pyroclastic deposit at Campi Flegrei caldera (Campanian region, Italy). Results from calculations indicate that the time required for partial or complete resorption of phenocrysts varies from ~0.5 to ~40 h, and that the effect of crystal dissolution may be relevant to estimate magma residence times whether significant dissolution occurs during magma mixing processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Play fairway analysis of geothermal resources across the State of Hawaii: 1. Geological, geophysical, and geochemical datasets

Phase 1 of a Play Fairway Analysis (PFA) of geothermal resource potential across the State of Hawaii was recently completed. The final products of this work include a statewide geothermal resource probability map, a map of confidence in this probability, an assessment of the viability of development in areas of interest, and a prioritized list of recommended future exploration activities. The intersection of subsurface heat (H), permeability (P), and fluid (F) is necessary for an optimum geothermal play. This study: (1) identified and compiled all legacy and current data relevant to Hawaii’s geothermal resource; (2) ranked these datasets in terms of their relevance to subsurface H, P, F; (3) developed a Bayesian statistical method to incorporate the data and their rankings, and produce a statewide resource probability map; (4) developed a method to assess confidence in the probability values; and (5) assessed what we term ‘development viability’ in resulting areas of interest across the state. This paper details the basic project workflow, and activities (1) and (2). It describes the rationale for including this study’s datasets: surface geologic mapping data (calderas, rift zones, volcanic vents, dikes, faults), groundwater data (temperature, Chloride:Magnesium, SiO 2 ) and geophysical data (gravity, magnetotelluric, seismic, geodetic strain), and justifies their relative rankings in terms of H, P, and F. A second paper describes activities (3) and (4) related to the statistical methodology, and a third paper describes activity (5), the development viability criteria, as well as our suggested roadmap for future exploration activities. Overall, we find that the likelihood of abundant of geothermal resources is highest on the youngest island of Hawaii, but groundwater indicators suggest there may also be resources on the other, older islands. Furthermore, the higher demand for renewable energy on the more populated islands of Maui and Oahu, as well as a high viability of development on Lanai, also motivate further exploration on these islands.

15 GEOTHERMAL ENERGY↗
DOE OSTI2013

Study of seismicity around Toba area based on relocation hypocenter result from BMKG catalogue

Toba area has complex tectonic setting attracting many earth scientists to study and understand tectonic and geological process or setting. The area is affected by oblique subduction zone, Renun Sumatran fault sub segment and some volcanoes that are near it. The earthquake catalogue provided by BMKG from April, 2009 to December, 2011 must be relocated firstly to get the precise hypocenter. We used catalogue data of P and S phase or P phase only and double-difference method to relocate the earthquakes. The results show hypocenter position enhancement that can be interpreted tectonically. The earthquakes after relocation relating to the Sumatran fault, subduction zone, volcanoes and seismic activities beneath Toba caldera can be mapped clearly. The relocated hypocenters in this study are very important to provide information for seismic hazard assessment and disaster mitigation study.

58 GEOSCIENCES↗
DOE OSTI2020

Complex subsurface hydrothermal fluid mixing at a submarine arc volcano supports distinct and highly diverse microbial communities

Hydrothermally active submarine volcanoes are mineral-rich biological oases contributing significantly to chemical fluxes in the deep sea, yet little is known about the microbial communities inhabiting these systems. Here we investigate the diversity of microbial life in hydrothermal deposits and their metagenomics-inferred physiology in light of the geological history and resulting hydrothermal fluid paths in the subsurface of Brothers submarine volcano north of New Zealand on the southern Kermadec arc. From metagenome-assembled genomes we identified over 90 putative bacterial and archaeal genomic families and nearly 300 previously unknown genera, many potentially endemic to this submarine volcanic environment. While magmatically influenced hydrothermal systems on the volcanic resurgent cones of Brothers volcano harbor communities of thermoacidophiles and diverse members of the superphylum “DPANN,” two distinct communities are associated with the caldera wall, likely shaped by two different types of hydrothermal circulation. The communities whose phylogenetic diversity primarily aligns with that of the cone sites and magmatically influenced hydrothermal systems elsewhere are characterized predominately by anaerobic metabolisms. These populations are probably maintained by fluids with greater magmatic inputs that have interacted with different (deeper) previously altered mineral assemblages. However, proximal (a few meters distant) communities with gene-inferred aerobic, microaerophilic, and anaerobic metabolisms are likely supported by shallower seawater-dominated circulation. Furthermore, mixing of fluids from these two distinct hydrothermal circulation systems may have an underlying imprint on the high microbial phylogenomic diversity. Collectively our results highlight the importance of considering geologic evolution and history of subsurface processes in studying microbial colonization and community dynamics in volcanic environments.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2016

Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)

Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufrière Hills deposits, indicating that the island experienced large-magnitude, sub-aerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit's combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly sub-rounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (< 2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water.

58 GEOSCIENCES↗
DOE OSTI2019

Supereruption quartz crystals and the hollow reentrants

Hollow reentrants in quartz phenocrysts from Yellowstone (western United States) caldera’s Lava Creek Tuff are preserved vestiges of bubbles in the supereruption’s pre-eruptive magma reservoir. We characterized the reentrants using a combination of petrographic techniques, synchrotron X-ray microtomography, and cathodoluminescence imagery. One or more reentrants occur in ~20% of quartz, and up to ~90% of those reentrants are hollow. The earliest-erupted parts of the Lava Creek Tuff have the most empty reentrants. The hollow reentrants provide direct, physical evidence for volatile saturation, exsolution, and retention in a magma reservoir. Quartz-melt surface tension permits bubbles to attach to quartz only when bubbles have been able to nucleate and grow in the melt. Prior to eruption, the Lava Creek Tuff existed as a bubbly, volatile-saturated magma reservoir. Finally, the exsolved volatiles increased magma compressibility, helping to prevent the ever-accumulating magma from reaching a critical, eruptive overpressure until it reached a tremendous volume.

58 GEOSCIENCES↗