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At least 127 records · Page 7Linked to original sources
USGS2020

Geographic and oceanographic influences on ferromanganese crust composition along a Pacific Ocean meridional transect, 14N to 14S

The major controls on the variability of ferromanganese (FeMn) crust composition have been generally described over the past 40 years; however, most compilation studies lack quantitative statistics and are limited to a small region of several seamounts or compare FeMn crusts from disparate areas of the global oceans. This study provides the first detailed research to address the geographic and oceanographic controls of FeMn crust composition from a line of seamounts across 30° of latitude in the west central Pacific. Element concentrations from the uppermost layer (<15 mm) of 57 FeMn crusts were evaluated for statistically significant variance and correlation with a variety of oceanographic and geographic parameters. Manganese, Co, Ni, Mo, and Zn concentrations in crusts in this region are highly anticorrelated with seawater oxygen concentrations, suggesting oxygen as the dominant controlling factor for these elements. Iron instead correlates with water depth, which we attribute to increased carbonate ion concentration with increasing water depth. Silicon and Al content in crusts demonstrate a potential meridional variance of detrital inputs and sources in the region. Iron, Ba, and Mg are enriched in FeMn crusts below the equatorial upwelling zone which is related to biological productivity. Fluctuations in the four oceanographic and geographic parameters, seawater oxygen content, detrital input, surface productivity, and deep sources of iron, are robustly recorded by FeMn crusts. Modern measurements of these primary parameters, as well as paleoceanographic reconstructions, can be used to define regions of interest for FeMn crust exploration.

Geochemistry, Geophysics, Geosystems↗
USGS2020

Hydrothermal activity in the southwest Yellowstone Plateau Volcanic Field

In 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 non-thermal water endmember compositions. The thermal water endmember equilibrated at 160-170 °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.

Yellowstone National Park↗
USGS2020

Linking subsurface to surface using gas emission and melt inclusion data at Mount Cleveland volcano, Alaska

Mount Cleveland is one of Alaska's most active volcanoes, yet little is known about the magmatic system driving persistent and dynamic volcanic activity. Volcanic gas and melt inclusion (MI) data from 2016 were combined to investigate shallow magmatic processes. SO 2 emission rates were between 166 and 324 t/day and the H 2 O/SO 2 was 600 ± 53, whereas CO 2 and H 2 S were below detection. Olivine‐, clinopyroxene‐, and plagioclase‐hosted MIs have up to 3.8 wt.% H 2 O, 514 ppm CO 2 , and 2,320 ppm S. Equilibration depths, based on MI H 2 O contents, suggest that a magmatic column extended from 0.5 to 3.0 km (~10–60 MPa). We used MI data to empirically model open‐system H‐C‐S degassing from 0 to 12 km and found that a column of magma between 0.5 and 3 km could produce the measured gas H 2 O/SO 2 ratio. However, additional magma deeper than 3 km is required to sustain emissions over periods greater than days to weeks, if the observed vent dimension is a valid proxy for the conduit. Assuming an initial S content of 2,320 ppm, the total magma supply needed to sustain the annual SO 2 flux was 5 to 9.8 Mm 3 /yr, suggesting a maximum intrusive‐to‐extrusive ratio of 13:1. The model predicts degassing of <50 t/day CO 2 for July 2016, which corresponds to a maximum predicted CO 2 /SO 2 of 0.2. Ultimately, frequent recharge from deeper, less degassed magma is required to drive the continuous activity observed over multiple years. During periods of recharge we would expect lower H 2 O/SO 2 and measurable volcanic CO 2 .

Alaska↗
USGS2020

Local earthquake Vp and Vs tomography in the Mount St. Helens region with the iMUSH broadband array

We present new 3-D P wave and S wave velocity models of the upper 20 km of the Mount St. Helens (MSH) region. These were obtained using local-source arrival time tomography from earthquakes and explosions recorded at 70 broadband stations deployed as part of the imaging Magma Under St. Helens (iMUSH) project and augmented by several data sets. Principal features of our models include (1) low P wave and S wave velocities along the St. Helens seismic zone to depths of at least 20 km corresponding to high conductivity imaged by iMUSH magnetotelluric studies. This delineates a zone of weakness that magma can exploit at the location of MSH; (2) a 5- to 7-km diameter, 6–15 km deep, 3–6% negative P wave and S wave velocity anomaly beneath MSH, consistent with previous estimates of the source region for recent eruptions. We interpret this as a magma storage region containing up to 15–20 km 3 of partial melt, which is about 5 times more than the largest documented eruption at MSH; (3) a broad region of low P wave velocity below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and possible presence of fluids or melt; (4) several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. Our results place further constraints on the geometry of these features at depth.

Washington↗
USGS2020

Distribution of earthquakes on a branching fault system using integer programming and greedy sequential methods

A new global optimization method is used to determine the distribution of earthquakes on a complex, connected fault system. The method, integer programming, has been advanced in the field of operations research, but has not been widely applied to geophysical problems until recently. In this application, we determine the optimal distribution of earthquakes on mapped faults to minimize the global misfit in slip rates for multi-fault ruptures. Integer programming solves for a decision vector composed of every possible location that a sample of earthquakes can occur on every fault, subject to slip-rate uncertainty constraints. Step over connections are straightforward to include, whereas branching fault connections are not. To include branching ruptures, we distinguish between individual multi-fault rupture paths, as opposed to formulating the integer-programming problem based on individual faults as in previous studies. The new method is applied to the complex fault system in the San Francisco Bay Area as a case study. Results from the integer-programming method are compared to those from a local optimization method, termed the greedy-sequential method. Several experiments using these two methods indicate that shape of the on-fault magnitude distributions and which branching faults are involved in multi-fault ruptures depend on how much emphasis is placed on fitting the target slip rate. In cases where the underlying data are not strong enough to warrant chasing the target slip rate, it is better to focus on the distribution of feasible results that better represents the uncertainty in the solutions imposed by the data.

Geochemistry, Geophysics, Geosystems↗
USGS2020

Focused fluid flow along the Nootka Fault Zone and continental slope, Explorer-Juan de Fuca plate boundary

Geophysical and geochemical data indicate there is abundant fluid expulsion in the Nootka fault zone (NFZ) between the Juan de Fuca and Explorer plates and the Nootka continental slope. Here we combine observations from > 20 years of investigations to demonstrate the nature of fluid-flow along the NFZ, which is the seismically most active region off Vancouver Island. Seismicity reaching down to the upper mantle is linked to near-seafloor manifestation of fluid flow through a network of faults. Along the two main fault traces, seismic reflection data imaged bright spots 100 300 m below seafloor that lie above changes inbasement topography. The bright spots are conformable to sediment layering, show opposite-toseafloor reflection polarity, and are associated with frequency-reduction and velocity push-down indicating the presence of gas in the sediments. Two seafloor mounds ~15 km seaward of the Nootka slope are underlain by deep, non-conformable high amplitude reflective zones. Measurements in the water column above one mound revealed a plume of warm water, and bottom-video observations imaged hydrothermal vent system biota. Pore fluids from a core at this mound contain predominately microbial methane (C1) with a high proportion of ethane (C2) yielding C1/C2 ratios < 500 indicating a possible slight contribution from a deep source. We infer the reflective zones beneath the two mounds are basaltic intrusions that create hydrothermal circulation within the overlying sediments. Across the Nootka continental slope, gas hydrate related bottom-simulating reflectors are widespread and occur at depths indicating heat-flow values of 80 90 mW/m2.

Oregon, Washington↗
USGS2021

In‐situ mass balance estimates offshore Costa Rica

The Costa Rican convergent margin has been considered a type erosive margin, with erosional models suggesting average losses up to −153 km 3 /km/m.y. However, three‐dimensional (3D) seismic reflection and Integrated Ocean Drilling Program data collected offshore the Osa Peninsula images accretionary structures and vertical motions that conflict with the forearc basal erosion model. Here we integrate such data to do an in‐situ accounting of material transfer at the plate boundary across the outermost 10 km of the forearc, characterized by active and inactive megathrusts. Our in‐situ budget finds an approximate balance between sediment recycling via accretion and underplating, 0.7–2.3 km 3 /km/m.y., and basal erosion, 0.7 km 3 /km/m.y., while subducting sediment volumes, 7.8 km 3 /km/m.y., greatly outpace either material transfer volumes. These budget results differ significantly from published estimates based on simple proxies of trench axis deflection and slope subsidence. These budget results are the summation of thin incoming hemipelagic sediments that variably accrete along the deformation front, underplating of hemipelagic sediments on the upthrown‐side and basal erosion on the downthrown‐side of active plate bending faulting landward of the trench axis, and sediment subduction primarily composed of pelagic sediments.

Geochemistry, Geophysics, Geosystems↗
USGS2020

Hydrothermal alteration on composite volcanoes: Mineralogy, hyperspectral imaging and aeromagnetic study of Mt Ruapehu, New Zealand

Prolonged volcanic activity can induce surface weathering and hydrothermal alteration that is a primary control on edifice instability, posing a complex hazard with its challenges to accurately forecast and mitigate. This study uses a frequently active composite volcano, Mt Ruapehu, New Zealand, to develop a conceptual model of surface weathering and hydrothermal alteration applicable to long‐lived composite volcanoes. The alteration on Mt Ruapehu was classified using ground samples as non‐altered, supergene argillic, intermediate argillic, and advanced argillic. The first two classes have a paragenesis that is consistent with surficial infiltration and circulation of low‐temperature (<40°C) neutral to mildly acidic fluids, inducing chemical weathering and formation of weathering rims on rock surfaces. The intermediate and advanced argillic alteration formed from hotter (≥100°C) hydrothermal fluids with lower pH, interacting with the andesitic to dacitic host rocks. The distribution of weathering and hydrothermal alteration has been mapped with airborne hyperspectral imaging through image classification, while aeromagnetic data inversion was used to map alteration to up to 500‐m depth. The joint use of hyperspectral imaging complements the geophysical methods since it can spectrally identify hydrothermal alteration mineralogy. This study established a conceptual model of hydrothermal alteration history of Mt Ruapehu, exemplifying a long‐lived and nested active and ancient hydrothermal system. This study's combination approach can be used to indicate the most likely sources of future debris avalanches, which are a significant hazard on Ruapehu.

Mt Ruapehu↗
USGS2021

Coseismic surface displacement in the 2019 ridgecrest earthquakes: Comparison of field measurements and optical image correlation results

A fundamental topic in earthquake studies is understanding the extent to which fault rupture at the surface is localized on primary fault strands as opposed to distributed tens to hundreds of meters away from primary ruptures through off‐fault deformation (OFD) via a combination of discrete secondary faulting and bulk deformation. The 2019 Ridgecrest, CA M w 6.4 and M w 7.1 earthquakes provide an opportunity to explore this problem via comparison of published field‐based and mostly on‐fault offset measurements, with new lateral displacement measurements made over length scales of hundreds of meters across the primary rupture using WorldView satellite images collected before and after the earthquakes. A mean fit to the field observations underestimates net slip (pixel‐correlation results) by an average of 0.4 m along the M w 6.4 rupture (∼41% of net left‐lateral displacement) and 0.7 m along the M w 7.1 rupture (∼65% of net right‐lateral displacement). We attribute these differences to substantial OFD along the lengths of the M w 6.4 (∼59%) and M w 7.1 (∼35%) ruptures. A maximum fit to the field observations provides an improved match to the pixel correlation results (difference of 0.1–0.2 m or ∼76%–98% of net displacement). OFD may in part depend on displacement magnitude, where locations along the rupture with smaller displacements (<2 m) are characterized by a greater percentage of OFD. Furthermore, OFD increases with rupture zone width, especially at the scale of individual rupture strands (hundreds of meters). These findings contribute to a growing understanding of the important role of OFD in accommodating deformation near the surface.

California↗
USGS2021

Basalt geochemistry and mantle flow during early backarc basin evolution: Havre Trough and Kermadec Arc, southwest Pacific

The Havre Trough (HT) backarc basin in the southwest Pacific is in the rifting stage of development. We distinguish five types of basalt there based on their amount and kind of slab component: backarc basalts (BAB) with little or no slab component, modified BAB with slight amounts, reararc (RA) with more, remnants of the preexisting arc (Colville Ridge horsts), and arc front volcanoes within the HT. Previous subarc mantle is quickly removed and replaced by more fertile mantle with less slab component. The ambient mantle is “Pacific” isotopically, and more enriched in Nb/Yb and Nd and Hf isotope ratios north of the Central Kermadec Discontinuity at 32°S than to the south. The contrast may reflect inheritance in the south of mantle that was depleted during spreading that formed the southern South Fiji Basin and a higher degree of melting because of a wetter slab-derived flux. The slab component also differs along strike, more like a dry melt in the north and a supercritical fluid in the south. The mass fraction of slab component increases southward in the backarc as well as the arc front. RA volcanoes have the most slab component (1%–2%) and form indistinct ridges at high angles to, and <50 km behind, frontal volcanoes. Backarc basalts have less and occur throughout the basin. Slab components are distributed further into the backarc, and more irregularly, during the rifting than spreading stage of backarc basin development. The rifting stage is disorganized geochemically as well as spatially.

Geochemistry Geophysics Geosystems↗
USGS2021

The systematics of chlorine, lithium, and boron and δ37Cl, δ7Li, and δ11B in the hydrothermal system of the Yellowstone Plateau Volcanic Field

Chlorine, 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 δ 37 Cl, δ 7 Li, and δ 11 B compositions of thermal water and rhyolite from Yellowstone Plateau Volcanic Field (YPVF). The Cl/B, Cl/Li, δ 37 Cl (−0.2‰ to +0.7‰), and δ 11 B (−6.2‰ to −5.9‰) values of alkaline‐chloride thermal waters reflect high‐temperature leaching of chlorine, lithium, and boron from rhyolite that has δ 37 Cl and δ 11 B 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 δ 7 Li (−1.2‰ to +3.8‰) values in thermal waters. The relatively large range in δ 7 Li 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, δ 7 Li (−0.3‰ to +2.1‰), and δ 11 B (−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.

Wyoming↗
USGS2021

Magnetic surveys with unmanned aerial systems: Software for assessing and comparing the accuracy of different sensor systems, suspension designs and compensation methods

A typical problem for magnetic surveys with small Unmanned Aerial Systems (sUAS) is the heading error caused by undesired magnetic signals that originate from the aircraft. This can be addressed by suspending the magnetometers on sufficiently long tethers. However, tethered payloads require skilled pilots and are difficult to fly safely. Alternatively, the magnetometer can be fixed on the aircraft. In this case, aircraft magnetic signals are removed from the recordings with a process referred to as magnetic compensation, which requires parameters estimated from calibration flights flown in an area with magnetically low-gradients prior to the survey. We present open-source software fully written in Python to process data and compute compensations for two fundamentally different magnetometer systems (scalar and vector). We used the software to compare the precision of two commercially available systems by flying dense grid patterns over a 135 × 150 m area using different suspension configurations. The accuracy of the magnetic recordings is assessed using both standard deviations of the calibration pattern and tie-line cross-over differences from the survey. After compensation, the vector magnetometer provides the lowest heading error. However, the magnetic field intensity recovered with this system is relative and needs to be adjusted with absolute data if absolute intensity values are needed. Overall, the highest accuracy of all suspension configurations tested was obtained by fixing the magnetometer 0.5 m below the sUAS onto a self-built carbon-fiber frame, which also offered greater stability and allowed fully autonomous flights in a wide range of conditions.

Geochemistry, Geophysics, Geosystems (G-Cubed)↗
USGS2022

Multidisciplinary constraints on the thermal-chemical boundary between Earth's core and mantle

Heat flux from the core to the mantle provides driving energy for mantle convection thus powering plate tectonics, and contributes a significant fraction of the geothermal heat budget. Indirect estimates of core-mantle boundary heat flow are typically based on petrological evidence of mantle temperature, interpretations of temperatures indicated by seismic travel times, experimental measurements of mineral melting points, physical mantle convection models, or physical core convection models. However, previous estimates have not consistently integrated these lines of evidence. In this work, an interdisciplinary analysis is applied to co-constrain core-mantle boundary heat flow and test the thermal boundary layer (TBL) theory. The concurrence of TBL models, energy balance to support geomagnetism, seismology, and review of petrologic evidence for historic mantle temperatures supports Q CMB ∼15 TW, with all except geomagnetism supporting as high as ∼20 TW. These values provide a tighter constraint on core heat flux relative to previous work. Our work describes the seismic properties consistent with a TBL, and supports a long-lived basal mantle molten layer through much of Earth's history.

Geochemistry, Geophysics, Geosystems↗
USGS2021

STEPS: Slip time earthquake path simulations applied to the San Andreas and Toe Jam Hill Faults to redefine geologic slip rate uncertainty

Geologic slip rates are a time-averaged measurement of fault displacement calculated over hundreds to million-year time scales and are a primary input for probabilistic seismic hazard analyses, which forecast expected ground shaking in future earthquakes. Despite their utility for seismic hazard calculations, longer-term geologic slip rates represent a time-averaged measure of the tempo of strain release and do not measure variability across earthquake cycles. We have developed a numerical approach called STEPS (Slip Time Earthquake Path Simulations), which is built upon field-based observations and explicitly incorporates realistic variations in displacement per event and variability in the recurrence interval between earthquakes. The STEPS approach, which simulates strain release through time, relies on representing earthquake cycles as stairsteps, rather than straight-line paths, connecting per earthquake time-displacement coordinates. We simulate earthquake histories based on these input constraints using two examples: the Carrizo section of the San Andreas fault and the Toe Jam Hill fault of the Seattle fault zone. We find that modeled slip rate distributions agree with slip rates reported for the sites of interest by the original investigators, while providing a slip rate distribution that reflects the variability of earthquake frequency and displacement. The STEPS approach provides an estimate of fault slip rate uncertainty based on a synthetic suite of plausible time-displacement paths resulting from individual earthquakes, rather than measurement uncertainties associated with offset features. When considering this simulated earthquake behavior between measurements, the uncertainty associated with earthquake paths is greater than that calculated by the long-term rate.

California, Washington↗
USGS2021

Isolating detrital and diagenetic signals in magnetic susceptibility records from methane-bearing marine sediments

Volume-dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post-depositional alteration by H 2 S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane-bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X-ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high-resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H 2 S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ 13 C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo-positions of the sulfate-methane transition zone, past enhancements of OSR or paleo-methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps.

Geochemistry, Geophysics, Geosystems↗
USGS2021

Quantifying non-thermal silicate weathering using Ge/Si and Si isotopes in rivers draining the Yellowstone Plateau Volcanic Field, USA

In active volcanic regions, high-temperature chemical reactions in the hydrothermal system consume CO 2 sourced from magma or from the deep crust, whereas reactions with silicates at shallow depths mainly consume atmospheric CO 2 . Numerous studies have quantified the load of dissolved solids in rivers that drain volcanic regions to determine chemical weathering rates and atmospheric CO 2 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 CO 2 consumption rates. Here we use the Ge/Si ratio and the stable silicon isotopes (δ 30 Si) 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 δ 30 Si 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 CO 2 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 CO 2 consumption in other volcanic regions. This study highlights the significance of quantifying seasonal variations in chemical weathering rates for improving estimates of atmospheric CO 2 consumption rates in active volcanic regions.

Wyoming↗
USGS2021

Synchronous emplacement of the anorthosite xenolith-bearing Beaver River diabase and one of the largest lava flows on Earth

New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U-Pb zircon date of 1091.83 0.21 Ma (2 ) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross-cutting Silver Bay intrusion (1091.61 0.14 Ma; 2 ), these data tightly bracket the age of the Beaver River diabase to be 1091.7 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 0.27 Ma; 2 )—which is further supported by indistinguishable tilt-corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

Michigan, Minnesota↗
USGS2021

Multidisciplinary constraints on magma compressibility, the pre-eruptive exsolved volatile fraction, and the H2O/CO2 molar ratio for the 2006 Augustine eruption, Alaska

Geodetically modeled reservoir volume changes during volcanic eruptions are commonly much smaller than the observed eruptive volumes. This discrepancy is thought to be partially due to the compressibility of magma, which is largely controlled by the presence of exsolved volatiles. The 2006 eruption of Augustine Volcano, Alaska, produced an eruptive volume that was ∼3 times larger than the geodetically estimated syn-eruptive subsurface volume change. In this study, we use a multistep methodology that combines constraints from geodetic, volcanic gas, geologic, and petrologic data together with equations relating physical processes to observable parameters. We apply a Monte Carlo approach to quantify uncertainties. Ultimately, we solve for the exsolved volatile volume fraction and the magma compressibility. We estimate Augustine's 2006 pre-eruptive exsolved volatile phase to be ∼5.5 vol% of the magma at storage depths, yielding a bulk magma compressibility of ∼3.8 × 10 −10 Pa −1 . We develop a novel approach to estimate the H 2 O/CO 2 ratio of the syn-eruptive gas emissions in the absence of direct H 2 O emission measurements which are hard to obtain due to the high background levels in ambient air. We find a best-fit H 2 O/CO 2 molar ratio of 29. We also investigate the effects of applying different equations of state to our model. We find that the Ideal Gas Law might be used as a first approximation due to its simplicity; however, it overestimates volatile density and compressibility significantly at storage depths. This project capitalizes on the insights that can be gained by integrating multidisciplinary data with models of physical processes.

Alaska↗