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OFM Research West

Seattle, WA, United States
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Tramontano S.,Vanderbilt University | Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West
Earth and Planetary Science Letters | Year: 2017

Understanding silicic eruption triggers is paramount for deciphering explosive volcanism and its potential societal hazards. Here, we use phase equilibria modeling to determine the potential role of internal triggering – wherein magmas naturally evolve to a state in which eruption is inevitable – in rhyolitic magma bodies. Whole-rock compositions from five large to super-sized rhyolitic deposits are modeled using rhyolite-MELTS. By running simulations with varying water contents, we can track crystallization and bubble exsolution during magma solidification. We use simulations with variable enthalpy and fixed pressure for the five compositions. The interplay between bubble exsolution and crystallization can lead to an increase in the system volume, which can lead to magma overpressurization. We find that internal triggering is possible for high-silica rhyolite magmas crystallizing at pressures below 300 MPa (<11 km depth in the crust), revealing a window of eruptibility within the upper crust from which high-silica eruptions emanate. At higher pressures, the critical overpressure threshold for eruption is only reached once crystallinities are high, >50 wt.%, which makes magma immobile – high-silica rhyolite eruptions from such depths would require external triggering, but examples are scarce or entirely absent. Calculated crystallinities at which the critical overpressure threshold is reached compare favorably with observed crystal contents in natural samples for many systems, suggesting that internal evolution plays a critical role in triggering eruptions. Systems in which fluid saturation happens late relative to crystallization or in which degassing is effective can delay or avoid internal triggering. We argue that priming by crystallization and bubble exsolution is critical for magma eruption, and external triggering serves simply as the final blow – rather than being the driving force – of explosive rhyolitic eruptions. © 2017 Elsevier B.V.

Pamukcu A.S.,Brown University | Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West | Miller C.F.,Vanderbilt University | McCracken R.G.,Iowa State University
Contributions to Mineralogy and Petrology | Year: 2015

Establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt, but developing methods to constrain these pressures is challenging. We apply the new rhyolite-MELTS phase-equilibria geobarometer—based on the equilibrium between melt, quartz, and two feldspars—to matrix glass compositions from Peach Spring Tuff (Arizona–California–Nevada, USA) high-silica rhyolite. We compare the results to those from amphibole geothermobarometry, projection of glass compositions onto the haplogranitic ternary, and glass SiO2 geobarometry. Quartz + 2 feldspar rhyolite-MELTS pressures span a relatively small range (185–230 MPa), consistent with nearly homogeneous crystal compositions, and are similar to estimates based on projection onto the haplogranitic ternary (250 ± 50 MPa) and on glass SiO2 (255–275 MPa). Amphibole geothermobarometry gives much wider pressure ranges (temperature-independent: ~65–300 MPa; temperature-dependent: ~75–295 MPa; amphibole-only: ~80–950 MPa); average Anderson and Smith (Am Mineral 80:549–559, 1995) + Blundy and Holland (Contrib Miner Petrol 104:208–224, 1990) or Holland and Blundy (Contrib Miner Petrol 116:433–447, 1994—Thermometer A, B) pressures are most similar to phase-equilibria results (~220, 210, 190 MPa, respectively). Crystallization temperatures determined previously with rhyolite-MELTS (742 °C), Zr-in-sphene (769 ± 20 °C), and zircon saturation (770–780 °C) geothermometry are similar, but temperatures from amphibole geothermometry (~450–955 °C) are notably different; the average Anderson and Smith + Holland and Blundy (1994—Thermometer B; ~710 °C) temperature is most consistent with previous estimates. The rhyolite-MELTS geobarometer effectively culls glass compositions affected by alteration or analytical issues; Peach Spring glass compositions that yield pressure estimates reveal a tight range of plausible Na2O and K2O contents, suggesting that low Na2O and high K2O contents of many Peach Spring samples are due to alteration. Use of altered whole-pumice compositions in rhyolite-MELTS simulations is likely the cause of the incorrect crystallization sequence reported previously for Peach Spring compositions. Using the rhyolite-MELTS geobarometer, we estimate a more realistic composition for Peach Spring Tuff high-silica rhyolite, and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization. © 2015, Springer-Verlag Berlin Heidelberg.

Gardner J.E.,University of Texas at Austin | Befus K.S.,University of Texas at Austin | Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West
Contributions to Mineralogy and Petrology | Year: 2014

Thermodynamic models are vital tools to evaluate magma crystallization and storage conditions. Before their results can be used independently, however, they must be verified with controlled experimental data. Here, we use a set of hydrothermal experiments on the Late-erupted Bishop Tuff (LBT) magma to evaluate the rhyolite-MELTS thermodynamic model, a modified calibration of the original MELTS model optimized for crystallization of silicic magmas. Experimental results that are well captured by rhyolite-MELTS include a relatively narrow temperature range separating the crystallization of the first felsic mineral and the onset of the ternary minimum (quartz plus two feldspars), and extensive crystallization over a narrow temperature range once the ternary minimum is reached. The model overestimates temperatures by ~40 °C, a known limitation of rhyolite-MELTS. At pressures below 110 MPa, model and experiments differ in the first felsic phase, suggesting that caution should be exercised when applying the model to very low pressures. Our results indicate that for quartz, sanidine, plagioclase, magnetite, and ilmenite to crystallize in equilibrium from LBT magma, magma must have been stored at ≤740 °C, even when a substantial amount of CO2 occurs in the coexisting fluid. Such temperatures are in conflict with the hotter temperatures retrieved from magnetite-ilmenite compositions (~785 °C for the sample used in the experiments). Consistent with other recent studies, we suggest that the Fe-Ti oxide phases in the Late Bishop Tuff magma body are not in equilibrium with the other minerals and thus the retrieved temperature and oxygen fugacity do not reflect pre-eruptive storage conditions. © 2014 Springer-Verlag Berlin Heidelberg.

Gualda G.A.R.,Vanderbilt University | Pamukcu A.S.,Vanderbilt University | Ghiorso M.S.,OFM Research West | Anderson Jr. A.T.,University of Chicago | And 2 more authors.
PLoS ONE | Year: 2012

Supereruptions violently transfer huge amounts (100 s-1000 s km3) 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 ~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. © 2012 Gualda et al.

Spera F.J.,University of California at Santa Barbara | Ghiorso M.S.,OFM Research West | Nevins D.,University of California at Santa Barbara
Geochimica et Cosmochimica Acta | Year: 2011

Liquid MgSiO3 is a model for the Earth's magma ocean and of remnant melt present near the core-mantle boundary. Here, models for molten MgSiO3 are computed employing empirical potential molecular dynamics (EPMD) and results are compared to published results including two EPMD studies and three first-principles molecular dynamics (FPMD) models and to laboratory data. The EPMD results derived from the Oganov (OG) potential come closest to the density of MgSiO3 liquid at the 1-bar melting point inferred from the melting curve. At higher P, EPMD densities calculated from the OG potential and FPMD broadly match shock wave studies, with the OG potential yielding the better comparison. Matsui (M) potential results deviate from other studies above ~50GPa. Overall, results based on the OG potential compare best to experimental densities over the P-T range of the mantle. Isothermally, upon increasing P the mean coordination numbers (CN̄) of oxygen around Si and Mg monotonically increase with pressure. Tetrahedral Si and octahedral Si monotonically increase and decrease, respectively, whereas pentahedral Si maximizes at 10-20GPa. Tetrahedral Mg decreases monotonically as P increases whereas pentahedral, octahedral and higher coordination polyhedra each show similar behavior first increasing and then decreasing after attaining a maximum; the P of the maximum for each polyhedra type migrates to higher P as the CN increases. Free oxygen and oxygen with one nearest neighbor of either Si or Mg decreases whereas Si or Mg with two or three nearest oxygens (i.e., tricluster oxygen) increases with increasing P isothermally. The increase of tricluster oxygen is consistent with spectroscopy on MgSiO3 glass quenched from 2000K and 0-40GPa and high-energy X-ray studies constraining the coordination of O around Mg and around Si at 2300K and 1bar. Coordination statistics from FPMD studies for O around Si and Si around O are in agreement with the EPMD results based on the M and OG potentials. Mg self-diffusivity is greater than O and Si self-diffusivities for both the M and OG potentials. All D values monotonically decrease with increasing pressure isothermally and all atoms are more diffusive in the M liquid compared to the OG liquid except at T>~5000K and P>100GPa. Previously published EPMD diffusivities fall between values given by the M and OG potentials, at least up to 45GPa. The M liquid is generally less viscous than the OG liquid except at P>~80GPa. Activation energy and volume are around 96kJ/mol and 1.5cm3/mol, respectively. The FPMD viscosity results at 120GPa and 4000 and 4500K are essentially identical to the values from the M and OG potentials. FPMD viscosity results are similar to the OG results for P<60GPa; at higher P, the FPMD viscosities are higher. At 4000K and 100GPa the shear viscosity of liquid MgSiO3 is ~0.1Pas. More extensive laboratory results are required to better define the thermodynamic, transport and structural properties of MgSiO3 liquids and for comparison with computational studies. © 2010 Elsevier Ltd.

Martin G.B.,University of California at Santa Barbara | Ghiorso M.,OFM Research West | Spera F.J.,University of California at Santa Barbara
American Mineralogist | Year: 2012

Empirical potential molecular dynamics (EPMD) simulations of 1-bar eutectic composition liquid in the system CaAl 2Si 2O 8-CaMgSi 2O 6 have been conducted using the interatomic pair-potential of Matsui (1998). Simulations using ∼10 000 atoms over a wide range of conditions (r: 2200-5000 kg/m 3; T: 1600-5500 K; P: 0-170 GPa) were used to derive an equation of state, determine self-diffusivities for all atoms, calculate melt viscosity, and investigate melt structures by coordination statistics. EOS results compare well to laboratory shock wave data up to ∼25 GPa, diverging at higher pressure. Based on simulations of the end-member compositions of the join using the same potential, non-ideality in the volume of mixing at pressures below 10 GPa disappears at higher pressures. Ideal volume mixing at elevated pressure is consistent with inferences from laboratory shock wave studies of liquids in this system. The non-ideal volume of mixing at low pressure is directly correlated to structural differences between the end-member liquids and the mixing of cation-anion coordination polyhedra of differing volume. Self-diffusivities show reasonable agreement with laboratory values, with activation energies and activation volumes in the range 90-100 kJ/mol and 1-3 cm 3/mol, respectively. Shear viscosities at 3500 K span from 1.8 × 10 -3 Pas at low P to ∼4.4 × 10 -3 Pas at ∼14 GPa.

Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West
Geochemistry, Geophysics, Geosystems | Year: 2015

The thermodynamic modeling software MELTS is a powerful tool for investigating crystallization and melting in natural magmatic systems. Rhyolite-MELTS is a recalibration of MELTS that better captures the evolution of silicic magmas in the upper crust. The current interface of rhyolite-MELTS, while flexible, can be somewhat cumbersome for the novice. We present a new interface that uses web services consumed by a VBA backend in Microsoft Excel © 2014. American Geophysical Union. All Rights Reserved.

Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West
Contributions to Mineralogy and Petrology | Year: 2013

The Bishop Tuff, one of the most extensively studied high-silica rhyolite bodies in the world, is usually considered as the archetypical example of a deposit formed from a magma body characterized by thermal and compositional vertical stratification-what we call the Standard Model for the Bishop magma body. We present here new geothermometry and geobarometry results derived using a large database of previously published quartz-hosted glass inclusion compositions. Assuming equilibrium between melt and an assemblage composed of quartz, ±plagioclase, ±sanidine, +zircon, ±fluid, we use Zr contents in glass inclusions to derive quartz crystallization temperatures, and we use (1) silica contents in glass, (2) projection of glass compositions onto the haplogranitic (quartz-albite-orthoclase) ternary, and (3) phase equilibria calculations using rhyolite-MELTS, to constrain crystallization pressures. We find crystallization temperatures of ~740-750 °C for all inclusions from both early- and late-erupted pumice. Crystallization pressures for both early- and late-erupted inclusions are also very similar to each other, with averages of ~175-200 MPa. We find no evidence of late-erupted inclusions having been entrapped at higher temperatures or pressures than early-erupted inclusions, as would be expected by the Standard Model. We argue that the thermal gradient inferred from Fe-Ti oxides-the backbone of the Standard Model-does not reflect equilibrium pre-eruptive conditions; we also note that H2O-CO2 systematics of glass inclusions yields overlapping pressure ranges for early- and late-erupted inclusions, similar to the results presented here; and we show that glass inclusion and phenocryst compositions show bimodal distributions, suggestive of compositional separation between early- and late-erupted populations. These findings are inconsistent with the Standard Model. The similarity in crystallization conditions and the compositional separation between early- and late-erupted magmas suggest that two laterally juxtaposed independent magma reservoirs existed in the same region at the same time and co-erupted to form the Long Valley Caldera and the Bishop Tuff. This hypothesis would explain the lack of mixing between early- and late-erupted crystal populations in pumice clasts; it could also explain the inferred eruption pattern-which resulted in early-erupted magmas being deposited only to the south of the caldera-if the early-erupted magma body resided to the south and the late-erupted magma body was located to the north. Our alternative model is consistent with the patchy distribution of thermal anomalies and the inference of co-eruption of distinct magma types in active volcanic areas such as the central Taupo Volcanic Zone. © 2013 Springer-Verlag Berlin Heidelberg.

Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West | Lemons R.V.,Vanderbilt University | Carley T.L.,Vanderbilt University
Journal of Petrology | Year: 2012

Silicic magma systems are of great scientific interest and societal importance owing to their role in the evolution of the crust and the hazards posed by volcanic eruptions. MELTS is a powerful and widely used tool to study the evolution of magmatic systems over a wide spectrum of compositions and conditions. However, the current calibration of MELTS fails to correctly predict the position of the quartz + feldspar saturation surface in temperature, pressure and composition space, making it unsuitable to study silicic systems. We create a modified calibration of MELTS optimized for silicic systems, dubbed rhyolite-MELTS, using early erupted Bishop pumice as a reference. Small adjustments to the calorimetrically determined enthalpy of formation of quartz and of the potassium end-member of alkali feldspar in the MELTS calibration lead to much improved predictions of the quartz + feldspar saturation surface as a function of pressure. Application of rhyolite-MELTS to the Highland Range Volcanic Sequence (Nevada), the Peach Spring Tuff (Arizona-Nevada-California), and the late-erupted Bishop Tuff (California), using compositions that vary from trachydacite to high-silica rhyolite, shows that the calibration is appropriate for a variety of fluid-bearing silicic systems. Some key observations include the following. (1) The simulated evolutionary paths are consistent with petrographic observations and glass compositions; further work is needed to compare predicted and observed mineral compositions. (2) The nearly invariant nature of silicic magmas is well captured by rhyolite-MELTS; unusual behavior is observed after extensive pseudo-invariant crystallization, suggesting that the new calibration works best for relatively small (i.e. <50 wt %) crystallization intervals, comparable with what is observed in volcanic rocks. (3) Our success with rhyolite-MELTS shows that water-bearing systems in which hydrous phases do not play a critical role can be appropriately handled; simulations are sensitive to initial water concentration, and although only a pure-H 2O fluid is modeled, suitable amounts of water can be added or subtracted to mimic the effect of CO 2 in fluid solubility. Our continuing work on natural systems shows that rhyolite-MELTS is very useful in constraining crystallization conditions, and is particularly well suited to explore the eruptive potential of silicic magmas. We show that constraints placed by rhyolite-MELTS simulations using late-erupted Bishop Tuff whole-rock and melt inclusion compositions are inconsistent with a vertically stratified magma body. © The Author 2012. Published by Oxford University Press. All rights reserved.

Gualda G.A.R.,Vanderbilt University | Ghiorso M.S.,OFM Research West
Contributions to Mineralogy and Petrology | Year: 2014

Constraining the pressure of crystallization of magmas is an important but elusive task. In this work, we present a method to derive crystallization pressures for rocks that preserve glass compositions (either glass inclusions or matrix glass) representative of equilibration between melt, quartz, and 1 or 2 feldspars. The method relies on the well-known shift of the quartz-feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure. The critical realization for development of the method is the fact that melt, quartz and feldspars need to be in equilibrium at the liquidus for the melt composition. The method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite-MELTS over a range of pressures, and searching for the pressure at which the expected assemblage (quartz+1 feldspar or quartz+2 feldspars) is found at the liquidus. We evaluate errors resulting from uncertainties in glass composition using a series of Monte Carlo simulations for a quartz-hosted glass inclusion composition from the Bishop Tuff, which reveal errors on the order of 20-45 MPa for the quartz+2 feldspars constraint and on the order of 25-100 MPa for the quartz+1 feldspar constraint; we suggest actual errors are closer to the lower bounds of these ranges. We investigate the effect of fluid saturation in two ways: (1) By applying our procedure over a range of water contents for three glass compositions; we show that the effect of fluid saturation is more important at higher pressures (~300 MPa) than at lower pressures (~100 MPa), but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant H2O concentrations >3 wt% and (2) by performing the same type of pressure determinations with a preliminary version of rhyolite-MELTS that includes a H2O-CO2 mixed fluid phase; we use a range of H2O and CO2 concentrations for two compositions characteristic of early-erupted and late-erupted Bishop Tuff glass inclusions and demonstrate that calculated pressures are largely independent of CO2 concentration (for CO2 <1,000 ppm), at least for relatively high H2O contents, as expected in most natural magmas, such that CO2 concentration can be effectively neglected for application of our method. Finally, we demonstrate that pressures calculated using the rhyolite-MELTS geobarometer compare well with those resulting from H2O-CO2 glass inclusion barometry and Al-in-hornblende barometry for an array of natural systems for which data have been compiled from the literature; the agreement is best for quartz-hosted glass inclusions, while matrix glass yields systematically lower rhyolite-MELTS pressures, suggestive of melt evolution during eruptive decompression. © 2014 Springer-Verlag Berlin Heidelberg.

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