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2020Journal of Volcanology & Geothermal Research
Rhyolite lava emplacement dynamics inferred from surface morphology
Master's student Tyler Leggett published his work using stunning drone imagery to better understand the emplacement dynamics of rhyolite lavas. We focused on the South Coulee lava flow in Mono Craters, CA. Ridge spacing and block size distributions were used to calculate flow criteria, including eruptive timescales, strain rates, and flow velocity.
Supersaturation nucleation and growth of plagioclase: a numerical model
We share a Matlab model that predicts the nucleation and growth of plagioclase during magmatic ascent. This tool is built upon experimental results and can be used to better understand natural textures observed in volcanic rocks.
X-ray micro diffraction measurements indicate that volcanic stresses are preserved in quartz crystals from Yellowstone and Long Valley caldera. The source of the <300 MPa of residual stress remains uncertain but may be produced by force chains in the magma reservoir or stresses associated with the brittle failure of viscous melt.
Supereruption quartz crystals and the hollow reentrants
Quartz phenocrysts from the Lava Creek Tuff, Yellowstone caldera, contain empty reentrants. The reentrants are preserved vestiges of bubbles in the pre-eruptive chamber. They tell us the magma was bubbly in the reservoir.
We performed high temperature, continuous decompression experiments using a starting material of pumice from Pinatubo's climactic 1991 eruption. Plagioclase microlites nucleate and grow systematically in response to supersaturation-dependent disequilibrium.
We calibrate the pressure-dependent Raman shift of feldspars using a diamond anvil cell coupled with Raman spectroscopy. The calibration allows Raman spectroscopy of feldspars to be used to quantify PT conditions for magmatic rocks, low- to high-grade metamorphic rocks, and the mantle.
We use compositional gradients around spherulites in obsidians from Yellowstone caldera to estimate the temperature-time interval of spherulite crystallization. We also use X-ray computer tomography to measure spherulite size distributions.
Oxygen isotope ratios in quartz and alkali feldspar crystals in spherulites from Yellowstone caldera preserve fractionations indicating that spherulites nucleate at ~600 C and continue to grow until ~300 C.
We studied the pre-eruptive magmatic storage conditions of lavas from the Central Plateau Member Rhyolites, the most recent eruptions from Yellowstone caldera. Mineral compositions, melt inclusions, and high temperature experiments indicate the magmas were stored at ~750 C and <5 km depth.
We measured microlite number densities and orientations in obsidian collected at lavas at Yellowstone Caldera and Mono Craters, CA. Microlites textures are not sensitive to emplacement, and instead must be controlled by conduit processes.
Trace element profiles surrounding spherulites in rhyolitic obsidian preserve a record of their thermal history. Our results constrain spherulite growth to a temperature interval of 750 and 400 C, and demonstrate the host lava cooled at ~1 C per day.
We performed high temperature phase equilibria experiments on the Late Bishop Tuff to evaluate the performance of rhyolite-MELTS. The rhyolite-MELTS software captures experimental results well above 110 MPa, but caution should be exercised at lower pressures.
Douglas Knob is a small obsidian lava dome that erupted from Yellowstone Caldera. To better understand its magma storage and ascent processes, we analyzed mineral compositions, melt inclusions, and microlite orientations. We conclude it was stored at ~760 C and ~50 MPa. Microlites align in the conduit.
We measured the Cl concentrations and Cl isotope compositions of obsidian pyroclasts and obsidian chips from lava domes. Neither measurement of Cl tracks degassing processes, which is likely caused by disequilibrium and slow Cl diffusion.
In our first foray into spherulites, we use diffusion modeling of trace element gradients (water, F, and Rb) to estimate spherulite growth rates and lava cooling timescales. Spherulites from Tequila volcano in Mexico likely grew with a radial growth law below the glass transition temperature.
We performed an experimental FTIR study on quartz-hosted melt inclusions from the Tuff of Bluff Point, Yellowstone Caldera to determine if melt inclusions must be doubly exposed prior to analyses. We conclude that single exposure is bad, but unexposed inclusions can be reliable in a sufficiently large population.
Eocene basaltic magmas were shallowly emplaced into unconsolidated mudstones in the Big Bend area of West Texas. These subvolcanic, phreatomagmatic interactions are preserved in outcrops, which demonstrate a wide array of explosive and nonexplosive processes.
We performed detailed field work on a complex succession of basaltic pyroclastic deposits from a sequence of overlapping phreatomagmatic maar volcanoes. The unit sits near the KT boundary in west Texas and has often been cited for age constraints for studies on dinosaurs and other fauna.