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Deciphering the enigmatic origin of Guyana's diamonds
Guyana's alluvial diamonds have been mined from rivers deep in the Amazon jungle for the past century but they had never previously been studied. PhD student Roy Bassoo used diamond texture and composition to demonstrate they are likely detrital in origin, shed from the Paleoproterozoic Roraima Supergroup.
Volcanic origin for the Younger Dryas geochemical anomalies ca. 12,900 cal B.P.
Osmium isotopes and Platinum Group elements spike in 5 sediment layers in Hall's Cave, Texas. One spike is at the same time as the start of the Younger Dryas cooling event. We show that the prevailing hypothesis of an extraterrestrial impactor is wrong. Instead, cryptotephra from distant volcanic eruptions are a much more likely explanation for the unradiogenic signature in the spikes in the sediments.
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2020 Gems and Gemology
Finders, Keepers: Field trip to Crater of Diamonds, USA
PhD student Roy Bassoo describes diamond characteristics from specimens collected at Crater of Diamonds, Arkansas. Roy uses a field trip narrative to share the science, which includes morphology, inclusion composition, and mantle conditions.
2020Geochemistry, Geophysics, Geosystems
Hydrogen isotope composition of a large silicic magma reservoir preserved in quartz-hosted glass inclusions of the Bishop Tuff plinian eruption
We measured the abundance of hydrogen isotopes in melt inclusion glasses trapped within quartz phenocrysts from the Bishop Tuff. The hydrogen isotope ratios range from -40 to -60 per mil. These values are not modified by degassing, AFC processes, or diffusion. Instead, the likely record the composition of the rhyolite reservoir.
2020Journal of Structural Geology
Rheology of a coaxial shear zone in the Virginia Blue Ridge: wet quartzite dislocation creep at ~250-280 C
A Cambrian quartzite in the Blue Ridge mountains of Virginia record penetrative strain at subgreenschist facies. Quartz was dynamically recrystallized in a wet environment that averages 100 to 400 ppm, but ranges up to 2,000 ppm. Hydrolytic weakening permitted the high strain in such cold rocks.
2020Geochemistry, Geophysics, Geosystems
Applications and limitations of elastic thermobarometry: insights from elastic modeling of inclusion-host pairs and example case studies
We present elastic thermobarometry using Raman spectroscopy to the petrology community. We provide good and bad case studies to transparently demonstrate the application of the technique. We also share a Matlab software that allows a potential user to explore any host-inclusion mineral pair.
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.
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2020 Contributions to Mineralogy and Petrology
Supersaturation nucleation and growth of plagioclase: a numerical model
We share an experimentally-calibrated numerical model that simulates the growth of plagioclase in silicic magma. Our experiments provide kinetic constraints on plagioclase growth rate relative to the degree of disequilibrium in the system. The model allows a user to interrogate natural plagioclase populations.
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.
2018 Contributions to Mineralogy and Petrology
Crystal nucleation and growth produced by continuous decompression...
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.
2018 American Mineralogist
Feldspar Raman shift and application as a magmatic thermobarometer
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.