Theses/Dissertations - Geosciences
Permanent URI for this collectionhttps://hdl.handle.net/2104/4784
Browse
Browsing Theses/Dissertations - Geosciences by Author "Befus, Kenneth S."
Now showing 1 - 3 of 3
- Results Per Page
- Sort Options
Item Embayments in explosive, silicic eruptions : a textural, numerical, and experimental assessment.(2021-08-09) Ruefer, Anna Cameron, 1997-; Befus, Kenneth S.I explore the occurrence and formation of embayments in quartz using textural, experimental, and numerical techniques. Embayments are pockets of melt partially trapped inside of volcanic crystals. During eruption these pockets of melt quench to glass. The concentration of water within embayment glass is the foundation for a powerful new technique in volcanology that is thought to record how quickly the magma ascended during an eruption. My goal was to determine how accurately embayments record changes in the magma during ascent. Using a high-pressure, high-temperature system, I performed decompression experiments on natural and synthetic embayments to test their ability to record magmatic conditions. Experimental work is thus far unsuccessful but highlights best practices for researchers going forward. My work on rhyolitic eruptions shows that embayments are common across eruptions. The range in embayment shapes and textures preserves a record of three separate timescales: diffusion speedometry, emptying, and faceting.Item Guiana Shield diamonds, the sub-cratonic lithosphere, and kimberlite emplacement in the upper crust.(2021-08-02) Bassoo, Roy Raymond, Jr., 1985-; Befus, Kenneth S.The origin of diamonds from Guyana have remained an enigma despite being mined for nearly 150 years within the Guiana Shield. These gemstones are an undescribed diamond suite within an understudied portion of the Amazonian Craton. Our study confirms they are likely eroding as paleo-placers from Roraima Supergroup rocks, but may have been primarily derived from older ultramafic rocks (≥ 2 Ga) with a high H2O/CO2 volatile budget. These ancient diamonds also contain inclusions that preserve high Cr, Al, Mg, and low Ca, suggesting a harzburgitic paragenesis. Forsterite with elevated Mn, and chromite with low Fe3+/Fe2+ ratios suggest metasomatic alteration within low oxygen fugacity conditions (log ƒO2 (ΔFMQ) -1.6±1.1). Raman thermobarometry suggests entrapment at 5 – 7 GPa within cratonic roots. Low OH active infrared absorbances indicate low ppm H2O contents of forsterite inclusions, from which we were able to estimate high viscosities of 1023.7±2.1 Pa∙s within the cratonic root which incidentally resists delamination through geologic time. We applied similar mineralogical and spectroscopic techniques to diamonds from the Prairie Creek lamproite in Arkansas, USA. Coesite and forsterite inclusions record entrapment conditions of 4.8±0.5 GPa, typical of cratonic settings. However, Arkansas diamonds preserve brown body colors and one example of purple luminescence suggesting vacancy clusters and a distorted cubic lattice. In a highly viscous and turbulent upper mantle of an edge-driven convection cell, as inferred for Arkansas lamproite magma genesis, diamonds may be subjected to extreme forces which distort the atomic lattice. Finally we conducted high temperature (300 – 900 °C) and cold seal experiments at 100-200 MPa on hypabyssal kimberlite from the Jericho kimberlite (Northwest Territories, Canada) to examine sub-solidus reactions that occur syn- to post- emplacement. We observe that olivine in the presence of calcite is unstable and monticellite is precipitated, within the span of hours, suggesting at least rapid syn-emplacement This decarbonation reaction is directly proportional to temperature and indirectly proportional to available fluid CO2, where +12 wt.% CO2 increases calcite stability by 100 °C. Additionally, calcite preservation in hypabyssal kimberlite provides an observational constraint that diamond grade has not been diminished by post-emplacement conditions.Item Rhyolite lava emplacement dynamics inferred from surface morphology captured with drones.(2019-07-12) Leggett, Tyler Nathan, 1995-; Befus, Kenneth S.The fluid dynamic properties of lava control the formation of morphologic features during emplacement. Therefore, fluid dynamic properties of the lava during emplacement can be inferred by measuring morphologic features on rhyolite lava flows. I analyzed surface features of rhyolite lava flows using high-resolution digital elevation models (DEMs) and orthomosaics made from unmanned aerial vehicle (UAV) structure-from-motion photogrammetry. I compare our newly acquired datasets to DEMs of rhyolitic lava flows from across the western USA, including Rock Mesa, Newberry, Obsidian Flow, Interlake Flow, and Banco Bonito. I measured ridge spacing, vergence angle of the axial plane of fold, block size distributions, and overall flow morphology. Pressure ridges range from 10 to 87 m, and block sizes from 0.5 to 0.7 m. These estimate interior viscosities from 109.7 to 1012.5 Pa s. Ranges of timescale can be estimated to be 30 – 548 days with eruptive fluxes from 8 – 135 m3 s-1.