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Shock Effects and Mineral Assemblages in the Genomict Eucrite Northwest Africa 8677

Description

Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid

Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid Vesta. Brecciated eucrites provide a record of the impact processes that occurred after the crustal formation of the parent body. Radiometric dating of HEDs has shown that they were affected by resetting events at 3.4 – 4.1 and 4.48 Ga. Therefore, shock effects in HEDs are windows into ancient impacts on asteroids early in solar system history. Northwest Africa (NWA) 8677 is a genomict eucrite with lithologies that are texturally different, but compositionally similar. The clasts in the breccia include strongly shocked (S5) gabbroic fragments and weakly shocked (S3) basaltic clasts. Coesite, a high-pressure polymorph of quartz, is preserved in the core of a large (~250 μm) silica grain, indicating the gabbro was strongly shocked. A large thermal overprint from the surrounding melt resulted in the transformation of coesite to low-pressure silica phases of quartz and cristobalite on the rims of this grain. The shock melt, interstitial to the breccia fragments, exhibits well-developed quench textures and contains a low-pressure mineral assemblage of plagioclase and pyroxene, implying that crystallization occurred after pressure release. The heterogeneity in shock features between the gabbroic and basaltic lithologies suggests that NWA 8677 experienced a variable impact history, which included at least two impact events. An initial impact strongly shocked and brecciated the gabbro and ejected both onto the regolith of the parent body where a more weakly shocked basalt was incorporated. A second impact produced the interstitial melt between the breccia matrix. The temperature of this shock melt remained high after pressure release, resulting in crystallization of a low-pressure assemblage of pyroxene and feldspar, as well as the transformation of quartz + cristobalite rims on coesite

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  • 2019-05

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Quantifying the Timing and Controls of Magmatic Processes Associated with Volcanic Eruptions

Description

Volcanic eruptions can be serious geologic hazards, and have the potential to effect human life, infrastructure, and climate. Therefore, an understanding of the evolution and conditions of the magmas stored

Volcanic eruptions can be serious geologic hazards, and have the potential to effect human life, infrastructure, and climate. Therefore, an understanding of the evolution and conditions of the magmas stored beneath volcanoes prior to their eruption is crucial for the ability to monitor such systems and develop effective hazard mitigation plans. This dissertation combines classic petrologic tools such as mineral chemistry and thermometry with novel techniques such as diffusion chronometry and statistical modeling in order to better understand the processes and timing associated with volcanic eruptions. By examining zoned crystals from the fallout ash of Yellowstone’s most recent supereruption, my work shows that the rejuvenation of magma has the ability to trigger a catastrophic supereruption at Yellowstone caldera in the years (decades at most) prior to eruption. This provides one of the first studies to thoroughly identify a specific eruption trigger of a past eruption using the crystal record. Additionally, through experimental investigation, I created a novel diffusion chronometer with application to determine magmatic timescales in silicic volcanic systems (i.e., rhyolite/dacite). My results show that Mg-in-sanidine diffusion operates simultaneously by both a fast and slow diffusion path suggesting that experimentally-derived diffusion chronometers may be more complex than previously thought. When applying Mg-in-sanidine chronometry to zoned sanidine from the same supereruption at Yellowstone, the timing between rejuvenation and eruption is further resolved to as short as five months, providing a greater understanding of the timing of supereruption triggers. Additionally, I developed a new statistical model to examine the controls on a single volcano’s distribution of eruptions through time, therefore the controls on the timing between successive eruptions, or repose time. When examining six Cascade volcanoes with variable distribution patterns through time, my model shows these distributions are not result of sampling bias, rather may represent geologic processes. There is a robust negative correlation between average repose time and average magma composition (i.e., SiO2), suggesting this may be a controlling factor of long-term repose time at Cascade volcanoes. Together, my work provides a better vision for forecasting models to mitigate potential destruction.

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  • 2020

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Transitions in Eruption Style at La Fossa Cone, Vulcano Island, Italy

Description

Volcanoes can experience multiple eruption styles throughout their eruptive histories. Among the most complex and most common are eruptions of intermediate explosivity, such as Vulcanian and sub-Plinian eruptions. Vulcanian eruptions

Volcanoes can experience multiple eruption styles throughout their eruptive histories. Among the most complex and most common are eruptions of intermediate explosivity, such as Vulcanian and sub-Plinian eruptions. Vulcanian eruptions are characterized by small-scale, short-lived, ash-rich eruptions initiated by the failure of a dense magma plug or overlying dome that had sealed an overpressured conduit. Sub-Plinian eruptions are characterized by sustained columns that reach tens of kilometers in height.

Multiple eruption styles can be observed in a single eruptive sequence. In recent decades, transitions in eruption style during well-documented eruptions have been described in detail, with some workers proposing explanatory mechanisms for the transitions. These proposed mechanisms may be broadly classified into processes at depth, processes in the conduit, or some combination of both.

The present study is focused on the Pietre Cotte sequence because it may have encompassed up to three different eruptive cycles, each representing different degrees of explosivity. The first deposits are composed of repeated layers of fine ash and lapilli composed of latite and rhyolite endmembers, efficiently mixed at sub-cm scales. The thin layers and bubble/crystal textures indicate that the magma underwent numerous decompressions and open-system degassing, and that the eruptions waned with time. The second phase of the sequence appears to have been initiated by cm-scale mixing between a volatile-rich, mafic magma from deeper in the system and a shallow silicic body. Textures indicate that the magma ascended rapidly and experienced little to no open-system degassing. The final phase of the sequence again produced repeated layers of fine ash and lapilli, of uniform trachyte composition, and waned with time. The first and last phases were likely produced in Vulcanian eruptions, while the pumice-rich layers were likely produced in Vulcanian to sub-Plinian eruptions.

In summary, the Pietre Cotte sequence is characterized by up to three magma recharge events in ~200 years. The differences in eruptive style appear to have been controlled by variations in the volatile content of the recharge magma, as well as the efficiency and scale of magma mixing and resulting overpressures.

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  • 2020

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Crustal Storage and Ascent Rates of the Mt. Shasta Primitive Magnesian Andesite

Description

Primitive arc magmas provide a critical glimpse into the geochemical evolution of subduction zone magmas, as they represent the most unadulterated mantle-derived magmas observed in nature in these tectonic environments

Primitive arc magmas provide a critical glimpse into the geochemical evolution of subduction zone magmas, as they represent the most unadulterated mantle-derived magmas observed in nature in these tectonic environments and are the precursors of the more abundant andesites and dacites typical in arcs. To date, the study of primitive arc magmas has largely focused on their origins at depth, while significantly less is known about pre-eruptive crustal storage and ascent history. This study examines the crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite (PMA), the demonstrated dominant parent magma for the abundant mixed andesites erupted at Mt. Shasta. Petrographic and geochemical observations of the PMA identify a mid-crustal magma mixing event with a less evolved relative of the PMA recorded in multiple populations of reversely zoned clinopyroxene and orthopyroxene phenocrysts. Prior phase equilibrium experiments and thermobarometric calculations as part of this study suggest the PMA experienced storage, mixing with a less evolved version of itself, and subsequent crystallization at 5kbar and 975°C. Modeling of Fe-Mg interdiffusion between the rims and cores of the reversely-zoned clinopyroxene and orthopyroxenes suggest this mixing, crystallization and subsequent ascent occurred within 10 years, or ~2.9 (+6.5 / -2.5) years, prior to eruption. Ascent from 5kbar or ~15 km, with no meaningful shallower storage, suggests minimum crustal transit rates of ~5 km/year. This rate is comparable to only a couple of other similar types of crustal transit rates (and slower than the much faster, syn-eruptive ascent rates measured through methods like olivine-hosted melt embayment volatile gradients and U-series isotope measurements on other arc magmas). The results of this study help to constrain the pre-eruptive history and ascent rates of hydrous primitive arc magmas, illuminating their magmatic processes during ascent. When combined with geophysical signals of magma movement, mixing to eruption timescales such as this have the power to inform volcanic hazard models for monogenetic, cinder cone eruptions in the Southern Cascades.

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  • 2019

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Strategies for (U-Th)/Pb Geochronology of Impact Structures: Lessons from the West Clearwater Lake Crater, Canada

Description

Establishing the timing of impact crater formation is essential to exploring the relationship between bolide impact and biological evolution, and constraining the tempo of planetary surface evolution. Unfortunately, precise and

Establishing the timing of impact crater formation is essential to exploring the relationship between bolide impact and biological evolution, and constraining the tempo of planetary surface evolution. Unfortunately, precise and accurate impact geochronology can be challenging. Many of the rock products of impact (impactites) contain relict, pre-impact phases that may have had their isotopic systematics completely reset during the impact event, only partially reset, or not reset at all. Of the many isotopic chronometers that have been used to date impactites, the U/Pb zircon chronometer (ZrnPb) seems least susceptible to post-impact disturbances, and ZrnPb dates are typically much more precise than those obtained using other chronometers. However, the ZrnPb system is so resistant to resetting that relict zircons in impactites often yield dates that reflect the igneous or metamorphic ages of the target rocks rather than the age of the impact itself. The present study was designed to answer a simple question: is there a straightforward sample collection and analysis strategy for high-accuracy ZrnPb dating of an impact structure if the impactites collected from it may contain inherited zircons? To study this, ZrnPb dates were determined for impactites from a single crater with a well-constrained impact age: the West Clearwater Lake impact structure, located at Lake Wiyâshâkimî, Québec, Canada.

The amount of ZrnPb resetting and the mechanisms responsible for resetting varied amongst the samples. Each sample characteristically contained either: newly crystallized zircons from the impact melt ("neocrystalline"), relict zircons ~50-100% reset, or, relict zircons ~0-50% reset. The variably reset relict zircons define a discordia line from ~2700 Ma to ~286 Ma – consistent with the ages of the target rock and the impact, respectively (Schmieder et al., 2015a; Simard, 2004). ZrnPb measurements from the neocrystalline zircons provided a new preferred impact age of 286.64 ± 0.35 Ma (2σ), a ~10x improvement in precision. The characteristics of the West Clearwater ZrnPb data vary between samples yet become easily interpretable as a whole, showing that efforts to measure robust, precise impact ages benefit from strategies that prioritize applying multiple analytical techniques to multiple types of impactite from the same crater.

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  • 2019