Matching Items (9)

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Modeling the mantle genesis of basalts from the Lassen Volcanic Center

Description

There are many outstanding questions regarding the petrologic processes that give rise to andesitic and basaltic magmas in subduction zones, including the specifics that govern their geographical distribution in a

There are many outstanding questions regarding the petrologic processes that give rise to andesitic and basaltic magmas in subduction zones, including the specifics that govern their geographical distribution in a given arc segment. Here I investigate the genesis of calc-alkaline and tholeiitic basalts from the Lassen Volcanic Center in order to determine the pressure, temperature, source composition, and method of melting that lead to the production of melt in the mantle below Lassen. To this aim, a suite of primitive basalts (i.e. SiO2<52 and Mg#>65) are corrected for fractional crystallization by adding minerals back to the bulk rock composition with the goal of returning them to a primary composition in equilibrium with the mantle. Thermobarometry of the primary melt compositions is conducted to determine temperature and pressure of melting, in addition to a forward mantle modeling technique to simulate mantle melting at varying pressures to constrain source composition and method of melting (batch vs. fractional). The results from the two techniques agree on an average depth of melt extraction of 36 km and a source composition similar to that of depleted mantle melted by batch melting. Although attempted for both calc-alkaline and tholeiitic basalts, the fractional crystallization correction and thus the pressure-temperature calculations were only successful for tholeiitic basalts due to the hydrous nature of the calc-alkaline samples. This leaves an opportunity to repeat this study with parameters appropriate for hydrous basalts, allowing for the comparison of calc-alkaline and tholeiitic melting conditions.

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Date Created
  • 2015-05

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Tecolote Cinder Cone Ballistics: Volcanic Bomb Formation and Dynamics

Description

Cinder cones are common volcanic structures that occur in fields, and on the flanks of shield volcanoes, stratovolcanoes, and calderas. Because they are common structures, they have a significant possibility

Cinder cones are common volcanic structures that occur in fields, and on the flanks of shield volcanoes, stratovolcanoes, and calderas. Because they are common structures, they have a significant possibility of impacting humans and human environments. As such, there is a need to analyze cinder cones to get a better understanding of their eruptions and associated hazards. I will approach this analysis by focusing on volcanic bombs and ballistics, which are large clots of lava that are launched from the volcanic vent, follow ballistic trajectories, and can travel meters to a few kilometers from their source (e.g. Fagents and Wilson 1993; Waitt et al. 1995).
Tecolote Volcano in the Pinacate Volcanic Field in Mexico contains multiple vents within a horseshoe-shaped crater that have all produced various ejecta (Zawacki et al. 2019). The objectives of this research are to map ballistic distribution to understand the relationship between the source vent or vents and the bombs and ballistics that litter the region around Tecolote, and interpret the eruption conditions that ejected those bombs by using their distributions, morphologies, and fine-scale textures.
The findings of this work are that these bombs are apparently from the last stages of the eruption, succeeding the final lava flows. The interiors and exteriors of the bombs display different cooling rates which can are indicated by the fabric found within. Using this, certain characteristics of the bombs during eruption were extrapolated. The ‘cow pie’ bombs were determined to be the least viscous or contained a higher gas content at the time of eruption. Whereas the ribbon/rope bombs were determined to be the most viscous or contained a lesser gas content. Looking at the Southern Bomb Field site, it is dominated by large bombs that were during flight were molded into aerodynamic shapes. The Eastern Rim site is dominated by smaller bombs that appeared to be more liquid during the eruption. This difference in the two sites is a probable indication of at least two different eruptive events of different degrees of explosivity. Overall, aerodynamic bombs are more common and extend to greater distances from the presumed vent (up to 800 m), while very fluidal bombs are uncommon beyond 500 meters. Fluidal bombs (‘cow pie’, ‘ribbon’, ‘rope/spindle’) show a clear trend in decreasing size with distance from vent, whereas the size-distance trend is less dramatic for the aerodynamic bombs.

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Created

Date Created
  • 2020-05

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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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Created

Date Created
  • 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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Agent

Created

Date Created
  • 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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Agent

Created

Date Created
  • 2019

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Interactions Between Fluids, Melts, and Rocks in Subduction Zones

Description

My dissertation research broadly focuses on the geochemical and physical exchange of materials between the Earth’s crust and mantle at convergent margins, and how this drives the compositional diversity observed

My dissertation research broadly focuses on the geochemical and physical exchange of materials between the Earth’s crust and mantle at convergent margins, and how this drives the compositional diversity observed on the Earth’s surface. I combine traditional petrologic and geochemical studies of natural and experimental high-pressure mafic rocks, with thermodynamic modeling of high-pressure aqueous fluids and mafic-ultramafic lithologies allowing for more complete understanding of fluid-melt-rock interactions. The results of the research that follows has important implications for: the role of lower crustal foundering in the geochemical origin and evolution of the modern continental crust (Chapter 2; Guild et al., under review), metasomatic processes involving aqueous metal-carbon complexes in high pressure-temperature subduction zone fluids (Chapter 3; Guild & Shock, 2020), natural hydrous mineral stability at the slab-mantle interface (Chapter 4; Guild, et al., in preparation) and water-undersaturated melting in the sub-arc (Chapter 5; Guild & Till, in preparation).

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Created

Date Created
  • 2020

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Timescales and Characteristics of Magma Generation in Earth and Exoplanets

Description

Volcanic eruptions are serious geological hazards; the aftermath of the explosive eruptions produced at high-silica volcanic systems often results in long-term threats to climate, travel, farming, and human life. To

Volcanic eruptions are serious geological hazards; the aftermath of the explosive eruptions produced at high-silica volcanic systems often results in long-term threats to climate, travel, farming, and human life. To construct models for eruption forecasting, the timescales of events leading up to eruption must be accurately quantified. In the field of igneous petrology, the timing of these events (e.g. periods of magma formation, duration of recharge events) and their influence on eruptive timescales are still poorly constrained.

In this dissertation, I discuss how the new tools and methods I have developed are helping to improve our understanding of these magmatic events. I have developed a method to calculate more accurate timescales for these events from the diffusive relaxation of chemical zoning in individual mineral crystals (i.e., diffusion chronometry), and I use this technique to compare the times recorded by different minerals from the same Yellowstone lava flow, the Scaup Lake rhyolite.

I have also derived a new geothermometer to calculate magma temperature from the compositions of the mineral clinopyroxene and the surrounding liquid. This empirically-derived geothermometer is calibrated for the high FeOtot (Mg# = 56) and low Al2O3 (0.53–0.73 wt%) clinopyroxene found in the Scaup Lake rhyolite and other high-silica igneous systems. A determination of accurate mineral temperatures is crucial to calculate magmatic heat budgets and to use methods such as diffusion chronometry. Together, these tools allow me to paint a more accurate picture of the conditions and tempo of events inside a magma body in the millennia to months leading up to eruption.

Additionally, I conducted petrological experiments to determine the composition of hypothetical exoplanet partial mantle melts, which could become these planets’ new crust, and therefore new surface. Understanding the composition of an exoplanet’s crust is the first step to understanding chemical weathering, surface-atmosphere chemical interactions, the volcanic contribution to any atmosphere present, and biological processes, as life depends on these surfaces for nutrients. The data I have produced can be used to predict differences in crust compositions of exoplanets with similar bulk compositions to those explored herein, as well as to calibrate future exoplanet petrologic models.

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Date Created
  • 2020

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Fluorine partitioning between nominally anhydrous minerals (olivine, clinopyroxene, and plagioclase) and silicate melt using secondary ion mass spectrometry and newly synthesized basaltic fluorine microanalytical glass standards

Description

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the melt during magma differentiation and rendering F less subject to disturbance during degassing upon magma ascent. Hence, the association between fluorine in basalts and fluorine in the mantle source region is more robust than for other volatile species. The ionic radius of F- is similar to that of OH- and O2-, and F may substitute for hydroxyl and oxygen in silicate minerals and melt. Fluorine is also incorporated at trace levels within nominally anhydrous minerals (NAMs) such as olivine, clinopyroxene, and plagioclase. Investigating the geochemical behavior of F in NAMs provides a means to estimate the pre-eruptive F contents of degassed magmas and to better understand the degassing behavior of H. The partition coefficients of F were determined for clinopyroxene, olivine, plagioclase, and hornblende within melts of olivine-minette, augite-minette, basaltic andesite, and latite compositions. The samples analyzed were run products from previously-published phase-equilibria experiments. Fluorine was measured by secondary ion mass spectrometry (SIMS) using an 16O- primary beam and detection of negative secondary ions (19F-, 18O-, 28Si-). SIMS ion intensities are converted to concentrations by analyzing matrix-matched microanalytical reference materials and constructing calibration curves. For robust F calibration standards, five basaltic glasses (termed Fba glasses) were synthesized in-house using a natural tholeiite mixed with variable amounts of CaF2. The Fba glasses were characterized for F content and homogeneity, using both SIMS and electron-probe microanalysis (EPMA), and used as F standards. The partition coefficients for clinopyroxene (0.04-028) and olivine (0.01-0.16) varied with melt composition such that DF (olivine-minette) < DF (augite-minette) < DF (basaltic andesite) < DF (latite). Crystal chemical controls were found to influence the incorporation of F into clinopyroxene, but none were found that affected olivine. Fluorine partitioning was compared with that of OH within clinopyroxenes, and the alumina content of clinopyroxene was shown to be a strong influence on the incorporation of both anions. Fluorine substitution into both olivine and clinopyroxene was found to be strongly controlled by melt viscosity and degree of melt polymerization.

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Agent

Created

Date Created
  • 2012

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Timing and structural control of gold mineralization, Santa Gertrudis, Sonora, Mexico

Description

The Santa Gertrudis Mining District of Sonora, Mexico contains more than a dozen purported Carlin-like, sedimentary-hosted, disseminated-gold deposits. A series of near-surface, mostly oxidized gold deposits were open-pit mined from

The Santa Gertrudis Mining District of Sonora, Mexico contains more than a dozen purported Carlin-like, sedimentary-hosted, disseminated-gold deposits. A series of near-surface, mostly oxidized gold deposits were open-pit mined from the calcareous and clastic units of the Cretaceous Bisbee Group. Gold occurs as finely disseminated, sub-micron coatings on sulfides, associated with argillization and silicification of calcareous, carbonaceous, and siliciclastic sedimentary rocks in structural settings. Gold occurs with elevated levels of As, Hg, Sb, Pb, and Zn. Downhole drill data within distal disseminated gold zones reveal a 5:1 ratio of Ag:Au and strong correlations of Au to Pb and Zn. This study explores the timing and structural control of mineralization utilizing field mapping, geochemical studies, drilling, core logging, and structural analysis. Most field evidence indicates that mineralization is related to a single pulse of moderately differentiated, Eocene intrusives described as Mo-Cu-Au skarn with structurally controlled distal disseminated As-Ag-Au.

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Agent

Created

Date Created
  • 2011