The development of young silicic magmas in the eastern San Francisco volcanic field based on geochemistry and u-series disequilibrium dating

Knott, Curt Christopher (2023) The development of young silicic magmas in the eastern San Francisco volcanic field based on geochemistry and u-series disequilibrium dating. Masters thesis, Northern Arizona University.

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Abstract

Silicic volcanic plugs erupted at O’Neill and Strawberry Craters in the San Francisco Volcanic Field of Northern, Arizona, provide an opportunity to investigate silicic magma development in the otherwise dominantly mafic eastern part of the volcanic field. U-Th disequilibrium ages were obtained on the interiors and the surfaces of zircon microphenocrysts along with new whole-rock geochemical data. Two surface age populations were identified for both locations, the younger of which are 72 ± 5 ka and 67 ± 3 ka (1σ errors) for O’Neill and Strawberry Craters, respectively. These U-Th disequilibrium zircon surface dates provide maximum timings for the eruptions and more tightly constrain them than previously determined eruption ages. Zircon interiors constrain a minimum of three age populations, ranging from 127 ± 5 ka to 70 ± 2 ka for O’Neill Crater and 107 ± 5 ka to 74 ± 1 ka for Strawberry Crater. KDE analysis of the ages show that zircons crystallized episodically. Magma residence times were estimated to be up to 60 kyr for O’Neill Crater and up to 35 kyr for Strawberry Crater based on comparisons between oldest interior dates and the surface ages. Zircon surfaces have U concentrations that cluster around 200 ppm, whereas some zircon interiors have similar concentrations but others define a population with > 1 wt.% U. There is no discernable age difference between the two populations. My interpretation of the combined age and chemical data is two magma reservoirs coexisted, one of which assimilated high U material, such as a hydrothermal U deposit or a high U sedimentary unit. Mixing between the two magmas occurred prior to eruption.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Geochemistry; Geology; U-Th Disequilibrium; Volcanology; Zircon; Arizona; San Francisco Volcanic Field
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	23 Jul 2026 22:05
Last Modified:	23 Jul 2026 22:05
URI:	https://openknowledge.nau.edu/id/eprint/6303

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