Uranium-series isotope constraints on magma generation and differentiation in the central Oregon Cascades
type: CreativeWork
description: The Cascade volcanic arc, extending for ~ 1250 km from northern California, USA, to southern British Columbia, Canada, is the surface expression of NE-directed subduction of the Juan de Fuca microplate beneath North America. Slow subduction of young, hot oceanic lithosphere creates an unusually warm subduction environment at depth. Fore-arc rotation and impingement of Basin and Range faulting create an extensional environment within much of the arc. The combination of these factors has been postulated as a cause of the wide variety of primitive basaltic compositions erupted within the arc. Within central Oregon, where intra-arc extension is highly focused, young mafic volcanism is particularly abundant, and eruption of true rhyolite is unusually common also. This dissertation seeks to address the generation and differentiation of young (< 20 ka) magmas in the central Oregon Cascades through application of uranium-series (U-series) isotope data, in conjunction with elemental and Sr-Nd isotope data. Trace element and isotopic (87Sr/86Sr, 143Nd/144Nd) data for the most primitive lavas suggest derivation by < 10% partial melting of a garnet-bearing mantle source. This source is interpreted to have experienced relatively minor modification by a slab-derived component, most likely via addition of a partial melt of subducted sediments. The 238U-230Th-226Ra U-series data are consistent with this interpretation. However, as a result of the minor (230Th/238U) disequilibrium created in the mantle wedge as a consequence of fluid addition, the data cannot resolve whether fluid modification of the source initiated melting or was an earlier (> 350 k.y. prior) event. (231Pa/235U) data for these samples, the first such data from the Cascades, are two to four times higher than any U-Pa data previously measured in lavas from any tectonic setting. The data cannot be reproduced from a secular equilibrium or U-enriched source with reasonable model parameters, implying significant pre-mel…
abstract: The Cascade volcanic arc, extending for ~ 1250 km from northern California, USA, to southern British Columbia, Canada, is the surface expression of NE-directed subduction of the Juan de Fuca microplate beneath North America. Slow subduction of young, hot oceanic lithosphere creates an unusually warm subduction environment at depth. Fore-arc rotation and impingement of Basin and Range faulting create an extensional environment within much of the arc. The combination of these factors has been postulated as a cause of the wide variety of primitive basaltic compositions erupted within the arc. Within central Oregon, where intra-arc extension is highly focused, young mafic volcanism is particularly abundant, and eruption of true rhyolite is unusually common also. This dissertation seeks to address the generation and differentiation of young (< 20 ka) magmas in the central Oregon Cascades through application of uranium-series (U-series) isotope data, in conjunction with elemental and Sr-Nd isotope data. Trace element and isotopic (87Sr/86Sr, 143Nd/144Nd) data for the most primitive lavas suggest derivation by < 10% partial melting of a garnet-bearing mantle source. This source is interpreted to have experienced relatively minor modification by a slab-derived component, most likely via addition of a partial melt of subducted sediments. The 238U-230Th-226Ra U-series data are consistent with this interpretation. However, as a result of the minor (230Th/238U) disequilibrium created in the mantle wedge as a consequence of fluid addition, the data cannot resolve whether fluid modification of the source initiated melting or was an earlier (> 350 k.y. prior) event. (231Pa/235U) data for these samples, the first such data from the Cascades, are two to four times higher than any U-Pa data previously measured in lavas from any tectonic setting. The data cannot be reproduced from a secular equilibrium or U-enriched source with reasonable model parameters, implying significant pre-mel…
author: Mitchell, Euan
language: English
date published: 2013-09-05T07:00:00Z
date modified: 2026-07-24T05:26:14Z
provider: University of New Mexico
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of New Mexico

PropertyValue
property ID: Handle
value: http://hdl.handle.net/1928/23350