The Post-caldera Magmatic System at Yellowstone Plateau

Download or read book The Post-caldera Magmatic System at Yellowstone Plateau written by Mark Evan Stelten and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The Yellowstone Plateau Volcanic Fields hosts a long-lived magmatic system that has produced three rhyolitic caldera-forming eruptions over the past 2.1 Ma, as well as numerous intracaldera rhyolitic eruptions between caldera-forming events. Whereas caldera-forming eruptions evacuate a large volume of magma near instantaneously, the eruption of intracaldera rhyolite flows intermittently over ≥ 100 kyr can be viewed as snapshots into an evolving magmatic system and allow the assessment of how large, multi-cyclical silicic magma systems evolve through time. I integrate age, trace-element, and isotopic data for minerals and volcanic glasses from the youngest post-caldera eruptive episode at Yellowstone, which generated the Central Plateau Member (CPM) of the Plateau Rhyolite from ca. 170 ka to 70 ka, to constrain [1] the physical nature of the Yellowstone magmatic system, [2] the processes controlling its compositional evolution, [3] the processes that produce compositional diversity within the eruptive sequence, and [4] the mechanisms/timescales of generating eruptible rhyolitic magmas. Linking 238U-230Th crystallization ages of zircon and sanidine hosted in the CPM rhyolites with their trace-element and isotopic compositions demonstrates that the magma reservoir that sourced the CPM rhyolites is characterized by a long-lived (150 - 250 kyr) and compositionally heterogeneous crystal mush, that is evolving over time to more fractionated trace-element compositions via cooling induced crystallization. While most of the CPM rhyolites show a coherent compositional evolution though time, compositionally anomalous rhyolites can be produced by the mixing of extracaldera magmas with the margin of the main Yellowstone magma reservoir. This highlights the dynamic nature of magmatic interactions that occur at the margins of large magma reservoirs, and represents a process by which compositional diversity within a magmatic suite can be produced. Although the crystal mush that sourced the CPM rhyolites is uneruptible due to its high degree of crystallinity, eruptible magmas are generated by extracting melt and antecrystic zircons (i.e., related to older stages of Yellowstone magmatism) from the crystal mush while the major phases (e.g., sanidine) remain trapped in the locked crystal network. The extracted liquids (plus zircon) are then amalgamated into a liquid dominated (i.e., eruptible) magma body from which autocrystic (i.e., akin to their host melt) sanidines and zircon rims crystallize. These liquid-dominated magma bodies reside in the crust for [less than/equal to] 2 kyr (conservatively