The Dynamics of the Post-Caldera Magmatic System at Yellowstone: Insights from Age, Trace Element, and Isotopic Data of Zircon and Sanidine

Abstract:

Yellowstone hosts a voluminous magmatic system that produced three silicic caldera-forming eruptions over the past 2.1 My. Following the most recent of these (the Lava Creek Tuff at 639 ka), the magma system at Yellowstone underwent two episodes of intracaldera eruptions, the latest of which produced the Central Plateau Member (CPM) rhyolites. The CPM rhyolites erupted intermittently from ca. 170 ka to ca. 70 ka and can be viewed as snapshots of the magma system through time, which provides a unique opportunity to study the dynamics of an evolving caldera system. To constrain the nature and timescales of magmatic processes at Yellowstone we examine four CPM rhyolites that erupted from ca. 116 ka to ca. 74 ka and present a comprehensive data set that integrates (1) ²³⁸U-²³⁰Th ages, trace-elements, and Hf isotope compositions of the surfaces and interiors of single zircons, (2) bulk ²³⁸U-²³⁰Th ages and in situ Ba and Pb isotope compositions of sanidines, (3) sanidine ⁴⁰Ar-³⁹Ar ages, and (4) trace-element and isotopic compositions of the CPM glasses. Zircon ²³⁸U-²³⁰Th ages and Hf isotope data demonstrate that isotopically juvenile magmas, derived from Yellowstone basalts, were added to the Yellowstone magma reservoir over time and were fundamental to its post-caldera isotopic evolution. We use zircon Hf isotope data along with new Hf isotope data (and existing O isotope data) for the Yellowstone basalts (whole-rocks), older Yellowstone rhyolites (glasses), and local crustal sources to quantify the role of isotopically juvenile magma in the evolution of the magmatic system. Additionally, linking age, trace-element, and isotopic data from zircon and sanidine demonstrates that eruptible CPM rhyolites were generated by extracting melt and antecrystic zircon from a long-lived (>200 ky) crystal mush, while sanidine remained trapped in the crystal network. The extracted melts amalgamated and then crystallized new sanidines and rims on the antecrystic zircons that were in equilibrium with their host melt. In this model, the ages of the zircon surfaces and sanidines constrain the residence times of the eruptible magma bodies to < 9 ky. This study highlights how detailed investigations linking the age and composition of minerals hosted in rhyolites can provide information on the dynamics of magmatic processes at a variety of scales.