TIMESCALES OF OPEN-SYSTEM MAGMATIC PROCESSES: INSIGHTS FROM TEXTURAL STUDIES AND DIFFUSION MODELING IN PLAGIOCLASE FROM MOUNT JEFFERSON, OREGON

Mount Jefferson, Oregon is a glaciated stratovolcano in the central Cascade arc and the locus of andesitic to rhyo-dacitic volcanism for at least 1 million years. To understand the eruption dynamics of Mount Jefferson, a young (<15 ka) representative lava dome, the Whitewater Creek Andesite, is investigated. A combination of field relations (multiple populations of mafic enclaves, basal scoriaceous component) and disequilibrium textures (zoning, sieving, resorption rims) documented using backscattered electron images suggests that the dome is the product of at least three mixing components. Calculations based on amphibole, feldspar, and melt inclusion compositions indicate that upper crustal storage was at ~2.5 to 4 kbar. Reverse zoning of anorthite content indicates that the disequilibrium textures are likely due to increasing temperature in the subvolcanic magma chamber during eruption. These observations are consistent with a model in which multiple magma recharge events took place prior to eruption. Here we investigate the diffusive equilibrium of major (Al, Si, Ca, Na, Fe, K, Mg) and trace (Sr, Ba) elements in plagioclase phenocrysts to determine the residence time of the minerals and duration of magma mixing events in the subvolcanic magmatic system. Concentration profiles of the elements are completed using the electron microprobe (EMP) for major elements and laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) for trace elements. The goals of this investigation are to 1) combine field observation, petrography, and in-situ geochemical analyses to quantify compositional changes over a progressive eruptive event 2) calculate the ambient conditions of mixing and eruption using mineral chemistry and volatile elements and 3) determine residence times of plagioclase in the residual melt, thus determining the duration between initial intrusion of the primitive magma and initiation of the subsequent eruption. By constraining these variables we aim to improve our understanding of the details of residence times of open system magmatic processes and contribute to discussions on the formation and evolution of continental arcs.