T184 Subduction Zone Magmatism from Source to Eruption, in the Lab and in the Field: In Honor of Christy Till, GSA Mineralogy, Geochemistry, Petrology, and Volcanology Division Early Career Awardee (Posters)

Tuesday, 24 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

DECOMPRESSION RATES VIA GEOSPEEDOMETRY OF PRIMITIVE MAGNESIAN ANDESITE AT MT. SHASTA, CALIFORNIA

Paper number: T184-4
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The volumetrically dominant lavas volcanism that composes the Mt. Shasta edifice in the southern Cascades display evidence for multiple stages of crustal level crystal fractionation, magma recharge, and magma mixing. These magmas have been shown to be derived from hydrous (4.5-6 wt% H2O) mantle melts, including an extremely primitive magnesian andesite (PMA) and a basaltic andesite. The PMA has a single eruptive center at a satellite vent ~12 miles north of the main stratocone summit that contains reversely zoned phenocrysts of clinopyroxene (Wo40, Fs8-16, En50-44) and orthopyroxene (Wo2-4, Fs7-17, En80-89). Based on its low crystallinity and high Mg#’s, the PMA is thought to have ascended relatively rapidly to the surface with minimal crustal storage, mixing, and fractionation. However, analysis of the crystal cargo reveals pyroxenes with multiple populations, chemical zoning, and resorption textures, all of which point to a multi-stage history for the Mt. Shasta PMA. Phase equilibria experiments on the PMA have been conducted over a range of P-T conditions, the results of which are used to help constraint crystallization conditions in our natural sample. Experimental results, in tandem with two-pyroxene geothermometry results, suggest that the compositional zoning in clino- and orthopyroxene represent crystallization and storage conditions of ~0.8-6 kbar and 900 –1200°C. Diffusion chronometry utilizing Fe-Mg exchange in the PMA zoned pyroxenes suggests timescales for magma recharge at ~15 km depth to eruption that are on the order of hundreds of days to a few years. This suggests that this hydrous primitive magma ascended through the upper crust at a range of maximum decompression rates from 10’s to ~100 m/day, which is comparable to estimates of decompression rates in other arc systems determined from diffusion chronometry and U-series disequilibria. These rates are two orders of magnitude slower than rates from melt embayments in similar tectonic settings.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-337564

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.