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

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, Fs_8-16, En_50-44) and orthopyroxene (Wo_2-4, Fs_7-17, En_80-89). 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.