Determining the Crustal Storage History of Mt. Shasta Primitive Magnesian Andesite to Assess Magmatic Ascent Rates

The volumetrically-dominant lavas erupted at Mt. Shasta 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), primitive mantle melts, including an extremely primitive magnesian andesite (PMA) and a basaltic andesite (Grove et al., 2005). The PMA has a single eruptive center at a satellite vent 12 miles north of the main stratocone summit that contains major phases such as clinopyroxene (Wo₄₀, Fs_8-16, En_50-44), orthopyroxene (Wo_2-4, Fs_7-17, En_80-89), olivine (Fo_87-92), and plagioclase (An₉₄). Based on its extremely primitive nature, the PMA is thought to have ascended relatively rapidly to the surface with little crustal storage, mixing, and fractionation. However, analysis of the crystal cargo reveals mafic minerals (clinopyroxene, orthopyroxene, and olivine) with multiple populations, chemical zoning, resorption textures, and in some cases skeletal growth, 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 (Grove et al., 2003; Krawczynski et al., 2012), the results of which are used to constraint crystallization conditions in our natural sample. When compared to the experimental phase equilibria, the compositional zoning in the mafic minerals suggest crystallization and storage at 0.1 - 476 MPa and 1020 -1250°C. Ongoing diffusion chronometry utilizing Fe-Mg exchange in the PMA zoned mafic minerals will determine the timescales associated with the crustal storage history. An understanding of these factors will constrain the amount of time the PMA took to transit the crust during its multi-stage history, which may be typical for primitive magmas in arcs.

Grove et al. (2003). Contrib Mineral Petrol, 145: 515-533. https://doi.org/10.1007/s00410-003-0448-z

Grove et al. (2005). Contrib Mineral Petrol, 148: 542-565. https://doi.org/10.007-s00410-004-0619-6

Krawczynski et al. (2012). Contrib Mineral Petrol, 164: 317-339. https://doi.org/10.007/s00410-012-0740-x