Crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite: implications for arc magma crustal flux rates

Primitive arc magmas provide our closest glimpse of the original mantle-derived magmas that produce the more ubiquitous andesites and dacites found in subduction zones and that ultimately construct Earth’s continental crust. This study examines the crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite (PMA), a demonstrated parent magma for the voluminous mixed andesites erupted at Mt. Shasta. Our petrographic and geochemical observations of the PMA identify a mid-crustal magma mixing event recorded in multiple populations of reversely zoned clinopyroxene and orthopyroxene phenocrysts. Thermobarometric calculations conducted as part of this study and prior phase equilibrium experiments (Grove et al., Contrib Miner Petrol 145:515–533, 2003; Krawczynski et al., Contrib Miner Petrol 164:317–339, 2012) suggest the PMA experienced storage, mixing, and subsequent crystallization at ~ 500 MPa and ~ 975 °C. Modeling of Fe–Mg interdiffusion between the rims and cores of the reversely zoned pyroxenes suggests this mixing event and the resulting crystal rim growth occurred less than 10 years prior to eruption (2.9-2.2+6.4). Ascent from 500 MPa (~ 15 km) during the calculated diffusion timescales suggests minimum crustal transit rates of ~ 170 MPa (~ 5 km)/year and cooling rates of ~ 5–7 °C/km, consistent with conductive cooling models. This ascent rate is slower than the handful of previously documented trans-crustal magmatic ascent rates and significantly slower than syn-eruptive decompression rates. If this behavior is representative, ~ the 10% mafic magmas erupted as part of the modern Mt. Shasta edifice fluxed through the crust within decades. Coupled with a review of the U–Th–Ra residence times for Shasta andesites to dacites, we suggest that crustal magma flux and assembly beneath modern Mt. Shasta occurred in discrete pulses that occupy a minority of the 700 k.y. period of edifice construction. The results of this study thus constrain the pre-eruptive history and ascent characteristics of a hydrous primitive arc magmas in the upper crust between their shallowest storage region in the mid-crust and volatile exsolution and provide constraints on crustal magma flux beneath continental arc volcanoes. Should future earthquake swarms indicative of magma movement in the middle to upper crust occur beneath Shasta, the results presented here also provide the first estimates of the possible magma ascent rates and the time intervals that could accompany related magma ascent to eruption at Mt. Shasta.