Crustal Storage and Ascent Rates of the Mt. Shasta Primitive Magnesian Andesite
2019
Primitive arc magmas provide a critical glimpse into
the geochemical evolution of subduction zone magmas, as they
represent the most unadulterated mantle-derived magmas observed in
nature in these tectonic environments and are the precursors of the
more abundant andesites and dacites typical in arcs. To date, the
study of primitive arc magmas has largely focused on their origins
at depth, while significantly less is known about pre-eruptive
crustal storage and ascent history. This study examines the crustal
storage and ascent history of the Mt. Shasta primitive magnesian
andesite (PMA), the demonstrated dominant parent magma for the
abundant mixed andesites erupted at Mt. Shasta. Petrographic and
geochemical observations of the PMA identify a mid-crustal magma
mixing event with a less evolved relative of the PMA recorded in
multiple populations of reversely zoned clinopyroxene and
orthopyroxene phenocrysts. Prior phase equilibrium experiments and
thermobarometric calculations as part of this study suggest the PMA
experienced storage, mixing with a less evolved version of itself,
and subsequent crystallization at 5kbar and 975°C. Modeling of
Fe-Mg interdiffusion between the rims and cores of the
reversely-zoned clinopyroxene and orthopyroxenes suggest this
mixing, crystallization and subsequent ascent occurred within 10
years, or ~2.9 (+6.5 / -2.5) years, prior to eruption. Ascent from
5kbar or ~15 km, with no meaningful shallower storage, suggests
minimum crustal transit rates of ~5 km/year. This rate is
comparable to only a couple of other similar types of crustal
transit rates (and slower than the much faster, syn-eruptive ascent
rates measured through methods like olivine-hosted melt embayment
volatile gradients and U-series isotope measurements on other arc
magmas). The results of this study help to constrain the
pre-eruptive history and ascent rates of hydrous primitive arc
magmas, illuminating their magmatic processes during ascent. When
combined with geophysical signals of magma movement, mixing to
eruption timescales such as this have the power to inform volcanic
hazard models for monogenetic, cinder cone eruptions in the
Southern Cascades.
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