Graduate Thesis Or Dissertation
 

Controls on eruption style and magma compositions at Mount Hood, Oregon

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  • This study is an effort to characterize the magma sources, plumbing system, and eruptive behavior of Mount Hood, a low-explosivity recharge-dominated volcano in the Oregon Cascades. The three manuscripts in this dissertation make use of melt inclusion data, phenocryst compositions, and whole rock petrology and geochemistry to build a schematic model of plumbing, mixing, and eruption at Mount Hood. Volatile contents in melt inclusions were measured by Fourier Transform Infrared Spectroscopy (FTIR) and Secondary Ion Mass Spectometry (SIMS). These measurements indicate that the pre-eruptive volatile contents at Mount Hood are comparable to concentrations in more explosive volcanoes, and do not sufficiently explain the low explosivity of Mount Hood. Measured H₂O contents were also used to test the validity of multiple different hygrometers. Various geothermobarometers were applied to the melt inclusions and phenocrysts from Mount Hood, and demonstrate that pre-eruptive temperatures increase by 100-150 ̊C immediately after mafic recharge, which occurs days to weeks prior to eruption and is accompanied by a 5-10 fold decrease in magma viscosity. Numerical simulations of magma ascent indicate that magma fragmentation is significantly delayed with this magnitude of pre-eruptive heating, which reduces the likelihood of explosive eruption. Analyses of amphibole demonstrate two markedly different populations, which correspond to different magma compositions, temperatures, and pressures. Pressure and temperature calculations were compared to other geothermobarometers to crosscheck the validity of these results and generally agreed well. Trace element concentrations in lavas, enclaves, and inclusions from Mount Hood confirm previous models for simple binary mixing at Mount Hood. A linear regression technique for extrapolating the major element contents of the mixing endmembers works acceptably well to characterize the trace element budgets of these endmembers. Additionally, we observe that the "recharge filter" that is responsible for the compositionally monotonous lavas at Mount Hood is also the likely cause of long-term low explosivity, and is indicative of a two-part magma plumbing system that may be a general model for a number of other recharge-dominated arc volcanoes. The results presented in this dissertation, in concert with previous results by other authors, converge on a generally consistent model for the production, hybridization, and eruption of intermediate lavas at Mount Hood and elsewhere.
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