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• Experimental And Petrologic Constraints On Magma Movement, Storage, And Interactions At Two Volcanoes In Katmai National Park, Alaska

Between 1953 and 1974, ~0.5 km3 of lava and tephra erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest Trident. The eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. The dacite lava flows contain andesitic enclaves as well as compositional banding. Dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ~3.5 wt % H2O, equivalent to 3 km depth. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of the earliest lava by approximately one month. The enclaves in the dacite had experienced a complex history by the time they were erupted. Quantitative analysis of groundmass microphenocrysts in enclaves from the lava shows that the enclaves underwent a textural maturation. I have run experiments that replicate the path taken by andesite during magma mixing in which the andesite was annealed at 1000�C, cooled at various rates to 890�C, held for residence time t, and then quenched. The andesite experimentally cooled at the slower rates (2�C/h and 10�C/h) most resembles enclave groundmass. This is consistent with cooling of the andesite below an andesite-dacite interface, suggesting that pre-enclave formation crystallization caused vapor exsolution and enclave flotation. Decompression experiments on the dacite suggest an average ascent time for the eruption of 30 hours. The high silica rhyolite erupted during the June 1912 eruption of Katmai is notable both for its large volume and evolved composition. Hydrothermal, water-saturated experiments constrain the magma's pre-eruptive storage condition to a region in P-T space between 800�C and 100 MPa and 850�C and 40 MPa. Amphibole is only present in the rhyolite of Novarupta dome, the last product of the eruption. Novarupta dome rhyolite probably was stored under the same conditions but underwent magma mixing with andesite and dacite prior to effusion.
Volcanic and non-volcanic partings occur in coal beds of the Neogene Beluga and Sterling Formations along the shores of the Kenai lowland, Alaska. The partings were systematically characterized to determine their potential geological applications: Two-thirds of the partings originated as air-fall tephra. Of these, partly altered, Pliocene tephra typically contain volcanic glass + feldspar $\pm$ montmorillonite $\pm$ quartz $\pm$ kaolinite $\pm$ opal-CT. Highly altered Miocene partings are characterized by feldspar $\pm$ kaolinite $\pm$ montmorillonite $\pm$ quartz $\pm$ crandallite $\pm$ altered volcanic glass, where crandallite appears to have formed by replacement of volcanic glass prior to clay formation. About one-third of the partings are of detrital origin and contain detrital chlorite + illite + smectite + quartz $\pm$ feldspar $\pm$ siderite $\pm$ kaolinite. A Pliocene pumice parting near the top of the Sterling Formation was correlated from the northwestern to the southeastern Kenai lowland on the basis of similar glass morphologies, an absence of opaque minerals, and geochemical similarities. A crystal-tuff near the middle of the section could be traced across the Kenai lowland as one or two ash-falls, based on inertinite contents of adjacent coal, mineralogy, and geochemistry. Some other prominent tephras could not be correlated. The tephra partings are time-equivalent to DSDP cores from the Gulf of Alaska and along the Aleutian Island chain. Tephras occur every 125-500 yr in the lower part of the Beluga Formation, and their deposition probably coincides with a volcanic pulse 10.5 m.y. ago. This pulse is not well recorded in nearby DSDP cores. In the upper part of the Beluga Formation, during volcanic quiescence, tephras are recorded at an average rate of one every 9,000 yr. Time equivalent DSDP cores show a near absence of tephras. A volcanic pulse occurred during the deposition of the lower Sterling Formation, about 7.5 m.y. ago, with intervals between volcanism which averages 11,000 yr or longer. Volcanic sources appear to have been distant, which is consistent with an absence of tephra layers in a Gulf of Alaska core. About 5 m.y. ago, concurrent with the deposition of the upper Sterling Formation, the thicknesses of the tephra layers dramatically increase and the frequency increases to an average of one tephra every 2,000 years. This increase is recorded in DSDP cores as well.