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

Abstract

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.