Magma storage and mixing conditions for the 1953-1974 eruptions of Southwest Trident Volcano, Katmai National Park, Alaska

Between 1953 and 1974, approximately 0.5 km(3) of andesite and dacite erupt ed from a new vent on the southwest flank of Trident volcano in Katmai Nati onal Park, Alaska, forming an edifice now known as Southwest (or New) Tride nt. Field, analytical, and experimental evidence shows that the eruption co mmenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four lava flows (58.3-65.5 wt% SiO2) are the dominant products of t he eruption; these contain discrete andesitic enclaves (55.8-58.9 wt% SiO2) as well as micro- and macro-scale compositional banding. Tephra from the e ruption spans the same compositional range as lava flows; however, andesite scoria (56-58.1 wt% SiO2) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe-Ti oxide pa irs from andesite scoria show a limited temperature range, clustered around 1000 degreesC. Temperatures from grains found in dacite lavas possess a wi der range; however, cores from large (>100 mum) magnetite and coexisting il menite give temperatures of similar to 890 degreesC, taken to represent a p re-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply tha t the two magmas last resided at a water pressure of 90 MPa, and contained similar to3.5 wt% H2O, equivalent to 3 km depth if saturated. Unzoned pyrox ene and sodic plagioclase in the dacite suggest that it likely underwent si gnificant crystallization at this depth; highly resorbed anorthitic plagioc lase from the andesite suggests that it originated at greater depths and un derwent relatively rapid ascent until it reached 3 km, mixed with dacite, a nd erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of a composi tional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, via disaggregation of enclaves, pheno cryst transfer from one magma to another, and direct mixing of compositiona lly distinct melt phases.