ARCUS Student Award | 4th Annual Award
4th Annual ARCUS Award for Arctic Research Excellence
|Submitted by:||Michelle Coombs|
|Authors:||Michelle Coombs, John C. Eichelberger, and Malcolm J. Rutherford|
|Title:||Magma storage and mixing conditions for the 1953-68 eruption of Southwest Trident volcano, Katmai National Park, Alaska|
|Affiliation:||Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska|
Between 1953 and 1968, approximately 0.5 km3 of andesite and dacite erupted from a newly formed vent on the southwest flanks of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest (New) Trident. Field, analytical, and experimental evidence shows that the eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four laval flows (58.3ñ65.5 wt % SiO2) are the dominant result of the eruption; these contain discrete andesitic enclaves (55.8ñ57.2 wt % SiO2) as well as micro- and macro-scale compositional banding. Tephra from the eruption spans the same compositional range as lava flows; however, andesite scoria (56ñ58.1 wt % SiO2) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe-Ti oxide pairs from andesite scoria show a limited temperature range clustered at 1000°C. Temperatures from grains found in dacite lavas possess a wider range; however, cores from large (>100 µm) magnetite and coexisting ilmenite give temperatures of ~890°C, taken to represent a pre-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibia experiments on the andesite show that the two magmas last resided at a water pressure of 90 MPa, and contained ~3.5 wt % H2O, equivalent to 3 km depth. Unzoned pyroxene and sodic plagioclase in the dacite indicate that it likely underwent significant crystallization at this depth; highly resorbed anorthitic plagioclase from the andesite indicates that it originated at greater depths and underwent relatively rapid ascent until it reached 3 km, mixed with dacite, and erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of any compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, probably due to low density and viscosity differences. Disaggregation of enclaves, phenocryst transfer from one magma to another, and direct mixing of compositionally distinct melt phases were the three mechanisms by which hybridization was accomplished.