Magma storage and mixing conditions for the 1953–1

Authors: Michelle L Coombs John C Eichelberger Malcolm J Rutherford

Publish Date: 2000/11/01

Volume: 140, Issue: 1, Pages: 99-118

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Abstract

Between 1953 and 1974 approximately 05 km3 of andesite and dacite erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park Alaska forming an edifice now known as Southwest or 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 lava flows 583–655 wt SiO2 are the dominant products of the eruption these contain discrete andesitic enclaves 558–589 wt SiO2 as well as micro and macroscale compositional banding Tephra from the eruption spans the same compositional range as lava flows however andesite scoria 56–581 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 around 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 premixing temperature for the dacite Water contents from 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 ∼35 wt H2O equivalent to 3 km depth if saturated Unzoned pyroxene and sodic plagioclase in the dacite suggest that it likely underwent significant crystallization at this depth highly resorbed anorthitic plagioclase from the andesite suggests 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 a compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly via disaggregation of enclaves phenocryst transfer from one magma to another and direct mixing of compositionally distinct melt phases

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