Lanterna Rally

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Published: 2018

Total Pages: 210

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Magma recharge events and subsequent mixing processes are understood to precede volcanic eruptions. Textural evidence of intrusion of hot, mafic magma into a cooler, rheologically locked silicic magma is commonplace. Solidified "blobs" of injected magma, called enclaves, are evidence of magma mixing, but the petrological and mechanical conditions during their formation are debated. Mount Hood, Oregon consistently erupts andesite bearing compositionally similar enclaves. These enclaves are evidence of mingling and mixing of two magmas. However, due to the compositional similarity between enclave and host lava (e.g. ~1-5 wt.% difference in SiO2), it is unclear whether the preserved enclaves represent; 1) partially hybridized mafic melt remaining after mixing with significant crystal exchange with the host magma or 2) the preserved remnants of the intruding magma during recharge, with no homogenization or crystal exchange with the host magma. The aim of this study is to understand how and why enclaves form in compositionally similar host magmas, such as those at Mount Hood. Building off previous research, we utilize a combination of field observations, chemical analyses, and numerical modeling to constrain the rheology of the magmas prior to and during mixing. The degree of magma mixing is dependent on the viscosity contrast between the host and intruding magmas. Since these magmas are similar compositionally, variations in other magmatic properties such as crystallinity, and therefore temperature, and density may drive the viscosity differences between the host and intruding magmas needed for enclave formation.