HR: 0800h
AN: V21C-0729    [Abstracts]
TI: Melt CO₂ Enrichment by Permeable Flow and Resorption
AU: * Rust, A
EM: glacr@bristol.ac.uk
AF: University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, United Kingdom
AU: Blundy, J
AF: University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, United Kingdom
AU: Cashman, K
AF: University of Oregon, 1272 University of Oregon, Eugene, 97403, United Kingdom
AB: Matrix and melt inclusion glasses commonly have CO₂/H₂O ratio values that are too high to be explained by standard closed- or open-system degassing. We explore the possibility of increasing CO₂/H₂O in melts through cycles of decompression, permeability development and gas flow, followed by repressurization and volatile resorption. As a case study we consider the spring and summer 1980 eruptions at Mt. St. Helens. Glasses from the climactic May 18 eruption show the least CO₂ enrichment whereas the next eruption, on May 25, as well as several other eruptions, show substantial CO₂ enrichment. We suggest that the episodic nature of the eruption sequence could have caused portions of the magma to experience multiple pressure cycles leading to overall increases in the CO₂/H₂O content of the melt. The proposed cycle begins with magma containing isolated bubbles with gas pressures similar to the surrounding melt. Depressurization of the magma (e.g., by edifice collapse and eruption of more shallow magma) causes sufficient vesiculation for bubbles to touch and form apertures, creating a connected network of gas. As gas escapes, the pressure of the vertically-connected gas falls from the melt pressure (related to the weight of magma above) down towards a gas-static pressure. The resulting discrepancy between the melt and gas pressures will cause 1) volatile diffusion from the melt into the relatively low-pressure bubble phase, and 2) bubble collapse and thus permeability reduction. Once the bubbles become isolated by bubble collapse, gas pressure will return to magmastatic values, driving volatile resorption back into the melt. CO₂/H₂O enrichment can occur if the gas added, which comes from deeper magma, has a higher CO₂/H₂O content than the gas lost by flow upwards. To assess the feasibility of increasing CO₂/H₂O ratios by this process, requires comparison of three timescales: 1) gas escape, 2) bubble collapse by viscous flow of melt and 3) diffusion of volatiles through melt into bubbles. As gas escapes, the gas pressure gradient decreases and gas flux reduces. In contrast, the pressure difference driving volatile diffusion into the bubbles, and bubble collapse (pressure of melt - pressure inside bubble) will initially be small, but will increase as gas escapes.
DE: 8400 VOLCANOLOGY
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting