Degassing of stratified magma by compositional convection |
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| Institution: | 1. Centre for Process Systems Engineering, Department of Chemical Engineering, University College London, WC1E 7JE, London, United Kingdom;1. National Research Institute for Earth Science and Disaster Resilience, Tennodai 3-1, Tsukuba, Ibaraki 305-0006, Japan;2. Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University, 6-3 Aoba, Sendai, Miyagi 980-8578, Japan;3. SPring-8, Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan;1. Department of Geosciences, Boise State University, 1295 W University Drive, Boise, USA;2. Instituto Geofísico, Escuela Politécnica Nacional, Ladrón de Guevarra E11-253 y Andalucía, Quito, Ecuador;3. Facultad de Ciencias Exactas y Naturales, Pontificia Universidad Católica del Ecuador, Avenida 12 de Octubre 1076 y Roca, Quito, Ecuador;1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Resources, China University of Geosciences, Wuhan 430074, China;2. Department of Earth Sciences, Carleton University, 2115 Herzberg Laboratories, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada |
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| Abstract: | A new model is proposed for passive degassing from sub-volcanic magma chambers. The water content in stably stratified shallow magma chamber will be equated to its solubility at the upper boundary by convection. Water from a lower layer high in water content can enrich the contact zone of the upper layer and lead to further convective overturn of this boundary layer. A complete set of equations describing convection with bubble formation and dissolution is reduced to a simplified form by assuming a small bubble content. The development and pattern of flow driven by vesiculation is modeled numerically in a 2D magma chamber for relatively low Raleigh numbers (5×105). Bubbles rising from the magma will collect near the roof in a layer of 8–10 vol% and then escape upward to fumaroles. The Stokes flux of bubbles escaping from an andesitic magma with viscosity 104 P and a top surface of about 500×500 m corresponds with observed total magmatic water fluxes of 35 kg/s. Pressure within the chamber is buffered by elastic (and local visco-elastic) deformations in the solid rocks bounding the chamber to the range between ambient and close to lithostatic values. In a chamber closed to fresh magma inputs, the decrease in volume due to such gentle volatile escape lowers the reference pressure. Bubbles flux from the lower layer induced by variation of the saturation level around stratification boundary may be efficient mechanism for the water transport between layers. |
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