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Towards a universal set of bubble coalescence laws in low viscosity magmas

Abstract : Bubble nucleation, growth, and coalescence control the ascent, degassing, and eruption of all types of magma. Nucleation and growth are consequences of magma decompression, and are satisfactorily described by solubility and physical models. Coalescence is a more complicated issue, and one that bears heavily on the eruptive fate of an ascending magma. If coalescence is efficient, the resulting permeable networks of interconnected void space may lead to efficient outgassing, and in turn, a reduction in explosive potential. Alternatively, if coalescence is inefficient, bubbles may remain isolated and overpressured, leading to cataclysmic explosive eruptions. Vesicle textures in pyroclasts preserve the only natural evidence of bubble-bubble dynamics. However, most (if not all) pyroclast textures either have lost their gas by bubble collapse, or record syn- to post-fragmentation processes. In both cases, natural products may not always be good indicators of processes at depth. There is thus a gap in our understanding of deep conduit coalescence processes. We present an x-ray computed microtomographic (µ-cT) study of coalescence at the single bubble-bubble pair scale in experimentally and naturally decompressed, low viscosity magmas. Experimentally decompressed phonolites allow us to essentially look "behind" fragmentation, by reproducing deep conduit conditions, and controlling decompression and quenching. Initial results indicate there is no single law of coalescence that can explain all the interactions observed in these low-viscosity magmas, where bubble buoyancy and mobility are key issues. Instead, we identify four different processes, acting concurrently. (1) Traditional capillary +/- gravitational drainage along flattened interbubble films. (2) Static "dimpling," or penetration of one bubble into an adjacent bubble, driven by growth dispersion-controlled differences in internal bubble pressures. (3) Dynamic "handshaking," similar to dimpling, but occurring between buoyantly moving bubbles. (4) "Suction," or entrainment of small trailing bubble into faster-rising larger bubbles. Initial analytical models indicate that in low viscosity magmas, all of these processes act on timescales shorter than the typical timescales of ascent and decompression. Examination of a range of natural submarine and subaerial pyroclasts permits discussion of how each coalescence mechanism is represented in nature, and on the caveats of using textural information in natural pyroclasts to quantify coalescence mechanisms.
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Submitted on : Tuesday, September 10, 2013 - 4:44:25 PM
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C. Ian Schipper, Alain Burgisser. Towards a universal set of bubble coalescence laws in low viscosity magmas. American Geophysical Union, Fall Meeting 2010, Dec 2010, San Francisco, United States. ⟨insu-00860542⟩



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