# Experimental constraints on degassing and permeability in volcanic conduit flow

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## Abstract

This study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions: degassing and permeability in magmas. We studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature. From the data collected, we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium. Equilibrium bubble growth occurs when the decompression rate is slower than 0.1 MPa s^{−1}, while higher rates cause porosity to deviate rapidly from equilibrium, defining the first disequilibrium regime of degassing. If the deviation is large enough, a critical threshold of super-saturation is reached and bubble growth accelerates, defining the second disequilibrium regime. We studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions. Under these open conditions, we observed that bubbles start to coalesce at ~43 vol% porosity, regardless of decompression rate. Coalescence profoundly affects bubble texture and size distributions, and induces the melt to become permeable. We determined coalescence to occur on a time scale (~180 s) independent of decompression rate. We parameterized and incorporated our experimental results into a 1D conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust. Compared to previous models that assume equilibrium degassing of the melt during ascent, the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by ~25%, the acceleration at high porosities (>50 vol%) by a factor 5, and the associated decompression rate by an order of magnitude. The integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed, and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape.

## Keywords

Bubble growth Conduit flow Coalescence Decompression Degassing Rhyolite## Notes

### Acknowledgments

We would like to thank P.J. Shamberger for helping us running the coalescence experiments, while he was a REU intern at the Geophysical Institute and M. Wahlen for letting us use his digital microphotography equipment. Discussions with L. Mastin helped to clarify many mysteries of conduit flow modeling. Thorough reviews by M. Mangan and D. Sahagian greatly improved the manuscript. Funding was provided by the NSF grant EAR-0087853 to J.E.G. and by the Volcano Hazards Program of the US Geological Survey, through the Alaska Volcano Observatory to A.B.

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