Abstract
Understanding degassing of mafic magmas is important for modeling eruptions and examining controls on eruption style. We conducted high-pressure-high-temperature isothermal decompression experiments to investigate the effects of decompression-induced crystallization on permeability development and magma degassing. Experiments were performed on hydrous basaltic andesite (54 wt% SiO2) decompression rates equivalent to magma ascent velocities of ~1-3 m s-1. We measured the gas flux of the quenched samples using a bench-top permeameter and calculated the Darcian (k1) and inertial (k2) permeabilities using the Forchheimer equation. The experimental samples developed permeability at a critical vesicularity (Φc) of 56.4 ± 2.7 vol% (at 0.125 MPa s-1) and 50.76 ± 5.6 vol% (at 0.083 MPa s-1), considerably lower than the Φc > 63 vol% permeability threshold determined for crystal-free basaltic andesite melts. The percolation threshold decrease is observed when the microlites comprise ≥~20 vol% and can be explained by the onset of yield strength, which occurs when the crystals form a loosely packed, touching framework. © 2017 Geological Society of America.
| Original language | English |
|---|---|
| Pages (from-to) | 831-834 |
| Number of pages | 4 |
| Journal | Geology |
| Volume | 45 |
| Issue number | 9 |
| Early online date | 10 Jul 2017 |
| DOIs | |
| Publication status | Published - Jul 2017 |
Bibliographical note
Export Date: 13 September 2017CODEN: GLGYB
Funding details: EAR-1145194, NSF, National Science Foundation
Funding details: University of Texas at Austin
Funding text: This work was funded by National Science Foundation grant EAR-1145194 (to Larsen) and by the AXA Research Fund and a Royal Society Research Merit Award (to Cashman). We thank Olivier Bachmann and two anonymous reviewers for comments and suggestions that greatly improved this manuscript. We gratefully acknowledge Owen Neill for building the bench-top permeameter, and Shingo Takeuchi for allowing us to use two samples as permeameter standards. We also thank the University of Alaska Fairbanks Advanced Instrumentation Laboratory for their analytical capabilities and the University of Texas at Austin for performing the X-ray computed tomography.
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Keywords
- Basalt
- High pressure effects
- Solvents
- Basaltic andesite
- Decompression rate
- Forchheimer equation
- High pressure high temperature
- Induced crystallization
- Isothermal decompression
- Magma degassing
- Percolation thresholds
- Degassing