Gas bubble formation in crystal-rich magma and its roles on eruption dynamics

Abstract:

Although theoretical and petrological studies indicate that the ascent and eruption of crystal-rich silicic magma is compromised by prohibitive rheology (e.g., Rubin, 1995; Takeuchi, 2004), such magma nonetheless erupts, both explosively and effusively. It is increasingly recognized that bubbles in the magma may play a key role by reducing magma viscosity and changing the rheology (e.g., Pistone et al., 2013). Here, we investigate bubble formation processes and behavior in crystal-rich magma during decompression and propose a model for the eruption of such magma based on experimental results.

Decompression experiments were performed for 50 vol% crystal-bearing, hydrous rhyolite magma at a temperature of 800 °C in a cold-seal pressure vessel. The microstructures of bubbles and crystals were quantified using the X-ray CT. In the first experiments, the magmas were decompressed with a single step and at ~8 MPa h^–1. In the second series, the crystal-bearing magma was sandwiched by crystal-free magma and subjected to decompression rates of 8 to 28800 MPa h^–1. Using this technique, gas segregation from crystal-rich to crystal-free regions in the magma was detected. These experiments reveal that bubble formation caused a reduction of crystal connectivity under rapid decompression, whereas crystal connectivity did not change and gas bubbles were channeled out of the rigid crystal framework under low decompression rates.

Our experimental results clearly indicate feedbacks between bubble retention/segregation, decompression rate, and integrity of the crystal framework. Based on these findings, we infer that the ascent of crystal-rich magma must be driven by large overpressure, resulting in high ascent rate, and the different styles, i.e., explosive and effusive, of eruption of crystal-rich magma is caused by the non-linear behavior between bubble formation, decompression rate, and crystal network integrity. We place these findings in the context of caldera-forming eruptions of crystal-rich magma where large overpressures are induced by caldera-collapse, resulting in magma plug-flow, rapid decompression facilitated by shear-localization at conduit margins, and explosive eruption. Post-caldera effusive eruptions on the other hand are consistent with slow ascent and gas channelization and loss.