Bubble coalescence in rhyolitic melts during decompression from high pressure

Abstract

When bubbly magma becomes permeable, its eruptive behavior is altered and gases are released that may be detected for monitoring. Permeability is produced by bubbles interacting and coalescing, but can be changed if the magmatic foam deforms. This study investigates how decompression rate and viscosity influence bubble coalescence through a series of hydrothermal experiments, in which bubble-bearing rhyolite is decompressed at temperatures ranging from 725° to 875 °C, producing viscosities of ∼ 10 5–10 7 Pa s. Most decompressions are at steady rates of 0.0064 to 0.025 MPa s − 1 , but a few are at rates that increase as pressure decreases; all are slow enough to maintain equilibrium as pressure drops. Bubble interaction and coalescence is recorded by variations in bubble sizes and bubble number density ( N B), and is found to be influenced strongly by melt viscosity, with more time needed for bubbles to coalesce as viscosity increases, yet the extent of coalescence appears limited by the distance between bubbles. The extent of coalescence is also influenced by decompression rate, with bubble interactions in lower viscosity melts being suppressed at decompression rates comparable to those expected for Plinian eruptions. In contrast, rates equivalent to lava dome extrusions are too slow to hinder bubble interactions from extensively altering the bubble population in lower viscosity melts. Deformation of coalescing permeable foam was observed when N B is on order of ∼ 10 5 cm − 3 , but not when it was on order of ∼ 10 7 cm − 3 , suggesting that magma degassing could be influenced by how many bubbles nucleate in the first place.