The role of decompression history in gas bubble formation in crystal-rich silicic magma: Gas retention versus segregation

In crystal-rich magma, gas bubble formation causes magma lubrication by loosening the connected crystal framework. In contrast, if the crystal framework behaves as rigid, the gas bubbles segregate. In this study, we experimentally investigated the bifurcation between retention and segregation of gas bubbles in crystal-rich magmas under decompression. In particular, we focused on the history of decompression such as continuous, single-step, and multi-step decompressions with various degrees of each decompression step to consider the processes in the natural system. For this, the decompression experiments for hydrous rhyolitic melts with 50 vol% crystal were conducted with average rates of 10 and 80 MPa h−1 at a temperature of 800 °C. Bubble and crystal microstructures were analyzed using micro X-ray CT. Our experiments observed both retention and expansion of gas bubbles in the crystal-rich zone and permeable gas segregation. At an average rate of 80 MPa h−1 under multistep decompression, the boundary between the retention and segregation was the stepwise decompression of 5 MPa. On the other hand, the gas bubbles are retained when the degree of the stepwise decompression is larger than 20 MPa under an average decompression rate of 10 MPa h−1, resulting in the collapse of the framework. At the decompression degree <10 MPa at the rate of 10 MPa h−1, permeable gas segregation becomes dominant. Based on the experimental results, we established a regime map that predicts gas bubble retention and segregation during decompression. By applying this map to decompression rates and numbers of caldera collapses (twice) during the 1912 Katmai and 1991 Pinatubo eruptions, we infer that the magma during these eruptions experienced gas bubble formation (retention) without permeable segregation.