石基ガラスの塩素濃度不均質から読み解く珪長質マグマの脱ガス現象

抄録

Despite its water-rich nature, silicic magma often erupts calmly to discharge bubble-poor lava without accompanying violent explosion. To allow a water-rich magma to avoid explosion, gas phase that forms as a result of decompressive exsolution needs to escape efficiently to the outside of the magma. However, mechanism of this gas escape has long been unclear, mainly because direct evidence of gas pathways is rarely preserved in natural lava samples. In this paper I show that chlorine-content mapping analysis successfully visualizes the gas pathways hidden in groundmass glass. This chlorine mapping analysis was developed based on an idea that chlorine diffusivity in rhyolitic melt is so low that chlorine heterogeneity formed through degassing processes may survive for a long time without being homogenized. Timescales of degassing processes are estimated by applying a diffusion model to the observed heterogeneity. Results of my recent chlorine diffusion experiments are also presented, as the chlorine diffusivity is the key for interpreting the observed heterogeneity.