[S-VC32] Dynamics of volcanic eruptions and their physical and chemical processes

[SVC32-07] The mechanism of gas and melt separation from crystal-rich magma under multistep decompression

*Masaki Kobayashi¹, Satoshi Okumura¹, Masato Hoshino², Kentaro Uesugi², Osamu Sasaki³ (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.JASRI/SPring-8 , 3.Tohoku University Museum)

Keywords:crystal mush, phase separation, decompression, crystal framework, bubble growth

Caldera-forming eruptions that have large impact on earth surface often release both crystal-rich and -poor magmas from the reservoir with high crystallinity (crystal mush). Gas driven filter pressing induced by decompression-induced vesiculation may cause melt separation in the reservoir and explain the formation of crystal-poor magma; however, no experimental studies have investigated whether this process causes the melt separation or not. Here, we show that gas and melt separation, which originates from the interaction between growing gas bubbles and the crystal framework, is controlled by decompression history based on decompression experiments. To investigate the effect of stepwise collapses of the caldera roof, we performed multi-step decompression experiments for rhyolite magma with a crystallinity of 50 vol%. Our experiments demonstrated that gas bubbles are retained in magma without the separation and crystal framework collapses because of bubble expansion when the decompression of each step is >20 MPa, while gas bubble and melt are extracted from crystal framework by permeable flow with the pressure drop <10 MPa. These results indicate that the strength of crystal framework in crystal-rich magmas is ~20 MPa under crystallinities of 50 vol% when the crystals have isotropic shape. Based on the experimental results, we infer that the decompression of crystal mush during caldera-forming eruptions can cause melt separation when the degree of the decompression during stepwise collapse is small, resulting in gas separation.