Pre-eruptive conditions prior to and after magma mixing at Unzen volcano: constraints from high pressure experimental investigations

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The 1991-1995 eruption of Unzen is initiated by a mixing event (as observed in many eruptive systems) between a low-temperature phenocryst-rich dacitic magma and a high temperature andesitic magma Thus, understanding pre-eruptive conditions requires to determine pre-mixing conditions (in the two end-member magmas) and post-mixing conditions, and to model the processes occurring during mixing and ascent. In Unzen volcano, only post-mixing conditions in the magma chamber are well-characterized. The proposed project should contribute to provide new constraints on the pre-mixing magmas, especially the high temperature end-member, (including volatile contents, melt and phenocryst compositions) and on processes occurring during mixing and magma ascent. These constraints will be obtained from high pressure - high temperature experimental investigations on phase relations and volatile solubilities in silicate melts. The comparison of natural samples from the volcanic dome (including andesitic enclaves) and from the ICDP-drilling with experimental results will be used to understand the role of mixing (and possible subsequent degassing) on magma forcing and the role of kinetic processes during magma ascent (degassing, crystallization in a dynamic system).