Experimental study on vesiculation and formation of groundmass microlites induced by decompression: Constraints on processes related to magma ascent at Unzen volcano

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This ongoing project is aimed to model crystallization and degassing processes occurring in the conduit of Unzen volcano during magma ascent (1991-1995 eruption). The main objective of this experimental approach is to reproduce experimentally the texture, mineral assemblage and mineral composition analyzed in groundmass of Unzen volcanic rocks sampled at the surface and at depth (core samples from ICDP-drilling). Decompression experiments are performed at different decompression rates (using the groundmass composition as starting material) to simulate magma ascent. The starting and final pressures are chosen to reproduce different conditions in the deep (1.5 to 10 km; below ICDP target) and shallow (< 1.5 km; above ICDP target) parts of the Unzen magmatic conduit. The textures of experimental products are analyzed to determine crystal and bubble number densities. The comparison of experimental products and natural samples from Unzen dome and ICDP-borehole (textures, composition and chemical zoning of microlites) is used to constrain dynamics of magma ascent. The interpretation of the preliminary results (isobaric experiments and decompression experiments from 300 to 200 and 50 MPa) provide quantitative data on the beginning of vesiculation and bubble growth, on the degree of undercooling and on decompression rates necessary to reproduce the textures of natural rocks and the sizes of natural microlites (especially plagioclase and amphibole). For example, it is concluded that magmas collected in the drilled conduit must have ascended at velocities much faster than the average magma ascent velocity of ~10-30 m/hour. Both the thermal history and the ascent dynamics of the drilled magmas may differ strongly from those of the magmas represented by the dome samples. Additional experiments (especially those conducted at low pressures) are planed and will be helpful to interpret the differences in textures and microlite compositions between samples from Unzen dome and ICDP-borehole.