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

这个正在进行的项目旨在模拟岩浆上升（1991-1995年喷发）期间云仙火山管道中发生的结晶和脱气过程。这种实验方法的主要目的是通过实验重现在云仙火山岩地面和深度取样（ICDP钻探的岩心样品）中分析的纹理、矿物组合和矿物成分。在不同的减压速率（使用的基质组成作为起始材料）进行减压实验，以模拟岩浆上升。选择起始压力和最终压力以再现云仙岩浆管道的深（1.5至10 km;低于ICDP目标）和浅（< 1.5 km;高于ICDP目标）部分的不同条件。实验产品的纹理进行分析，以确定晶体和气泡数密度。通过云仙穹隆和ICDP钻孔的实验产物和天然样品的比较（微晶岩的结构、成分和化学分带），来约束岩浆上升的动力学。初步结果的解释（等压实验和减压实验，从300至200和50 MPa）提供了定量数据的开始泡和气泡的生长，过冷度和减压率必须再现天然岩石的纹理和天然微晶石（特别是斜长石和角闪石）的大小。例如，得出的结论是，在钻孔管道中收集的岩浆上升的速度必须远远快于平均岩浆上升速度约10-30米/小时。钻孔岩浆的热历史和上升动力学可能与穹隆样品所代表的岩浆有很大不同。计划进行额外的实验（特别是在低压下进行的实验），这将有助于解释云仙穹和ICDP钻孔样品之间的纹理和微晶成分差异。

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.