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）期间云岑火山管道中发生的结晶和脱气过程。该实验方法的主要目的是通过实验再现云岑火山地表和深层取样（icdp钻探的岩心样本）的结构、矿物组合和矿物组成分析。在不同的减压速率下进行减压实验（以地面物质组成为起始材料），模拟岩浆上升。在Unzen岩浆管道的深层（1.5 ~ 10 km，低于ICDP目标）和浅层（< 1.5 km，高于ICDP目标）部分，选择起始压力和最终压力来重现不同的条件。对实验产物的织构进行分析，以确定晶体和气泡数密度。通过对比云仙丘和icdp钻孔的实验产物和自然样品（微岩的结构、组成和化学分带），对岩浆上升动力学进行了约束。对初步结果（300 - 200 MPa和50 MPa的等压实验和减压实验）的解释提供了关于水泡和气泡生长的开始、过冷程度和减压速率所需的定量数据，以重现天然岩石的纹理和天然微长岩（特别是斜长石和角闪洞）的大小。例如，在钻孔管道中收集的岩浆上升速度一定比平均上升速度（~10-30 m/h）快得多。钻孔岩浆的热史和上升动力学可能与穹窿样品所代表的岩浆有很大的不同。计划进行更多的实验（特别是在低压下进行的实验），这将有助于解释Unzen圆顶和icdp -钻孔样品之间的纹理和微岩石组成差异。

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.