ERUPTION MECHANISMS AND ROCK TEXTURES

(1)高压高温实验研究：云英安质英安岩的相稳定性实验表明，在880℃条件下，基质英安岩的结晶压力要求大于60-100 Mpa，表明它们在3公里以上的深度结晶。基团浮晶岩的氯含量表明，在基团结晶之前已有2%的水从岩浆中脱气，表明英安岩的初始水含量约为6wt.%。通过含水熔体的降压进行了初步的气泡形成实验，并与Kikai-Akahoya长花岗岩的天然浮质喷射物6500年进行了比较。在富士火山1707A.D.玄武岩上进行了另一组实验，测定了液线斜长石的Ca/(Ca+Na)比随熔体含水量的变化，得到190 Mpa下的方程An=6*(H2O%)+67。利用这个关系，我们成功地从1707年斜长石斑晶和864个公元864年玄武岩的成分分带中获得了岩浆的脱气/混合历史。(2)天然岩石结构分析1991-1995年未岑英安岩中斑晶(斜长石和斜闪石)的成分分带表明，斑晶边缘有岩浆混合，斑晶核心有岩浆挥发输入/排气。后者是从斜长石中的Fe、Mg、K和均闪石中的Cl等元素的分布推断出来的，为岩浆室中挥发物的行为提供了新的证据。(3)数值模拟。我们用Hagen-Poiseuille定律计算了1991年温泉火山穹顶喷发的流量。岩浆粘度对含水率的强烈依赖性导致英安岩浆在700米以上的深度上升速度快得多。

(1) High pressure and high temperature experimental studies :Phase stability experiments on Unzen dacite showed that the pressure of groundmass pargasite crystallization requires more than 60-100 MPa at 880 C, suggesting they crystallized at depth of more than 3 km. Chlorine content of the groundmass pargasite indicates that degassing of 2% of water from the magma had occurred before the groundmass crystallization, indicating the initial water content of the dacite is ca. 6 wt.%.Preliminary vesiculation experiment through depressurization of hydrous melts were conducted, and compared with natural pumiceous ejecta of Kikai-Akahoya plinian erutpion, 6500 yr. b.p., in terms of vesicle size distribution and number density of vesicles in a unit mass of melt.Another suite of experiments were conducted on the 1707A.D.basalt of Fuji volcano, to determine the Ca/(Ca+Na) ratio of liquidus plagioclase as a function of water content of the melt, yielding an equation An=6* (H2Owt%)+67 at 190 MPa. Using this relationship, we successfully obtained the degassing/mixing history of magmas from the compositional zoning of plagioclase phenocyrsts in the 1707 as well as 864 A.D.basalts.(2) Analyses of natural rock texturesCompositional zonings of phenocrysts (plagioclase and homblende) in the Unzen 1991-1995 dacite showed both evidences of magma mixing at the rim of phenocrysts and of volatile input/degassing of magmas in the core of phenocrysts. Latter point is deduced from the minor element distributions (Fe, Mg, K in plagioclase and Cl in homblende), and provide new evidence concerning the behavior of volatiles in the magma chamber.(3) numerical modeling.We calculated the flow rate of the 1991 dome eruption of Unzen volcano usuing Hagen-Poiseuille law. Strong dependence of the viscosity of magma on the water content results in much faster uprise velocity of the dacite magma at depth more than 700 meters.