ERUPTION MECHANISMS AND ROCK TEXTURES

(1)高压和高温实验研究：对Unzen英安岩的相稳定性实验表明，在880 ℃下，基质Pargasite结晶的压力需要超过60-100 MPa，这表明它们在超过3 km的深度结晶。基块闪长岩的氯含量表明，在基块结晶之前，岩浆中有2%的水已经被脱气，表明英安岩的初始含水量约为10%。6重量%。对6500年前Kikai-Akahoya plinian erutpion的含水熔融物进行了减压泡囊形成的初步实验，并与天然浮石喷出物进行了比较。b.p.，另一组实验测定了富士火山1707年玄武岩液相线斜长石的Ca/（Ca+Na）比值与熔体含水量的关系，在190 MPa下得到方程An=6*（H_2Owt%）+67。利用这种关系，我们成功地获得了脱气/混合历史的岩浆成分分带的斜长石斑晶在1707年，以及864公元玄武岩。(2)1991-1995年云仙英安岩中斑晶（斜长石和均长石）的成分分带显示了斑晶边缘岩浆混合和斑晶核部岩浆挥发分输入/脱气的证据。后一种观点是从斜长石中微量元素Fe、Mg、K和钙钛矿中微量元素Cl的分布推断出来的，为岩浆房中挥发分的行为提供了新的证据。(3)数值模拟计算了1991年云仙火山圆顶喷发的流量。岩浆粘度对含水量的强烈依赖性导致了英安质岩浆在700米以上的上升速度大大加快。

(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.