Eruption mechanisms and mineral textures of the ejecta

(1)富士火山喷发物对比岩石学研究：考察了3000ybp omurroyama侧翼喷发、AD.864 Jyogan喷发和AD.864 Jyogan喷发的岩石学特征。1707年会惠火山喷发，了解喷发方式与富士火山喷出物晶体结构之间的关系。斜长石斑晶和微斑晶的成分分带揭示了岩浆隆起和喷发过程中含水干湿岩浆的脱气和补给过程。大urroyama火山喷发3000年时为下卧期火山碎屑陷落沉积，其中含有少量斜长石微晶，表明火山喷发前由于水体脱气，岩浆的结晶时间有限。AD.864喷发产生20-30%的斜长石斑晶，在边缘处常出现反分带，表明在喷涌喷发前，含水干性岩浆对浅层脱气岩浆进行了补充。(2)磁袋对岩浆混合影响的模拟实验：本实验旨在考察低雷诺数下两个岩浆的混合过程。以前的实验（如Blake和Campbell， 1986）只考察了高雷诺数下的混合条件。岩浆袋的存在增加了层流环流的不稳定性，这是由于重力对粘性力的增加。结果表明，当无量纲参数I（=粘性力/重力）小于0.1时，会发生流动失稳。对端段岩浆条件的分析表明，云仙火山下的岩浆穴存在流动不稳定。(3)火山喷出物的含水量和水化作用：我们研究了公元1783年的安山岩-流纹岩玻璃。1108年和13ka bp麻麻火山喷发产物。AD.1783和AD.1108喷出物的含水量为03-0.7 wt%，而13ka喷出物的含水量为15-4 wt%。在13ka火山玻璃的背散射电子图像中，我们观察到在气泡壁玻璃的核心处有明显的水化前缘，表明老沉积物的高含水量受到了玻璃二次水化的影响。(4)亚流岩浆粘度测定：A基拉韦厄闪长岩：晶相以辉长石和斜长石为主，少量橄榄石。含晶岩浆的相对粘度几乎可以用Einstein-Roscoe-Marsh方程很好地解释，表明晶体形态各向异性的影响被晶体尺寸弥散的影响所抵消。B: MORB拉斑岩：ODP Leg 206 MORB样品用于粘度测量。晶相也由辉长石和斜长石组成，橄榄石的含量与之相当。含晶岩浆的相对粘度比Einstein-Roscoe-Marsh方程的值大3-5倍，这可能是由于MORB电荷中斜长石的长径比较高所致。少

(1) Comparative petrological study of volcanic ejecta of Fuji volcano: We examined petrographic features of the 3000ybp Omuroyama flank eruption, AD.864 Jyogan eruption, and AD. 1707 Hoei eruption to understand the relations between the mode of eruptions and crystal textures of the ejecta at Fuji volcano. Compositional zoning of plagioclase phenocryst and microphenocryst tells us the processes of degassing and recharge of hydrous aphyric magmas during the magma uprise and eruptions. The 3000 yr by Omuroyama eruption had subplinian pyroclastic fall deposit which contains limited amount of plagioclase microphenocryst, suggesting only limited time for crystallization of magmas due to degassing of water just before the eruption. The AD.864 eruption products 20-30% of plagioclase phenocryst often with reverse zoning near the rim, indicating magma replenishment of degassed shallow magma by hydrous aphyric magmas just before the effusive eruptions.(2) Analogue experiments of the effect of mag … More ma pocket on the mixing of magmas: This experiment intends to examine the mixing processes of two magmas at low Reynolds number. Previous experiments (e g. Blake and Campbell, 1986) only examined the mixing conditions at high Reynolds number. Presence of a magma pocket increases the instability of laminar annular flow because of the increase of gravitational force against viscous force. It is shown that flow instability occurs when the dimensionless parameter I (=viscous force/gravitational force) is less than 0.1. Analyses of the condition of end member magmas indicate that flow instability occurs in a magma pocket beneath Unzen volcano.(3) Water content and hydration of volcanic ejecta: We examined andesitic-rhyolitic glasses of the AD.1783, AD. 1108, and 13ka bp eruption products of Mama volcano. Water contents are 03-0.7 wt % for AD.1783 and AD.1108 ejecta, whereas they are 15-4 wt% for 13ka ejecta. In back-scattered electron images of 13ka volcanic glasses, we observed clear hydration front in the core of the vesiculated bubble wall glass, suggesting that high water content of the old deposits are affected by secondary hydration of the glass.(4) Viscosity measurement of subliquidus magmas:A Kilauea thoeiite: Crystal phases are mainly augite and plagioclase with minor amount of olivine. The relative viscosity of the crystal-bearing magma is almost well explained by the Einstein-Roscoe-Marsh equation indicating that the effect of anisotropic crystal morphology is counterbalanced by the effect of crystal size dispersion.B: MORB tholeiite: An ODP Leg 206 MORB sample is used for the viscosity measurement. Crystal phases also consist of augite and plagioclase with mirror amount of olivine. The relative viscosity of the crystal-bearing magma is larger than the values of Einstein-Roscoe-Marsh equation by a factor of 3-5, which may be due to the higher aspect ratio of the plagioclase in the MORB charge. Less