Variation of volatile concentration in a magma system of Satsuma-Iwojima volcano deduced from melt inclusion analyses
Variation of volatile concentration in a magma system of Satsuma-Iwojima volcano deduced from melt inclusion analyses
复制标题
DOI：
10.1016/s0377-0273(00)00276-6
复制
发表时间：
2001-08-15
影响因子：
2.9
通讯作者：
Kawanabe, Y
中科院分区：
文献类型：
--
作者：
Saito, G;Kazahaya, K;Kawanabe, Y
本文对日本萨摩岩岛火山的30个熔融包裹体进行了化学分析，研究了火山岩浆房从距今6300年到现在的挥发分演化。大的变化，熔体中的挥发分浓度进行了观察。(1)流纹质熔体的水浓度随时间而降低; 3-4.6 wt.%在竹岛火山碎屑流存款（约200万美元）的最新火山口形成喷发时，6300年BP），3重量%对于小的火山碎屑流（约. 1300年BP），破火山口后流纹岩圆顶，和0.7-1.4重量% 1934年的海底火山喷发（昭和-岩岛）。(2)竹岛和岩岳喷发的流纹岩熔体中的CO2含量低于40 ppm，而昭和岩岛熔体中的CO2浓度更高，高达140 ppm。(3)稻村岳火山口后玄武岩火山渣锥的玄武质到安山质熔体的水和CO2浓度为1.2-2.8重量%且小于或等于290 ppm。分别岩浆房中的挥发分演化解释如下：（1）最后一次形成破火山口的流纹岩岩浆（约。6300年BP）是气体饱和的，由于压力变化的岩浆房，因为大的变化，水浓度的熔体是由于在喷发前的岩浆中的挥发分的出溶。岩岳喷发（Iwodake eruption）1300年BP）是由残余的破火山口形成流纹质岩浆，建议从这些岩浆之间的主要元素组成的相似性。(2)昭和岩岛流纹岩熔体的挥发分组成与岩岳山顶目前排出的岩浆气体一致，表明低压脱气条件。(3)岩浆房的岩浆对流在岩岳的管道在非喷发，但活跃的脱气期超过800年的水浓度降低流纹质岩浆的脱气。(4)地质和岩石学观察表明，在后破火山口阶段，可能存在一个由下部玄武岩层和上部流纹岩层组成的分层岩浆房。CO2从下伏玄武质岩浆向上部气体欠饱和（脱气）流纹质岩浆的加入增加了流纹质岩浆的CO2浓度。((C)2001 Elsevier Science B. V.保留所有权利。
Chemical analyses of 30 melt inclusions from Satsuma-lwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300 yr BP to the present. Large variations in volatile concentrations of melts were observed. (1) Water concentration of rhyolitic melts decreases with time; 3-4.6 wt.% at the time of latest caldera-forming eruption of Takeshima pyroclastic flow deposit (ca. 6300 yr BP), 3 wt.% for small pyroclastic flow (ca. 1300 yr BP) of Iwodake, post-caldera rhyolitic dome, and 0.7-1.4 wt.% for submarine lava eruption (Showa- Iwojima) in 1934. (2) Rhyolitic melts of the Takeshima and Iwodake eruptions contained CO2 of less than 40 ppm, while the Showa-Iwojima melt has higher CO2 concentration of up to140 ppm. (3) Water and CO2 concentrations of basaltic to andesitic melt of Inamuradake, a post-caldera basaltic scoria cone, are 1.2-2.8 wt.% and less than or equal to 290 ppm. respectively. Volatile evolution in the magma chamber is interpreted as follows: (1) the rhyolitic magma at the time of the latest caldera-forming eruption (ca. 6300 yr BP) was gas-saturated due to pressure variation in the magma chamber because the large variation in water concentration of the melt was attributed to exsolution of volatile in the magma prior to the eruption. Iwodake eruption (ca. 1300 yr BP) was caused by a remnant of the caldera-forming rhyolitic magma, suggested from the similarity of major element composition between these magmas. (2) Volatile composition of the Showa-Iwojima rhyolitic melt agrees with that of magmatic gases presently discharging from a summit of Iwodake, indicating the low pressure degassing condition. (3) The degassing of the magma chamber by magma convection in a conduit of Iwodake during non-eruptive but active degassing period for longer than 800 years decreased water concentration of the rhyolitic magma. (4) Geological and petrological observations indicate that a stratified magma chamber, which consists of a lower basaltic layer and an upper rhyolitic layer, might have existed during the post-caldera stage. Addition Of CO2 from the underlying basaltic magma to the upper gas-undersaturated (degassed) rhyolitic magma increased CO2 concentration of the rhyolitic magma. ((C) 2001 Elsevier Science B.V. All rights reserved.