The H2O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene
The H2O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene
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DOI：
10.1127/0935-1221/2011/0023-2141
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发表时间：
2011-10
影响因子：
2.1
通讯作者：
S. Okumura
中科院分区：
文献类型：
--
作者：
S. Okumura
在这项研究中，我们测定了日本三座火山[公元1929年北海道-小松(Ko-a)喷发、浅间天美(As-A)喷发和樱岛大正(Sz-ts/P1)喷发]火山喷发的安山岩岩浆的H2O含量。通过分析单斜辉石斑晶的H2O含量，并利用H2O在熔体和单斜辉石之间的分配系数，估算了岩浆室熔体中的H2O量。红外光谱分析表明，Ko-A、As-A和Sz-ts/P1的单斜辉石的H_2O含量分别为329±65、247±72和213±36 ppm。根据Ko-a单斜辉石的H2O含量和单斜辉石(3.1wt%H2O)中流纹岩熔体包裹体的H2O含量计算分配系数。假定单斜辉石与熔体包裹体平衡，分配系数为0.011±0.003，使As-A和Sz-ts/P1岩浆房熔体的H2O含量分别为2.3±0.9和2.0±0.6wt%。根据地球物理观测估算的岩浆房深度计算的饱和状态下岩浆房中的H2O含量与As-A的实测值吻合较好，Ko-a和Sz-ts/P_1的值略高于实测值。这些结果表明，弧安山质岩浆在喷发前已被H2O饱和或准备饱和。
In this study, we have determined the H 2 O content of arc andesitic magmas erupted from three volcanoes in Japan [the 1929 A.D. eruption of Hokkaido-Komagatake (Ko-a), Tenmei eruption of Asama (As-A), and Taisho eruption of Sakurajima volcanoes (Sz-Ts/P1)]. The amount of H 2 O dissolved in the melt at magma chamber was estimated by analyzing the H 2 O content of clinopyroxene phenocrysts and using the partition coefficient of H 2 O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H 2 O contents of clinopyroxene were 329 ± 65, 247 ± 72, and 213 ± 36 ppm for Ko-a, As-A, and Sz-Ts/P1, respectively. The partition coefficient was calculated from the H 2 O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H 2 O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 ± 0.003, which results in H 2 O contents of the melts at the magma chamber of 2.3 ± 0.9 and 2.0 ± 0.6 wt% for As-A and Sz-Ts/P1, respectively. The H 2 O content of magma in the magma chamber calculated at saturation on the basis of magma-chamber depth estimated from geophysical observation shows good agreement with the measured value for As-A; those of Ko-a and Sz-Ts/P1 are slightly higher than the measured values. These results indicate that arc andesitic magmas are saturated with H 2 O or are ready to saturate before the eruptions.