Understanding of magmatic eruptions and prediction of future eruptions in an active volcano, Zao

在Zao火山最年轻的阶段（30 ka至今），有三个活动期（约100 ka）。31 ~ 29 ka、7.5 ~ 4.1ka和2.0至今）。火山碎屑岩堆积是火山活动的主要产物。根据产物的岩石学特征，研究了三个时期的岩浆补给系统，从岩石学和矿物学特征判断，三个时期的喷发岩石均为橄榄+辉石玄武安山岩-安山岩，是两种端元岩浆混合形成的。估计的长英质端员是相似的时期，安山岩（约60%的二氧化硅含量）与斜方辉石（镁#=约64），单斜辉石（镁#=约68），斜长石（安=约65）斑晶。估计的镁铁质端员是玄武岩与橄榄石（Fo=约80）和斜长石（An=约90）斑晶在所有时期，然而，整体MgO，Cr和Ni含量的喷发岩石是在第二个时期高于其他两个时期。在第二和第三阶段，岩石的硅含量从58%下降到55- 56%，并恢复到58%，这些变化可以解释为不同百分比的玄武岩浆参与混合。这些特征表明，镁铁质端元岩浆在不同时期有明显的差异，可能在地壳深部储存了约3.5 - 2.0ky。近距离观察过去约0.8 - 2.0ky喷发的岩石化学成分，Zr逐渐减少，（和Cr）含量向上部增加至少两次，这可能对应于玄武质岩浆向浅部安山质岩浆房的递进注入，并且估计每次喷射的持续时间小于0.2ky。

In the youngest stage (30 ka to present) of the Zao volcano, three active periods (ca. 31 to 29 ka, 7.5 to 4.1 ka, and 2.0 to present) can be seen. Piles of pyroclastic rocks by numerous small to medium sized eruptions are main products of the activities. In this study we examined the magma feeding system in the three periods, based on the petrologic features of the products.Rocks erupted in the three periods are olivine+-pyroxene basaltic andesite to andesite, and these were formed by mixing of two end-member magmas, judged from the petrographic and mineralogic features. The estimated felsic end-members are similar among the periods, andesite (ca.60% in silica content) with orthopyroxene (Mg#=ca.64), clinopyroxene (Mg#=ca.68), plagioclase (An=ca.65) phenocrysts. The estimated mafic end-members are basalt with olivine (Fo=ca.80) and plagioclase (An=ca.90) phenocrysts in all periods, however, the bulk MgO, Cr and Ni contents of the erupted rocks are higher in the second period than in the other two periods. During the second and third periods, silica contents of the rocks decreased temporally from 58 to 55-56 % and recovered up to 58 %, and these variations can be explained by the different percentages of the basaltic magma involved in the mixing. Those features are suggesting that the mafic end-member magmas are distinct among periods, and may have been stored in the deeper part of the crust for ca.3.5 to 2.0 ky.Looking at the chemical compositions of rocks in the past ca.0.8-ky eruptions closely, gradual decrease in Zr (and increase in Cr) contents toward upper part can be seen at least twice, which may correspond to the progressive injection of the basaltic magma to the shallower andesitic magma chamber, and it is estimated that the duration of each injection is less than 0.2 ky.