Study on magma plumbing system of Aso volcano by methods of mare drilling isotope analysis

阿索火山活动具有4个大火山碎屑喷发旋回（LPEC）和其间多次小喷发的特征。为了解阿索火山岩浆成因，对150 ~ 90 ka 3个大喷发旋回的岩浆进行了岩石学和地球化学研究。在每个LPEC中，岩浆都是从一个层状岩浆房中喷发出来的，岩浆上覆较致密的镁铁质岩浆，岩浆中的Sr同位素组成与岩浆中的Sr同位素组成基本一致。总结了岩浆和镁铁质岩浆的重要地球化学特征。1.在每个大的喷发旋回中，镁铁质岩浆的全岩镍含量等于或大于镁铁质岩浆的全岩镍含量。2.硅质和镁铁质岩浆在每个LPEC具有相同的Sr同位素比值在全岩，玻璃，斜长石斑晶。3.三个LPEC的岩浆具有不同的Sr同位素比值。4.质量含水量 关于我们 至少在阿索-2和阿索-3期，铁基岩浆的体积大于镁铁质岩浆的体积。5.阿索火山岩浆的岩石学特征随时间发生了系统的变化：岩浆温度降低，K2 O/SiO2含量降低，氧逸度增加，特征3表明各LPEC岩浆的生成过程与其它LPEC岩浆的生成过程不同。特征2强烈地表明，每个LPEC的镁铁质岩浆和镁铁质岩浆来自相同的源物质。另一方面，特征1则表明岩浆不是由基性岩浆分异作用产生的。这些地球化学数据要求，每个LPEC的镁铁质岩浆和镁铁质岩浆来自同一个源物质，但他们没有父母-女儿的分馏关系。这些镁铁质和基性岩浆的岩浆成因可以定性地解释如下。源材料是辉长岩。基性岩浆是由辉长岩中部分熔融程度较高的分凝部分熔融岩浆分离结晶而成。辉长岩岩浆是从辉长岩中分离出来的部分熔融岩浆，部分熔融程度较低，分离后没有分馏。辉长岩部分熔融的岩浆生成可能发生在下地壳：来自地幔的热岩浆上升并加热下地壳。镁铁质岩浆产生于地幔热岩浆附近。地幔岩浆是由距离地幔岩浆较远的下地壳低程度部分熔融形成的。特征5解释为下地壳含水量的增加（即，岩浆源物质）。少

Activities of Aso volcano are characterized by four large pyroclastic eruption cycles (LPEC) and many minor eruptions between them after 300 ka. In order to understand magma genesis of Aso volcano, we investigated petrological and geochemical features of magmas of three large eruption cycles between 150 ka and 90 ka. It is emphasized that systematic data of Sr isotope ratio of whole-rock, glass, and plagioclase phenocryst (micro-drilling method) have been obtained.In each LPEC, the magmas erupted from a layered magma chamber with silicic magmas overlying denser mafic magmas. Important geochemical features of silicic and mafic magmas are summarized as follows. 1. Whole-rock Ni content of a silicic magma is equal to or larger than that of a mafic magma in each large eruption cycle. 2. Silicic and mafic magmas in each LPEC have identical Sr isotope ratios in whole-rock, glass, and plagioclase phenocryst. 3. Magmas of the three LPECs have different Sr isotope ratios. 4. Water content of ma … More fic magma is larger than that of silicic magma at least in Aso-2 and Aso-3. 5. petrological features of magmas of Aso volcano systematically change with time : magmatic temperature decreases, K2O content to SiO2 content decreases, oxygen fugacity increases.The feature 3 indicates that the magmas of each LPEC were produced by distinct processes from those of magma generation of the other LPECs. The feature 2 strongly suggests that the silicic and mafic magmas of each LPEC are from a same source material. On the other hand, the feature 1 indicates the silicic magma was not generated by fractionation of the mafic magma. These geochemical data request that the silicic and mafic magmas of each LPEC were generated from a same source material whereas they have no parent-daughter relationship on fractionation. The magma genesis of these silicic and mafic magmas can be explained qualitatively as follows. The source material is a gabbro. The mafic magma was produced by fractional crystallization of segregated partially-molten magma from the gabbro with high degree of partial melting and then fractional crystallization. The silicic magma was generated as segregated partially-molten magma from the gabbro with low degree of partial melting that was not fractionated after segregation. Magma generation of partial melting of a gabbro probably occurs in a lower crust : hot magmas from mantle rise and heat up the lower crust. The mafic magma is produced near the hot mantle magma. The silicic magma is generated by low degree of partial melting of the lower crust relatively far from the mantle magma. The feature 5 is explained by increase of water content in lower crust (i.e., source material of magma). Less