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Long-term evolution of a subduction zone volcano : Geology, petrology, and geochemistry of Rishiri Volcano, southern Kuril Arc
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| Title: | Long-term evolution of a subduction zone volcano : Geology, petrology, and geochemistry of Rishiri Volcano, southern Kuril Arc |
| Other Titles: | 沈み込み帯火山の長期進化過程 : 千島弧南部, 利尻火山の地質学・岩石学・地球化学 |
| Authors: | 谷内, 元1 Browse this author |
| Authors(alt): | Taniuchi, Hajime1 |
| Issue Date: | 25-Mar-2021 |
| Publisher: | Hokkaido University |
| Abstract: | Subduction zone is one of the most active areas of magmatism on the Earth, and many
geophysical and petrological studies have been carried out. However, the debates on the longterm
evolutionary processes and their controlling factors of subduction zone volcanoes have not
been put to rest. In this study, I carried out geological, geochronological, petrological, and geochemical
studies at the Rishiri Volcano, southern Kuril Arc, to understand the long-term evolution
of a subduction zone volcano. In Chapter 2, I obtained new K-Ar, Ar/Ar, and 14C ages to create
the time–cumulative volume step diagram and show that the eruption rate was high in the Early-
1 and Middle stages, intermediate in the Early-2 and Late-1 stages, and low in the Late-2 stage.
Besides, it was indicated that there was a clear relationship between the eruption rate and the
chemical composition of volcanic products. In Chapter 3, I showed that the calc-alkaline andesites
that erupted during the Middle stage were essentially formed through the magma mixing
between the mantle-derived magma and crust-derived magma. The water content of the primitive
end-member magma of the calc-alkaline andesite was obtained to be ~5 wt.%, which is predominantly
higher than other primitive magmas that produced basalts and tholeiitic andesites to
dacites. It was suggested that the water content of primary magma control the differentiation
process in the crust. In Chapter 4, I carried out a geochemical study of the adakitic dacites in the
Middle stage to clarify the nature and behavior of slab-derived fluids. The results showed that
the adakitic dacites were not produced by partial melting of crustal rocks and fractional differentiation
from mantle-derived magma, but rather by the involvement of slab-derived materials. It
was indicated that the slab-derived supercritical liquid separated into aqueous fluid and hydrous
melt and the former erupted as dacitic magma. In Chapter 5, it was clarified that the elemental
partitioning induced by the separation of slab-derived supercritical liquid into aqueous fluid and
hydrous melt would play an important role in causing the Na/K variation of primary basaltic
magma. In Chapter 6, I summarized the results in Chapters 2 to 5 and discuss the controlling
factors in the long-term evolution of the Rishiri Volcano. This study finally proposed that whether
or not the slab-derived supercritical liquid separated into aqueous fluid and hydrous melt would
determine the chemical composition of primary magma, differentiation processes in the crust,
and the eruption rate at the surface and play an important role in the long-term evolution of
subduction zone volcanoes. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第14364号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 准教授 栗谷 豪, 教授 中川 光弘, 教授 永井 隆哉, 助教 吉村 俊平 |
| Degree Affiliation: | 理学院（自然史科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/82852 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
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