230Th-238U radioactive disequilibrium constraint on genesis of volcanoes along Kyushu arc

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K03695
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: The University of Tokyo
• Principal Investigator: Nakai Shun'ichi 東京大学, 地震研究所, 教授 (50188869)
• Co-Investigator(Kenkyū-buntansha): 三好 雅也 福岡大学, 理学部, 教授 (50557353)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 九州 / ウラン―トリウム放射非平衡 / 阿蘇火山 / 地殻同化作用 / ザクロ石 / 霧島火山 / 開聞火山

Outline of Final Research Achievements

How the temperature of descending slab influences the geochemical characteristics of volcanic rocks in a subduction zone is an important problem. We tackled this problem by using the along- and across arc variations of 230Th-238U radioactive disequilibrium of volcanic rocks from the Kyushu island. We found most of the analyzed samples have (230Th/238U) varying from one to more than one. (238U/232Th) were less than one. It is likely that 1. the interactions of magmas with the lithospheric mantle or 2. the addition of sediment melt during preceding subductions increased U and Th concentrations and lowered (238U/232Th) of the mantle in this area, that made it difficult to recognize the addition of fluid-derived U. Regarding the Aso volcano, model calculation showed that the (230Th/238U) disequilibrium greater than one was originated by an assimilation of local crustal rocks and fractional crystallization process.

Free Research Field

地球化学

Academic Significance and Societal Importance of the Research Achievements

本研究では，阿蘇火山のウラン―トリウム放射非平衡が，地殻の同化作用によって起こり，(230Th/238U)の放射能比が1以上になっていることが明らかになった．地殻同化作用の影響を明確にした成果は，カスケードなどでも少なく，学術的意義がある．また，これまで九州の火山の火成岩については地球化学的データが十分には報告されていなかった．本研究の成果は，地球化学コミュニティに貢献すると期待できる．