Quaternary plutonic magma activities in the southern Hachimantai geothermal area (Japan) inferred from zircon LA-ICP-MS U-Th-Pb dating method

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• 作者：Hisatoshi Ito ; Akihiro Tamura ; Tomoaki Morishita ; Shoji Arai ; Fumiaki Arai ; Osamu Kato
• 关键词：Quaternary plutonic magma ; U&ndash ; Th&ndash ; Pb dating ; LA-ICP-MS ; Zircon ; Hachimantai geothermal area
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：1 September, 2013
• 年：2013
• 卷：265
• 期：Complete
• 页码：1-8
• 全文大小：1759 K

文摘

In order to constrain the timing of plutonic magma emplacements relevant to current geothermal activity in the Hachimantai geothermal area, northeast Japan, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Th-Pb analyses were performed on zircons from two different origins. One is from the Quaternary Kakkonda Granite in the Kakkonda geothermal field, whose present temperature at ~ 3700 m depth is 500 掳C, and the other is from a clastic dike in the Matsukawa geothermal field. Zircons from the Kakkonda Granite were obtained at a depth of 1998 m and their ²³⁸U-²⁰⁶Pb and U-Th ages showed ~ 0.1 Ma, which probably indicates that the Kakkonda Granite is the world's youngest emplaced granitic magma body ever confirmed. The clastic dike in the Matsukawa geothermal field contains dioritic to tonalitic xenoliths; therefore zircons extracted from the dike should also be of plutonic origin. ²³⁸U-²⁰⁶Pb age of 1.08 卤 0.08 Ma (after correction for initial ²³⁰Th disequilibrium; error shown as 95% confidence level) was obtained from the zircons. Therefore it was found that there were at least two Quaternary plutonic magma activities (~ 1.1 Ma and ~ 0.1 Ma) in the Hachimantai geothermal area. Through this study, it was also found that U-Th-Pb dating using a quadrupole type ICP-MS coupled with excimer laser is applicable to Quaternary zircons as young as 0.1 Ma and Fish Canyon Tuff zircon is usable as a standard material for U-Th-Pb dating method.