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

• LA-quadrupole-ICP-MS U-Pb is a viable method to date zircons as young as ~ 0.1 Ma.
• The emplacement time of the Kakkonda Granite was ~ 0.1 Ma, which probably indicates it is Earth’s youngest dated pluton.
• At least two Quaternary episodes of plutonism (~ 1.1 Ma and ~ 0.1 Ma) were confirmed in the Hachimantai geothermal area.

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 °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 ± 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.