Quaternary plutonic magma activities in the southern Hachimantai geothermal area (Japan) inferred from zircon LA-ICP-MS U–Th–Pb dating method Hisatoshi Ito a, ⁎, Akihiro Tamura b , Tomoaki Morishita b , Shoji Arai b , Fumiaki Arai c , Osamu Kato d a Geosphere Science Sector, Central Research Institute of Electric Power Industry, Chiba 270-1194, Japan b Department of Earth Sciences, Kanazawa University, Ishikawa 920-1192, Japan c Exploration Department, JMC Geothermal Engineering Co., Ltd., Iwate 020-0151, Japan d Tohoku Hydropower & Geothermal Energy Co., Inc., Iwate 020-0021, Japan abstract article info Article history: Received 10 May 2013 Accepted 25 August 2013 Available online 3 September 2013 Keywords: Quaternary plutonic magma U–Th–Pb dating LA-ICP-MS Zircon 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 238 U– 206 Pb 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; there- fore zircons extracted from the dike should also be of plutonic origin. 238 U– 206 Pb age of 1.08 ± 0.08 Ma (after correction for initial 230 Th 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Elucidating the timing of magmatic activity is a prerequisite to un- derstanding the development of geothermal systems (e.g., Schmitt et al., 2003). Finding very young plutons may stimulate geothermal exploration, which is urgently needed in Japan since the Fukushima- Daiichi nuclear disaster in 2011. Zircon U–Pb ( 238 U– 206 Pb) dating method can play an important role for this purpose because its datable range covers as young as ~ 0.1 Ma (Bacon et al., 2000; Sano et al., 2002) and its closure temperature is as high as or higher than consolidation temperature of a magma (Cooper and Reid, 2008). In this study, U–Pb dating was performed using zircons of plutonic origin from the Hachimantai geothermal area, northeast Japan (Fig. 1), which is an im- portant geothermal area for electricity generation (Muraoka et al., 1998; Tamanyu, 2000). Recently, laser ablation-inductively coupled plasma-mass spectrom- etry (LA-ICP-MS) has been used to obtain zircon U–Pb ages (e.g., Kosler and Sylvester, 2003), partly because this method requires less rigorous sample treatments and is much more cost-effective compared with conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS) and high-resolution ion microprobe (SIMS or SHRIMP). This method like ion microprobe has another advantage in that in-situ dating is possible whereas ID-TIMS requires sample dissolution. With regard to dating young zircons by U–Pb methods, SHRIMP methods can yield reliable ages as young as ~0.1 Ma (Bacon et al., 2000; Sano et al., 2002). Bacon et al. (2000) reported a SHRIMP U– Pb zircon age of ~0.1 Ma from granodiorite blocks ejected during a Holocene caldera-forming eruption of Mount Mazama (USA). They confirmed the reliability of U–Pb age by comparing with a U–Th dis- equilibrium zircon age. Sano et al. (2002) obtained a SHRIMP U–Pb zircon age of ~0.1 Ma from pyroclastic rocks ejected during the 1991 Mount Unzen (Japan) eruption. They attributed the discrepan- cy between U–Pb age and the eruptive age to a magma residence time in a shallow reservoir. Reid and Coath (2000) and Simon and Reid (2005) reported ion microprobe zircon U–Pb age of the Bishop Tuff (USA) as ~0.8 Ma and Crowley et al. (2007) applied ID-TIMS method to the Bishop Tuff and indicated that ID-TIMS is also applicable to young zircons with a much higher precision than the ion microprobe method. Cocherie et al. (2009) showed that dating zircon as young as ~0.6 Ma is possible using multi-collector (MC) LA-ICP-MS. Journal of Volcanology and Geothermal Research 265 (2013) 1–8 ⁎ Corresponding author. Tel.: +81 70 6568 9621; fax: +81 4 7183 3182. E-mail address: ito_hisa@criepi.denken.or.jp (H. Ito). 0377-0273/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2013.08.014 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores