[S-CG57] Hybrid Geochronology from Date to Age

[SCG57-P07] Development of LA-ICP-MS U−Th disequilibrium/fission-track double dating for zircon and its application to Holocene volcanics

*Hideki Iwano¹, Sota Niki², Hisashi Asanuma³, Shuhei Sakata⁴, Yuji Orihashi⁵, Takafumi Hirata¹ (1.Geochemical Research Center, The University of Tokyo, 2.Nagoya University, 3.Kyoto University, 4.Eathquake Research Institute, The University of Tokyo, 5.Hirosaki University)

Keywords:LA-ICP-MS, U-Th disequilibrium,, fission track, Holocene, zircon

Crystal records in volcanology can provide fundamental information on the timescale, temperature, pressure and chemical composition of a magma process. Since the discovery of the U-series disequilibrium, radiometric dating using the nuclides ²³⁸U, ²³⁰Th, ²²⁶Ra and ²¹⁰Pb has been conventionally used to decipher the subsurface history of various magmatic processes. Among these nuclides, the ²³⁸U–²³⁰Th dating method (hereafter referred to as the U–Th method) can be used to decode the crystal residence time prior to eruption. In addition to the U–Th method, the spontaneous fission of ²³⁸U occurs at a given rate, and the counting of the resulting fission tracks (FT) is a benchmark for dating the latest stage of magma activity due to the low closure temperature of its system. The difference between U–Th and FT ages should constrain the timescale of magma transport from the magma chamber to the surface. We have applied the U–Th and FT double-dating method with femtosecond laser ablation-ICP-mass spectrometry (fsLA-ICP-MS) to Late Pleistocene volcanic rock samples (Toya and Sanbe volcanoes, Japan) and demonstrated the potential of the method.
Our next challenge is the application of the double-dating method to Holocene volcanic rocks. The sample for a proof-of-concept study is the Mayuyama volcano in Unzen volcanic area, Kyushu, Japan. We selected this sample because (1) numerous TL and carbon-14 dates showed the appearance of the Mayuyama volcano about 46,000 years ago (Ozeki et al., 2005), (2) a previous zircon FT age (5.1 ± 1.5 ka) was reported (Danhara et al., 1993), and (3) the sample contains apatite, which is also utilized for the U-Th chronometer. Firstly, two U-Th isochrons were identified from the zircon data, with the older age group calculated at 125+212/-68 ka (95% confidence level) and the younger at 8.6+9.5/-8.7 ka. With the combination of the young zircon-group data and preliminary U-Th isotope data of apatite, a zircon-apatite mineral isochron yielded 12+/-3 ka, which is significantly younger than the older-group age. In this presentation, we will further demonstrate zircon LA-ICP-MS FT ages, temperature information, and compositional data from the two zircon groups to provide a detailed insight into magmatic processes at Mayuyama.

[1] Ozeki N., OKUNO M. and KOBAYASHI T. (2005) Bulletin of the Volcanological Society of Japan, 50, 441-454. [2] Danhara T, Iwano H., Hoshizumi H. and Watanabe K. (1993) Abstract of the Volcanological Society of Japan (Fall Meeting), p.51.