Thermochronology of non-volcanic hydrothermal activity in the Kii Peninsula, Southwest Japan: Evidence from fission track dating and helium isotopes in paleo-hydrothermal fluids

Abstract

Although there is no indication of volcanism during the Pliocene and Quaternary in the Kii Peninsula, Southwest Japan, the peninsula has long been recognized to be unusual and atypical of non-volcanic regions, with anomalous high-temperature hot springs and high 3He content in hot spring gases. Recently, it has been suggested that aqueous fluids generated by dehydration of the subducting Philippine Sea plate carry mantle helium from the subcrustal lithosphere to terrestrial water, resulting in the emanation of high 3He gases at high-temperature hot springs [e.g., Matsumoto T., Kawabata T., Matsuda J., Yamamoto K. and Mimura K. 2003. 3He/ 4He ratios in well gases in the Kinki district, SW Japan surface appearance of slab-derived fluids in a non-volcanic area in Kii Peninsula. Earth Planet. Sci. Lett. 216, 221–230]. Fission track dating of zircon and apatite samples from the altered host rocks around three hot springs (Yunomine Hot Spring, Kamiyu Hot Spring and Tosenji Hot spring) was carried out in order to elucidate the thermochronology of the hydrothermal system in this non-volcanic region. Additionally, new helium isotope data from paleo-fluids trapped in fluid inclusions in hydrothermal minerals were obtained for comparison with the present-day hydrothermal fluids from the above hot springs. The apatite FT ages from the altered host rocks around the hot springs ranged from 2.5 Ma to about 5 Ma, which are significantly younger than the 12 Ma age of the non-altered host rocks far from the hot springs. In the southern Kii Peninsula, therefore, it is obvious that the non-volcanic hydrothermal activity already has been ongoing since Pliocene or early Pleistocene time. The 3He/ 4He ratios of quartz veins and pyrite disseminated in the altered host rock around the hot springs ranged from 1.1 to 3.6 R A , generally in agreement with those of the present-day hydrothermal fluids. Almost constant values of 3He/ 4He ratios over time indicate that no important changes have occurred in deep source gases, at least during the last several million years.