Helium isotope variations along the niigatakobe tectonic zone, Central Japan

A linear zone with high strain rates along the Sea of Japan coast, the Nigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneity in the lower crust and/or upper mantle, which may be caused by the upwelling of aqueous fluid and/or melt related to subduction of the Philippine Sea and Pacific Plates. In order to elucidate the geographic distribution of ³He/⁴He ratios along the NKTZ, new helium isotope data from hot spring gases and water samples were determined. In the southern NKTZ, ³He/⁴He ratios lower than the atmospheric value indicate that radiogenic helium dominates over any mantle helium input from aqueous fluids generated during the dehydration of the subducting Philippine Sea slab because a mantle wedge, the potential source of mantle helium, appears to be absent. Higher ³He/⁴He ratios are observed in the central NKTZ where active volcanoes are concentrated, suggesting the existence of magmatic fluids in the lower crust and upper mantle. The ³He/⁴He ratios of most hot springs in the northern NKTZ, a non-volcanic region, can be interpreted as a three-component mixture of mantle helium associated with magmatism of Middle Miocene age, radiogenic crustal helium and atmospheric helium. However, the ³He/⁴He ratios of gases close to active faults in the northern NKTZ are similar to those near active volcanoes in the central NKTZ, suggesting that active faults may facilitate transfer of mantle helium carried by aqueous fluids derived from the subducting Pacific Plate slab from the lower crust to the Earth's surface.