Geographical distribution of 3He/4He ratios in north Kyushu, Japan: Geophysical implications for the occurrence of mantle-derived fluids at deep crustal levels

We measured 3He/4He ratios in hot spring gases collected from sites in north Kyushu, Japan that represent an island arc setting associated with the subduction of the Philippine Sea plate beneath the Eurasian plate. Our results revealed relatively high 3He/4He ratios (about 4 to 8 Ra) in the Beppu-Shimabara Graben (a continuation of the Okinawa trough) and relatively low ratios (about 0.1 to 2 Ra) in areas located to the north and south of the graben. Slightly elevated ratios (about 3 to 4 Ra) were observed in areas further north of the graben. North Kyushu hosts unusual He isotopic signatures considering that its backarc location would predict higher 3He/4He ratios than the low ratios observed here. Instead of reflecting a regional tectonic signature, the geographic distribution of 3He/4He ratios appears to vary with seismic velocity perturbations as detected at 20–30 km depth. The correspondence between these two parameters suggests that 3He/4He ratios in this case relate to the spatial distribution of fluids at the base of the crust.