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

Abstract

A total of 27 well gas samples collected from SW Japan were subjected to precise analysis of He, Ne and Ar isotopic compositions. The sampling area covers localities where mantle-derived helium emanates in the fore-arc of the Kii Peninsula, and is known as the ‘Kinki spot’ [e.g., Sano and Wakita, J. Geophys. Res. 90 (1985) 8728–8741]. The Kinki spot apparently is located within the fore-arc with no obvious magmatic activity, which contrasts to the occurrence of similarly high 3He/ 4He ratios along the volcanic front of NE Japan. Our high density sampling revealed more clearly that the high 3He/ 4He ratio wells are distributed in the Kii Peninsula above the area where the young and hot Philippine Sea Plate is subducting with a relatively steeper dip than in the adjacent area; this situation favors the near-trench side of the subducting slab entering eclogite facies conditions involving slab dehydration, without inducing melting of the mantle wedge. The aqueous fluids derived from the slab acquire mantle He during their passage through the mantle wedge. Thus, the observed association of mantle He and the fore-arc dehydration of the Philippine Sea Plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle He in well gases of the Kii Peninsula. The high 3He/ 4He region of Kii Peninsula also coincides with the occurrence of long-period tremors. These are caused by the movement of fluids derived from the subducting slab. Localized stresses cause significant uplift in the area, interpreted as indicating that intra-crustal fractures provide pathways through the crust for the fluids derived from depth. Thus, magma is not necessarily required as the carrier of mantle-derived helium to the surface of the fore-arc region of Kii Peninsula.