New helium isotope data from hot springs on eastern Kyushu Island were obtained in order to elucidate the geographic distribution of 3He/4He ratios in the fore‐arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios are clearly distinguished across the fore arc from Kyushu Island (0.093–0.79 RA, where RA denotes the atmospheric 3He/4He ratio of 1.4 × 10−6), to Shikoku Island (0.17–3.6 RA) and to the Kii Peninsula (0.24–7.4 RA). The distribution of mantle helium relative to total helium reveals a close association with the geophysical evidence of a serpentinized fore‐arc mantle wedge including the occurrence of deep, nonvolcanic, low‐frequency tremors (LFTs), believed to be caused by upwardly mobile aqueous fluids derived from slab dehydration. Flux of mantle helium carried by aqueous fluids via a non serpentinized fore‐arc mantle wedge is very low on Kyushu Island where LFTs have not been detected and 3He/4He ratios are lower than the atmospheric ratio. On the other hand, hot springs above the regions where LFTs occur in the Kii Peninsula and northern Shikoku Island have high 3He/4He ratios, indicating a significant contribution of mantle helium carried by aqueous fluids via a serpentinized fore‐arc mantle wedge. The helium isotopes in hot and natural spring gases may be a useful geochemical indicator of a serpentinized fore‐arc mantle wedge, especially in areas with sparse seismic station coverage.

[1] Regional and local variations in mantle helium provide insight into the coupling of mantle-crust tectonics, and heat and/or mass transfer from the Earth's interior. In order to further elucidate the geographic distribution of 3 He/ 4 He ratios in southwest Japan, the data from a total of 924 sites were compiled and synthesized. These include data from 48 additional hot spring and drinking water well sites on the northern Kyushu Island and in the northern Chugoku region. There appears to be good correlation between variations in helium isotope ratios and the geophysical evidence used to determine the configuration of the subducting Philippine Sea plate (PHS). Seismological studies reveal that the leading edge of the aseismic slab does not extend to the northern Chugoku region nor to the Osaka Bay area, where gas samples with significantly elevated 3 He/ 4 He ratios occur. This is consistent with a mantle-derived helium in these areas, from melts and/or mantle fluids ascribed to upwelling asthenosphere without being hindered by the descending PHS slab. In contrast, gas samples in the regions where the overriding crust comes into direct contact with the subducting PHS are dominated by radiogenic helium derived from the crust because of the absence of a mantle wedge, the most plausible source of mantle helium. Owing to the abrupt changes in the seismicity and focal mechanisms of intraplate earthquakes, the PHS is considered to have slab tears beneath the Kii Channel and/or the eastern Kii Peninsula oriented in a NW-SE direction. However, the lenear alignment of anomalously high 3 He/ 4 He ratios does not appear to be NW-SE trending along the assumed slab tears but rather forms an broad, ENE-WSW trending zone between the tears where low-frequency events occur. The emanation of gas with elevated 3 He/ 4 He ratios in the central peninsula can be explained by the upward mobilization of mantle volatiles derived from the mantle wedge above the PHS and/or transferred from the hydrated slab mantle through the N-S trending fractured medium within the PHS. Accordingly, the helium isotopes observed on the Earth's surface may be efficient geochemical indicators of the configuration of a relatively younger, warm aseismic slab, and be especially useful in seismically inactive areas.

[1] Recently, two shallow M W 6.6 inland earthquakes, the 2004 mid-Niigata prefecture earthquake and the 2007 Niigataken Chuetsu-oki earthquake, occurred in the Niigata sedimentary basin near the boundary between the Eurasian and the North American Plates. In order to elucidate the geographic distribution of the 3 He/ 4 He ratios in and around the seismic source regions, new helium isotope data from hot spring gases and water samples from much denser sampling than in previous studies were determined. Although temporal changes in the 3 He/ 4 He ratios obtained from each well were unknown before and after the earthquakes, higher 3 He/ 4 He ratios were observed in the earthquake source regions compared to other regions. Geochemical evidence for the presence of hot spring gases and natural gases with anomalously high 3 He/ 4 He ratios (>3.8 RA) indicates infiltration of mantle fluids derived from the subcrustal lithosphere. A plausible source of mantle helium can be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab, resulting in the acquisition of primordial helium from a mantle wedge as the fluids into the crust. In addition, aqueous fluids may raise the Earth's surface along permeable conduits leading to the reactivation of high-angle inverted normal faults and the triggering of recent earthquakes.

The East Bull Lake Pluton, a layered gabbro–anorthosite intrusion 90 km west of Sudbury, Ontario, is in an inward-dipping lopolith and is 780 m thick in the centre and elliptical in plan view (13.5 km × 3.5 km). It intruded Archean metavolcanic and metaplutonic rocks of the Superior Province during the Early Proterozoic (2480 Ma).The intrusion consistes of a basal anorthositic unit, succeeded by rhythmic-layered gabbro, troctolite, layered gabbro, and massive and dendritic gabbro units. It is offset by the west-northwest-striking, subvertical Folson Lake fault. South of this fault, only anorthosite and massive and dendritic gabbro are exposed. North of the fault, subsurface lithologies intersected by me boreholes correlate with surface lithologies. Troctolite, the most distinctive marker that can be correlated between boreholes and surface exposures, confirms the general attitude and shape of the layers and lopolith.Chemical composition of the intrusion ranges from high-Mg tholeiite to calc-alkaline high-alumina basalts.Fractures occur in several preferred orientations, the most common being northwesterly, subparallel to the Folson Lake fault, numerous mafic dykes, and topographic lineaments. Complex fracture-filling and alteration mineralogies formed under a wide range of P–T conditions representative of epidote-amphibolite – greenschist facies, pumpellyite–prehnite facies, zeolite facies, and low-temperature rock–water interaction processes.The last movement on the Folson Lake fault was a dextral strike slip of up to 3.0 km that postdates most mafic dykes.

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