Release of mantle helium from forearc region of the Southwest Japan arc

Shikoku, Japan, is an island located on the forearc region of the western sector of the Southwest (SW) Japan arc that was formed by subduction of the Philippine Sea plate beneath the Eurasian plate. Noble gas isotope analyses were carried out for 30 bubbling gas samples and spring water samples collected in Shikoku. In addition, chemical and carbon isotope analyses were conducted for six gas samples. Observed 3He/4He ratios were 0.17 R/RA to 3.56 R/RA, showing that several samples contain mantle helium components, although some samples contain helium of crustal origin. Geographical distributions of 3He/4He ratios show that high 3He/4He ratios originate mainly from two areas: one along the Median Tectonic Line (MTL), a major active fault in Japan; the other coinciding with a region in which non-volcanic long-period tremors occur, about 30 km deep [Obara, K., 2002. Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296, 1679-1681]. This fact indicates that fluids liberated from the slab in the forearc region cause deep tremors and fracturing within the crust, thereby easing the transfer of fluids to the surface that are mixed with mantle helium to the surface. An active fault system can also provide an efficient path for mantle helium transfer.