Relationship between ³He/⁴He ratios and subduction of the Philippine Sea plate beneath southwest Japan

Abstract

[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.

Mantle-derived fluids in the Nankai Trough Kumano forearc basin

“…3 He/ 4 He ratios in agreement with these values were, e.g. observed in samples from hot springs and water wells in the Kinki district (Sano et al 2009) and in southwest Japan (Chuguko region, Umeda et al 2012). Supported by seismic tomography and magnetotelluric investigations, which identified a large low-velocity and conductive zone extending from the upper mantle to the middle crust, Umeda et al (2012) interpreted the high 3 He/ 4 He ratios in southwest Japan close to the volcanic front with upward migration of mantle fluids from an upwelling astenospheric body.…”

“…We explore possible contributions of mantle-derived and crustal helium in the depth interval 950-3050 mbsf and discuss our findings in the context of fluid migration through the accretionary prism. Our data also provide a seaward extension of previously compiled He isotope ratio dataset for southeast Japan (Umeda et al 2012).…”

“…observed in samples from hot springs and water wells in the Kinki district (Sano et al 2009) and in southwest Japan (Chuguko region, Umeda et al 2012). Supported by seismic tomography and magnetotelluric investigations, which identified a large low-velocity and conductive zone extending from the upper mantle to the middle crust, Umeda et al (2012) interpreted the high 3 He/ 4 He ratios in southwest Japan close to the volcanic front with upward migration of mantle fluids from an upwelling astenospheric body. For the Kinki district, where high helium isotope ratios were found up to 200 km away from the volcanic front, Sano et al (2009) suggest fluid or magma rise from an extinct and subducted mid-ocean ridge.…”

“…in this depth interval is lower than for sample #950 mbsf (3.3 Ra). If we assume that the helium isotope composition of #950 mbsf is close to the actual values of the subducting PSP in southwest Japan (Umeda et al 2012) and mantle helium in the interval 3050-1700 mbsf derives from the same source, then admixing of crustal helium must have occurred at depths between~9 kmbsf (depth of the subducting PSP, Strasser et al 2014c), and 3050 mbsf (C0002P borehole depth) to yield a mean helium isotope ratio of2 .4 Ra. Admixing with crustal helium is obvious for samples #1400 and #1800 with 3 He/ 4 He ratios of 0.44 ± 0.24 and 1.33 ± 0.34, resp.…”

“…6 km, Moore et al 2015), ρ S = 2.6 cm 3 /g is the density of the sediments, ρ f = 1.025 cm 3 /g is the fluid density, [ 4 He] f,ma is the concentration of helium in the subducting Philippine Plate = 1.5 × 10 − 7 cm 3 /g [STP] (Sano and Nakajima 2008). (R/Ra) meas , crust, mantle is, relative to the atmospheric ratio, the measured 3 He/ 4 He ratio (2.4 Ra), the average helium production ratio in the crust (~0.02 Ra, Ballentine and Burnard 2002) and the helium isotope ratio of subducting Philippine Sea Plate (~3.4 Ra, Umeda et al 2012). P (He) is the annual radiogenic production and depends on the concentrations of the parent element nuclides (U, Th).…”