Helium isotopes as geochemical indicators of a serpentinized fore‐arc mantle wedge

Abstract

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.

Crustal tomography under the Median Tectonic Line in Southwest Japan using P and PmP data

“…3 He/ 4 He ratio is high along the MTL in Shikoku Island and particularly higher in Kii Peninsula (Dogan et al, 2006;Umeda et al, 2007). Our obtained velocity images in the MTL region are in good agreement with the results of these previous geophysical and geochemical studies conducted for SW Japan.…”

“…Salah and Zhao (2003a) found low Vp, low Vs and high Poisson's ratio anomalies beneath the Quaternary volcanoes and western Kii Peninsula and eastern Shikoku. High 3 He/ 4 He ratios are related with the presence of mantle helium (possible from the serpentinized forearc mantle wedge) transported by aqueous fluids to the Earth's surface through the fractures within the crust (Dogan et al, 2006;Umeda et al, 2007). Therefore, with support from these studies the low-V anomalies can be indicative of possible presence of fluids.…”

Low-frequency earthquakes and tomography in western Japan: Insight into fluid and magmatic activity

“…LF events also appear in a few non-volcanic areas, such as Kii Peninsular, Osaka Bay and Kobe area (Figs. 6c and d and 7c and d), where high He 3 /He 4 is recognized (e.g., Umeda et al, 2007). A zone of low-velocity and high Poisson's ratio was detected in the lower crust right beneath the source area of the 1995 Kobe earthquake (M 7.2) under Kobe and Osaka Bay, which was interpreted to be a fluid-filled, fractured rock matrix that triggered the rupture of the Kobe earthquake (Zhao et al, 1996).…”

The Leaking Mantle

“…It is thought that one of the most important ways to introduce foreign DNA into an organism is by stealth: repressing the expression of the introduced gene until Lateral gene transfer may have spurred microbial evolution, producing a pattern of evolution that resembles a network, rather than a tree. stresses in the upper crust and triggering earthquakes remains unclear, yet these two very different tectonic regimes share a common feature: 3 He/ 4 He ratios much higher than expected for regions virtually devoid of mantle melting (8,9). Looking more closely at the helium isotope characteristics of such regions-spatially and temporally-may provide evidence coupling the leaking mantle to one of our greatest natural hazards.…”