Mantle helium emission related to the 2011 M9.0 Tohoku-Oki earthquake

The 11 March 2011 moment magnitude 9.0 Tohoku-Oki Earthquake occurred on the Japan Trench plate boundary off the eastern shore of northern Honshu. In order to study the coseismic and after shock impacts on the geochemical state of the main shock region, we have collected bottom seawater samples with several depths at four sites, N1 (38.18N, 143.55E), R (38.21N, 143.79E), N2 (38.15N, 143.17E) and N3 (38.11N, 143.08E) on 15 April 2011 by CTD-CMS hydrocasts during the MR11-03 cruise of R/V Mirai. In the laboratory helium and neon were extracted from seawater and purified using hot Ti-getters and charcoal traps held at 77 K. The 4He/20Ne ratios were measured by a QMS. Then helium was separated from neon by a charcoal trap held at 40 K. The 3He/4He ratios were measured by a high-precision mass spectrometer (Helix SFT, GV instrument) and calibrated against our in-house standard (HESJ; Matsuda et al., 2002). Observed helium isotopic ratios are ranging from 1.16 Ra to 1.20 Ra (where Ra is the atmospheric ratio, 1.39x10-6). The 3He/4He ratios increase with the depth at the site N1and N2, while the ratios are constant at the site R. Observed 4He/20Ne ratios are comparable to the value of air saturated water. Helium isotopic signatures of North Pacific bottom water was well documented with a 15% excess 3He relative to the air (1.15 Ra) due to the mantle He derived from the mid-ocean ridge. In contrast the 3He/4He ratios of pore waters in oceanic sediments of non-volcanic fore arc regions showed crustal values (~0.2Ra; Sano and Wakita, 1985). If the significant amounts of pore water were expelled by the M9.0 earthquake, the 3He/4He ratio of the bottom seawater may decrease. However this is not the case. The ratios increase slightly toward the bottom, suggesting the mantle He emission in the main shock region. Within the experimental error margin of 4He/20Ne ratios, it is difficult to identify the end member by a simple binary mixing. However the value is at least higher than 2 Ra. There may be a pass and a carrier of the enhanced isotopic He from the upper mantle. Although the interpretation is admittedly oversimplified, the M9.0 earthquake and its after shocks may have induced the fluid-flowing from the wedge mantle beneath the island arc to the trench region through the plate boundary.