10:00 AM - 10:30 AM
[SGL23-04] Variations of 3He/4He ratios in pore water and fluid circulations at the outer slope of the Japan Trench
★Invited Papers
Keywords:Fluid circulation, Helium isotopes, Japan Trench, Plate subduction processes
Geochemical characteristics of pore fluids may reflect geochemical cycles associated with physicochemical processes at convergent plate boundaries such as fault-bending and petit-spot volcanism, which is though to be related to evolutionary histories of the Earth’s deep and surface reservoirs through plate subduction processes. For investigation of fluid origins and cycles, 3He is one of the most useful tracers because of its chemical inertness and high sensitivity for mantle-derived components. Park et al. (2021) observed 3He/4He ratios in sediment pore water at the outer slope of the Japan Trench, and suggested the presence of hydrothermal circulation across the Moho based on spatial variations of 3He/4He anomaly reflecting mixing of seawater and mantle-derived components. However scales and temporal variations of such hydrothermal circulation are not well constrained because datasets of pore water have been sparse.
In order to investigate fluid origins and circulations, and their temporal changes at the outer slope of the Japan Trench, we collected sediment and seawater samples at three sites (MC1, MC2 and MC3) in the off Sanriku region during the KH-20-8 cruise onboard R/V Hakuho Maru conducted in August 2020. It should be noted that MC1 and MC3 are the same sites as PC 7 and PC9 in Park et al. (2021), respectively, which is important for investigating temporal variations. Using the multi-corer system, sediment and bottom seawater samples were collected at three sites which were close to faults. The obtained samples were immediately transferred into copper tubes and both ends were sealed by metal clamps on the ship in order to avoid air contamination. Subsequent sample preparation and analyses were conducted in Atmosphere and Ocean Research Institute, University of Tokyo. Pore fluids were extracted from the sediment by centrifugation. Then gases dissolved in the pore fluid and seawater samples were extracted and introduced into the vacuum line connected to a QMS and a noble gas mass spectrometer (Helix SFT). In the line, the sample gases were purified and 4He/20Ne ratios were measured with the QMS, and 3He/4He ratios were measured with Helix SFT. The obtained ratios were calibrated against those of atmosphere and air-saturated water. At three sites in the off Sanriku region, vertical profiles of 3He/4He ratios in pore fluid and seawater samples indicated existence of deep fluid end-members with 3He/4He ratios apparently higher than atmosphere. In addition, the datasets at MC1 and MC3 (PC7 and PC9 in Park et al. (2021)) indicated a possibility that fluid circulations continuously provide mantle-derived volatiles to the ocean at the outer slope of the Japan Trench. We are planning to conduct additional observations at the outer slope of the Japan Trench in order to constrain spatial and temporal variations of fluid circulations precisely.
(References)
Park et al. (2021) Scientific Reports 11, 12026.
In order to investigate fluid origins and circulations, and their temporal changes at the outer slope of the Japan Trench, we collected sediment and seawater samples at three sites (MC1, MC2 and MC3) in the off Sanriku region during the KH-20-8 cruise onboard R/V Hakuho Maru conducted in August 2020. It should be noted that MC1 and MC3 are the same sites as PC 7 and PC9 in Park et al. (2021), respectively, which is important for investigating temporal variations. Using the multi-corer system, sediment and bottom seawater samples were collected at three sites which were close to faults. The obtained samples were immediately transferred into copper tubes and both ends were sealed by metal clamps on the ship in order to avoid air contamination. Subsequent sample preparation and analyses were conducted in Atmosphere and Ocean Research Institute, University of Tokyo. Pore fluids were extracted from the sediment by centrifugation. Then gases dissolved in the pore fluid and seawater samples were extracted and introduced into the vacuum line connected to a QMS and a noble gas mass spectrometer (Helix SFT). In the line, the sample gases were purified and 4He/20Ne ratios were measured with the QMS, and 3He/4He ratios were measured with Helix SFT. The obtained ratios were calibrated against those of atmosphere and air-saturated water. At three sites in the off Sanriku region, vertical profiles of 3He/4He ratios in pore fluid and seawater samples indicated existence of deep fluid end-members with 3He/4He ratios apparently higher than atmosphere. In addition, the datasets at MC1 and MC3 (PC7 and PC9 in Park et al. (2021)) indicated a possibility that fluid circulations continuously provide mantle-derived volatiles to the ocean at the outer slope of the Japan Trench. We are planning to conduct additional observations at the outer slope of the Japan Trench in order to constrain spatial and temporal variations of fluid circulations precisely.
(References)
Park et al. (2021) Scientific Reports 11, 12026.