Origin of Magnetic High at Basalt-Ultramafic Hosted Hydrothermal Vent Field in the Central Indian Ridge

Friday, 19 December 2014
Masakazu Fujii1, Kyoko Okino1, Taichi Sato2, Hiroshi Sato3 and Kentaro Nakamura4, (1)AORI, Univ. Tokyo, Kashiwa, Japan, (2)AIST, Ibaraki, Japan, (3)Senshu University, Kanagawa, Japan, (4)University of Tokyo, Bunkyo-ku, Japan
Hydrothermal alteration processes can change crustal magnetization by destruction and creation of magnetic minerals. In the Yokoniwa hydrothermal vent field (YHVF), located at the NTO-massif in the Central Indian Ridge, a high magnetization zone (with ~12 A/m in ~200 m-scale) was discovered by previous deepsea AUV survey. Basalts and ultramafic rocks were found around the YHVF, however the origin of magnetic high and its relationship with hydrothermal activity are remains to be investigated. Therefore, we conducted additional magnetic field measurement, rock sampling, and geological observation using submersible Shinkai 6500 during the YK09-13 and YK13-03 cruises.

Vector geomagnetic field were obtained along the dive tracks at an altitude of ~ 10 m. The crustal absolute magnetization is estimated using the 2D and 3D forward modeling technique. The values of magnetization show ~10 A/m just around the YHVF. This value is consistent with that of equivalent magnetization deduced from AUV data.

Rock magnetic characters were measured for 8 basalts, 4 dolerites, 5 sulfides, and 30 serpentinized peridotites (SPs). The measurements of NRM, magnetic susceptibility, magnetic hysteresis, low (6–300K) and high (50–700°C) temperature magnetization curves were performed. The estimated magnetization values are 0.1–6 A/m in basalts, 0.2–0.6 A/m in dolerites, and <0.1 A/m in sulfides. The SPs show strong magnetization of 0.4–11 A/m. The magnetic grain sizes ranges over single domain to pseudo-single domain. The temperature–magnetization curves clearly show the Verway transition and Currie temperature of 580 °C, therefore magnetic carrier of SPs is supposed as pure magnetite, which is created during serpentinization process.

Serpentinization degree (Sd) was also estimated by grain density measurement based on empirical formula from Oufi et al., 2002. Amount of magnetite was also estimated from saturation magnetization. The results show that the values of Sd vary in a range from 17 to 100 % and the highly serpentinized samples (>85%) have high magnetization (> 6 A/m).

Consequently, we proposed the presence of highly SP bodies at the YHVF as the origin of the high magnetizaion zone. It was formed by locally alteration of upper mantle section due to the hydrothermal activity.