HR: 0800h
AN: V41B-1389    [Abstracts]
TI: Hydrothermal circulation system in the central Mariana illustrated by Magnetometoric Resistivity experiments
AU: * Tada, N
EM: noriko@kobe-u.ac.jp
AF: Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501 Japan
AU: Seama, N
EM: seama@kobe-u.ac.jp
AF: Research Center for Inland Seas, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501 Japan
AU: Goto, T
EM: tgoto@jamstec.go.jp
AF: JAMSTEC, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kido, M
EM: kido@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AB: Hydrothermal vent fields are known to exist on the spreading axis, where sea water penetrates into the crust and upwells through the hydrothermal vents. Understanding of the hydrothermal circulation system is extremely important to reveal the cooling process of the oceanic crust. The thermal structure beneath the hydrothermal vent reflects the extent of underground activity and the convection scale of the hot water. Temperature in the crust can be estimated from the electrical conductivity because the conductivity depends on the water volume, the salinity concentration and the temperature of the sea water in the crust. The Alice Spring Field (18 $^{\circ}$12.9'N, 144 $^{\circ}$42.5'E and 3600m deep), on the spreading axis in the central Mariana Back-Arc Basin, is a suitable site for this purpose. Hydrothermal vent in this field was firstly discovered by Alvin in 1987 (Hawkins et al., 1990). Shinkai6500 also confirmed the hydrothermal activity in 1992 and 1996 (Gamou et al., 1994; Fujikura et al., 1997). In November, 2002, we conducted Magnetometric Resistivity (MMR) survey using R/V Kairei, JAMSTEC in this field. In the MMR method, controlled electric current was applied from a pair of electrodes; one is just beneath the sea surface and the other is close to the seafloor. To record electoromagnetic responses of the crust to the inputed current, we deployed six ocean bottom electromagnetometers (OBEMs), which can measure 3-components of magnetic and electric fields simultaneously. Measurements were conducted at 34 sites around the field, each of which consists of 30 minutes stacking for repeated current signals to keep better S/N ratio. Apparent resistivity is given by a function of amplitudes of magnetic field variation and source-receiver distance. We recovered the data from four OBEMs (two were on the spreading axis and other two were off axis). The plot of magnetic amplitudes to source-receiver distances shows different trend between OBEMs on-axis and off-axis. Therefore, we applied the Occam's inversions (Constable et al., 1987) to estimate 1D conductivity structure separately in these two areas. We obtained similar resistivity at upper part (0--100m) in both the two area, however, the resistivity at 100--300m in the on-axis is lower than that in the off-axis by roughly two orders of magnitude. This indicates that the temperature of crust under 300m depth on-axis is higher than that of off-axis. We will present further discussion on this meeting.
DE: 8194 Instruments and techniques
DE: 8135 Hydrothermal systems (8424)
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 1515 Geomagnetic induction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting