HR: 1340h
AN: V23B-0625    [Abstracts]
TI: Magnetization Of Gabbroic Rocks and Peridotites Recovered From Mid Atlantic Ridge 14N - 16N, ODP Leg 209
AU: * Kikawa, E
EM: kikawa@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Garces, M
EM: mgarces@ub.edu
AF: Univ. Barcelona, Facultat de Geologia, Campus de Pedralbes , Barcelona, 08028 Spain
AU: Gee, J
EM: jsgee@ucsd.edu
AF: Scripps Institution of oceanography, Lajolla, California, CA 92093-0 United States
AB: ODP Leg 209 was performed from May to July, 2004 at Mid Atlantic Ridge 14N - 16N. The precruise site survey studies performed by Alvine and Shinkai 6500 submersibles indicated the existence of fresh mantle peridotites for the drilling sites. However, the recovered rocks were basalts, gabbroic rocks and peridotites few of which were unaltered. Natural remanent magnetization (NRM) intensities calculated from ODP Leg 209 gabbroic rocks and peridotites indicated very large variations from 0.001 A/m to 50 A/m. Many of the rocks recovered were observed to have drilling-induced remanent magnetizations (DIRM). The DIRM was in general easily removed by alternating field (AF) demagnetization up to 20mT, so for most of the rocks recovered, it was possible to determine stable characteristic remanent magnetization vector directions. The DIRM which was unstable against AF demagnetization showed nearly vertical direction (downward). In many cases, the DIRMs possess more than 90 percent of NRMs of the rocks recovered. The existence of the DIRM in the rocks recovered made it difficult to obtain original natunal remanent magnetization intensities not altered by drilling. So care must be taken in considering in-situ magnetization of ODP Leg 209 gabbroic rocks and peridotites before recovered from the sea-floor, specially as remanent magnetization source for overlying sea-floor spreading magnetic anomalies, though some of the ODP Leg 209 gabbroic rocks and peridotites that were not affected by DIRM indicated reasonably high magnetization of 1 - 2 A/m to contribute to lineated marine magnetic anomalies . Similar DIRMs have been found to exist among gabbroic rocks and peridotites recovered during Legs 118, 147, and 176. Non-magnetic drilling tools including drill pipes, core barrels, BHA and so on would be essential to further studies for magnetizations for deep ocean crustal rcoks and mantle peridotites that contain relatively coarse grain magnetites easily affected by DIRM of essentially isothermal remanent magnetization (IRM) origin.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1517 Magnetic anomaly modeling
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting