21. PALEOMAGNETISM OF THE SERPENTINIZED PERIDOTITE FROM ODP HOLE 670A1

Paleomagnetic studies on the serpentinized peridotites recovered from ODP Hole 670A were conducted in three laboratories. High NRM intensities and magnetic susceptibilities were observed in the serpentinized peridotites, which suggest that the remanent and the induced magnetizations of the peridotites cannot be neglected as a source of the magnetic anomalies observed at sea surface. The in situ low inclination of the magnetization indicated from the laboratory studies suggests that the peridotite body has been subjected to a large-scale deformation after the acquisition of the magnetization. During Leg 109 of the Ocean Drilling Program, serpen­ tinized peridotites were drilled down to 92.5 m sub-bottom depth at Hole 670A, situated in the median valley of the Mid-Atlantic Ridge south of the Kane Fracture zone. The rocks around the hole were divided into four lithological units as determined from petrological study. The top unit consists of serpentinized harzburgite with the degree of serpentiniza- tion decreasing with depth. The second unit contains 959^100% serpentinized peridotite. The third unit consist of cpx-bearing harzburgite; the degree of serpentinizat ion in this unit is similar to that in the top unit. The fourth unit is almost completely serpentinized and similar to the second unit. As reviewed by Bonatti and Hamlyn (1981), oceanic ultra- mafic rocks have been recovered in relatively young oceanic crust away from large fracture zones, in crustal sections exposed along the large fracture zones, and in sections from trenches at the margins of ocean basins. During the Deep Sea Drilling Project, the Glomar Challenger sampled ultramafic rocks at Sites 334 and 895 (Aumento et al., 1977; Melson et al., 1978). The paleomagnetic and rock magnetic studies of these ultramafic rocks (Johnson, 1979; Dunlop and Prevot, 1982) indicated that the magnetization of the oceanic ultramafic rocks are likely contributors to oceanic magnetic anomalies. Because the entire 92.5 m of the drilled section in Hole 670A consists of the serpentinized peridotites, paleomagnetic study on these rocks is important for the discussion of the source layer of the magnetic lineations. For the shipboard study, 10 minicore samples were taken from the recovered peridotite and paleomagnetic studies were made. The results of the shipboard study is given in Detrick, Honnorez, Brian, Juteau, et al. (1988). The results indicated the high NRM intensity and the high magnetic susceptibility of the serpentinized peridotites. The NRM intensity is compara­ ble to the basalts drilled and dredged in the median valley of the Mid-Atlantic Ridge and is much higher than the average intensity of the basalt samples drilled in most of the DSDP and ODP holes (see, for example, Lowrie, 1977; Harrison, 1981). The susceptibility of the highly serpentinized peridotite is an order of magnitude larger than any oceanic basalts. These magnetic properties of the serpentinized peridotites are impor­ tant to discuss in terms of the effect on the magnetic lineations observed at sea surface. The low inclination values suggested from the shipboard measurements are also important because they have implications for the rotation or the deformation of the serpentinized peridotite body related to the process of seafloor spreading. It is also to be explained why the peridotite body is observed at the western side of the median valley more than 25 km away from the Kane fracture zone. Unfortunately, the samples employed in the shipboard study were not sufficient to resolve all of the above mentioned aspects. In the present paper, some shore-based studies on the peridotite samples were added to confirm the results of the shipboard observations. SAMPLES AND EXPERIMENTS In Hole 670A, the recovered total length of the core was about 6 m from the 92.5 m drilled section. As evident from the recovery rate, most of the recovered peridotites are in small pieces, and pieces longer than the aperture of the core barrel are very few. Hence, the number of oriented samples is very limited. In the paleomagnetic study, minicore samples were taken from all the oriented rock pieces, and some more samples were taken from unoriented small pieces. From some large pieces, two or more minicore-size samples were taken in order to check the reliability of the remanence properties. Since most of the large oriented pieces are highly serpenti­ nized peridotites, the inclusion of the unoriented samples is important to obtain an unbiased estimate of the magnetic properties of the serpentinized peridotite. The results from the small unoriented pieces are not included for the discussion of the direction of the magnetization. The procedure for the shore-based paleomagnetic study made in three different laboratories was very similar to the shipboard study. All the samples were progressively demag­ netized by the alternating magnetic fields, and the magnetiza­ tions were measured by Spinner magnetometers. From the measurements, the NRM intensity, the MDF (Median De­ structive Field), and the stable inclination were obtained. The magnetic initial susceptibility was also measured for all the samples.