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Downhole magnetometer logs obtained during Ocean Drilling Program Leg 144 from Holes 873A (Wodejebato Guyot), 878A (MIT Guyot), and 879A (Takuyo-Daisan Guyot) in the western Pacific Ocean were used to constrain the nature and origin of the igneous sections of the seamounts. Magnetic boundaries, apparent inclination, polarity of magnetization (normal or reverse), and the hemispheric origin (Northern or Southern) was determined by inspecting the variations and the amplitudes of the horizontal and the vertical magnetic field. Magnetization of the basalts between 175 and 200 mbsf in Hole 873A (Wodejebato Guyot) was acquired during a reversed polarity chron in the Southern Hemisphere; the apparent inclination is about 21.6°. Magnetization below about 200 mbsf is most likely dominated by induced or viscous magnetization. The apparent inclination above 200 mbsf is in good agreement with that derived from paleomagnetic studies of the core sample. Basalts from Hole 878A in MIT Guyot below 860 mbsf show that magnetization originated during a reversed polarity chron in the Southern Hemisphere. The apparent inclination below 860 mbsf is 21.8°. Magnetization of basalts above 860 mbsf possibly was acquired during a normal chron in the Southern Hemisphere. Magnetization of the basalts and volcaniclastic breccias in Hole 879A (Takuyo-Daisan Guyot) indicate an origin in the Southern Hemisphere during a normal polarity chron. The apparent inclination (-20.6°) is almost the same as that of the core-sample measurements. Many magnetic boundaries were obtained from the logs of Holes 873A (Wodejebato Guyot) and 878A (MIT Guyot). These boundaries were not caused by changes in magnetic polarity but by intensity variations within the same polarity. In this study, we suggest that induced magnetization in the upper basalt section is not significant in Wodejebato (Site 873), MIT (Site 878), and Takuyo-Daisan (Site 879) guyots. INTRODUCTION If one is to understand the origin of seamounts and absolute plate motion, one must know the magnetic properties of seamounts. In spite of numerous studies on magnetization of seamounts from magnetic anomalies (e.g., Sager and Pringle, 1988; Sager et al., 1993), the magnetic structure of seamounts is still unclear. Previous paleomagnetic studies on dredged and drilled samples from seamounts and islands show that the magnetic properties of seamount rocks are highly variable (Furuta et al., 1980; Kono, 1980; Gee et al., 1988, 1989). Gee et al. (1989) suggest that induced magnetization significantly contributes to seamount magnetization. Sager and Han (1993) found a low Koenigsberger ratio (Q ratio; i.e., the ratio of remanent magnetization to induced magnetization) in the southern Shatsky Rise. On the other hand, Sager and Pringle (1988) suggest that the magnitude of seamount magnetization derived from the seamount magnetic anomaly is almost the same as that obtained from dredged samples and that the induced magnetization of seamounts is negligible. Pillow basalts dredged from seamounts show high Q ratios (Furuta et al., 1980; Gee et al., 1988). Downhole measurements of the magnetic field in boreholes are a useful counterpart to paleomagnetic studies if one is to obtain an insight into the nature of the magnetic structure of the oceanic crust and seamounts and of magnetic polarity stratigraphy. Because core samples may acquire a secondary magnetization effect during the drilling process (Kikawa and Pariso, 1991; Nogi et al., in press a), the measured natural remanent magnetization (NRM) for recovered core samples may not accurately represent the in situ magnetization (Pariso and Johnson, 1993). Downhole logging can Haggerty, J.A., Premoli Silva, I., Rack, R, and McNutt, M.K. (Eds.), 1995. Proc. ODP, Sci. Results, 144: College Station, TX (Ocean Drilling Program). 2 Geological Survey of Japan, Tsukuba, Ibaraki 305, Japan. 3 Meteorological Research Institute, Tsukuba, Ibaraki 305, Japan. (Present address: National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo 173, Japan.) collect a continuous record of downhole magnetic field, which can be related to the magnetization of the surrounding material in order to accommodate poor recovery of paleomagnetic samples from the holes and to give independent information on the magnetization. Therefore, it is necessary to combine the results from the core-sample measurements and downhole logging to understand the magnetic structure and paleomagnetic stratigraphy fully. Many attempts have been made to develop a three-component downhole magnetometer and methods to analyze the three-component data (Bosum and Scott, 1988; Gallet and Courtillot, 1989; Kinoshita et al., 1989; Bosum and Kopietz, 1990; Hamano and Kinoshita, 1990; Pariso and Johnson, 1993; Nogi et al., in press b and c; Ito et al., this volume). It has been demonstrated that three-component downhole magnetometer logging results can reveal the magnetic structure of the oceanic crust and the origin of the magnetic anomaly lineations (Kinoshita et al., 1989; Hamano and Kinoshita, 1990; Pariso and Johnson, 1993; Ito et al., this volume), the magnetic structure of seamounts (Nogi et al., in press b), and the magnetization of sediments (Nogi et al., in press c). Three-component downhole magnetometer logs were conducted for the first time in the holes of seamounts during Ocean Drilling Program (ODP) Leg 143 at Sites 865 and 866 (Nogi et al., in press b), which penetrated the upper igneous sections of the seamounts. Nogi et al. (in press b) suggest significant contribution of induced magnetization to seamount magnetization from the comparison of the threecomponent downhole data with paleomagnetic results for discrete core samples. ODP Leg 144 drilled five guyots in the northwestern Pacific Ocean, and downhole measurements in the basement section were taken in Holes 873A (Wodejebato Guyot), 878A (MIT Guyot), and 879A (Takuyo-Daisan Guyot). Because the Japanese three-component downhole magnetometer that was used during Leg 143 (Nogi et al., in press a and b) encountered serious trouble during the operation at Hole 873A, we present the results of an analysis of the data obtained with the Schlumberger general purpose inclinometer tool (GPIT), which was attached to the Formation MicroScanner (FMS).

Magnetic Structures of Seamounts in the Western Pacific Ocean Deduced from Leg 144 Downhole Magnetometer Logs