Hydrothermally formed sulfide samples recovered from the Juan de Fuca Ridge during Ocean Drilling Program Leg 139 were investigated to determine the carrier of remanent magnetization and to study rock magnetic properties. The samples were examined by isothermal remanent magnetization acquisition, alternating field demagnetization, back field measurements, thermomagnetic and hysteresis measurements, X-ray diffraction, and ore microscopy. Grain sizes for magnetite and pyrrhotite particles were estimated from hysteresis parameters on samples containing only one magnetic phase. Magnetomineralogical analyses made on whole-rock samples show that massive sulfides recovered from Holes 856G and 856H are composed predominantly of pyrite, pyrrhotite, magnetite, and minor amounts of sphalerite and chalcopyrite. Magnetite together with two types of pyrrhotite determines the magnetic properties of these rock samples. The first type of pyrrhotite is ferrimagnetic up to 320°C. X-ray diffraction data are in good agreement with those for monoclinic pyrrhotite. The second type of pyrrhotite is antiferromagnetic at room temperature and ferrimagnetic between 200°C and 270°C. X-ray diffraction data show typical peaks for hexagonal pyrrhotite. Thermomagnetic analyses indicate that samples from Hole 856G contain mainly magnetite, whereas parameters from Hole 856H were also influenced by pyrrhotite. Pyrrhotite and magnetite were distinguished by the large J rs /J s and the small B cr /B c ratios for pyrrhotite compared with those for magnetite, and through direct methods such as ore microscopy and X-ray diffraction.

Rock magnetic properties of mafic rocks from Sites 855 and 857 of Ocean Drilling Program Leg 139, in the Middle Valley of the northern Juan de Fuca Ridge, were evaluated as a source of marine magnetic anomalies. Magnetic mineralogy and magnetic properties of the basalt flows from Site 855, located on the eastern boundary of the valley, are similar to those of typical young oceanic basalts. In contrast, the diabase sills at Site 857, situated well within the valley, are quite different from typical young oceanic basalts both in magnetic mineralogy and many magnetic properties. By thermomagnetic analyses and other magnetic methods, nearly pure magnetite was found to be a dominant magnetic carrier in the sills at Site 857. Saturation magnetization and susceptibility values, which are about two orders of magnitude lower than those of typical young oceanic basalts, suggest that the content of magnetic minerals in the Site 857 samples is much lower than in young oceanic basalts. As a result, natural remanent magnetization intensities are much lower than in young oceanic basalts. This low magnetization at Site 857 is consistent with the absence of expected marine magnetic anomalies over Middle Valley. We attribute the presence of magnetite and extremely low magnetization intensity to pervasive hydrothermal alteration of the rocks beneath a thick sediment blanket. Hydrothermal fluids leached iron from the original Fe-Ti oxides and greatly diminished the content of magnetic minerals. Our investigation suggests that hydrothermal alteration can cause subdued marine magnetic anomalies over sedimented rifts, as observed in several areas besides Middle Valley.

Rock magnetic studies on sediment samples from Ocean Drilling Program Sites 855 and 857, Middle Valley, Juan de Fuca Ridge, were carried out during Leg 139. Results of natural remanent magnetization intensity and susceptibility show that the concentration of magnetic minerals is high. The thermomagnetic analyses and the thermal demagnetization of composite isothermal remanent magnetizations (Lowrie, 1990) were also conducted.

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Rock magnetic studies on sediment samples from Ocean Drilling Program Sites 855 and 857, Middle Valley, Juan de Fuca Ridge, were carried out during Leg 139. Results of natural remanent magnetization intensity and susceptibility show that the concentration of magnetic minerals is high. The thermomagnetic analyses and the thermal demagnetization of composite isothermal remanent magnetizations (Lowrie, 1990) were also conducted.