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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 Properties of Mafic Rocks from the Middle Valley Sedimented Rift