Although the general concept of linear magnetic anomalies generated by seafloor spreading processes is well established, the details of the source distribution responsible for these anomalies remain uncertain. We summarize here magnetic properties from variably altered and deformed olivine gabbro, gabbro, and less abundant troctolite, gabbronorite, and oxide gabbros sampled at four sites drilled on the western median valley wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone. The overall mean natural remanent magnetization (NRM) intensity (1.54 ± 2.6 A/m) and Koenigsberger ratio (8.05 ± 15.2) for these samples suggest that lower crustal gabbros are a significant contributor to marine magnetic anomalies. However, dual magnetic polarities were recorded at all four sites, with apparent polarity reversals sometimes occurring over spatial scales of tens of centimeters. Detailed demagnetization and rock magnetic studies of one such interval suggest that the complex remanence, dual polarities, and the occurrence of spurious well-defined magnetization components are related to production of magnetite during high-temperature alteration and/or cooling in periods of opposite polarity. These complexities, if generally applicable to oceanic gabbros, may reduce the integrated contribution from the gabbroic layer to marine magnetic anomalies.

Date of initial receipt: 2 August 1995
Date of acceptance: 22 March 1996

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