Magnetic properties of dikes from the oceanic upper crustal section

Recovery of dredge samples of upper crustal dikes from the steep north wall of the Blanco Fracture Zone (West Blanco Deep) allows an evaluation of this rock type as a possible contributor to the magnetic anomaly source layer. Magnetic data from these dike/sill samples indicate that the majority of the rocks meet the essential criteria as a contributor to the source layer, including a high magnetic intensity (2.7 A/m), a ratio of remanent to induced magnetization that is greater than 1, and a relative stability against large-scale viscous remanent magnetization acquisition over long geological time periods. However, because the natural remanent magnetization intensities of the dike samples are an order of magnitude lower than the extrusive rocks within the axial zone, the presence of abundant near-surface dikes would lower the net magnetization of a crustal section and may be responsible for the axial magnetic low that has been observed on medium to fast spreading centers. If the magnetization of the dikes does not change with time due to low-temperature oxidation, then the decreasing ratio of extrusive/intrusive contribution as the pillow basalts undergo systematic oxidation will cause the magnetic layer to appear to thicken and the polarity transition zones to widen as the crust ages and moves off-axis.

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