Magnetic changes in pillow basalt due to sea floor weathering

Theoretical considerations appear to exclude viscous demagnetization as the cause of the gradual decay of the linear magnetic anomalies away from oceanic ridges. Oxidation of titanomagnetite to titanomaghemite during the weathering of submarine basalt seems to be the main cause of the lateral decrease in magnetization. In a detailed study of a partially weathered fragment of pillow basalt from the Juan de Fuca ridge, the remanent and saturation magnetizations of the weathered rim were found to be 40% less than those of the unweathered interior. The Curie temperature increases from 170°C in the interior to 235°C in the weathered rim, and the magnetic coercive force increases from 145 to 270 oe. The cubic cell edge shrinks from 8.460 to 8.434 A during oxidation, and the titanomagnetite grains become less brown. The direction of remanence is the only magnetic parameter unaffected by weathering. The magnetic properties of the basalt fragment vary with weathering in the same way that the average magnetic properties of dredged basalts vary with distance from the mid-Atlantic ridge. This study confirms the conclusion that these lateral magnetic variations are indeed due to the oxidation of titanomagnetite to titanomaghemite.

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