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The magnetization of titanomagnetites in pillow basalts is important as the major source for the magnetic anomalies observed over the ocean basins. The magnetization is generally reduced as the titanomagnetites are altered through reaction with sea-water. This alteration and its effects have been studied in three ways: by the examination of young pillow basalts from the Mid-Atlantic Ridge, by laboratory alteration of these pillows and by examination of highly altered lavas from Bermuda.

During alteration at sea-bottom temperatures the titanomagnetites remain single phase and while intensity of NRM is reduced the direction of NRM is preserved. At higher temperatures, 150- 200^oC, in the laboratory, alteration is much more rapid so that a decrease in NRM which might take a million years at sea- bottom temperatures would need but a few thousand years. After prolonged heating, phase splitting occurs sometimes accompanied by self-reversals.

The accelerated alteration due to even slight heating, the possible self-reversals which may accompany phase splitting, and the scattering of NRM directions as seen in the Bermuda core combine to produce an effective magnetization at depth in layer 2 less than those inferred from dredge haul samples. Because of this lower magnetization it is likely that all of layer 2 will contribute to the magnetic anomalies.

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Supervisor: J. Hall

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