GEOMAGNETIC PALEOINTENSITIES BY THE THELLIERS METHOD FROM SUBMARINE PILLOW BASALTS - EFFECTS OF SEAFLOOR WEATHERING

Measurements of geomagnetic paleointensity using the Thelliers' double-heating method in vacuum have been made on 10 specimens of submarine pillow basalt obtained from seven fragments dredged from localities 700 000 yrs old or younger. In the magnetic minerals, the titanium/iron ratio parameter x and the cation deficiency (oxidation) parameter z were determined by X-ray diffraction and Curie temperature measurement. Fresh material (z=0) provided excellent results: most of the natural remanent magnetization (NRM) could be thermally demagnetized before the magnetic minerals became altered, and the NRM-TRM lines were straight and well constrained, and geologically reasonable paleointensities were obtained. Somewhat oxidized material (z=0.2) also provided apparently valid paleointensities: values were similar to those from fresh specimens cut from the same fragments, although only half or less of the NRM could be thermally demagnetized before alteration of the magnetic minerals. More highly oxidized material (z=0.6) gave a result seriously in error: the paleointensity value is much too low, because of continuous disproportionation of titanomaghemite during the heating experiments and because seafloor weathering had decreased the NRM intensity. From limited published data, the extent of oxidation of titanomagnetite to cation deficient titanomaghemite in pillow basalt exposed on the seafloor appears to be approximately z=0.3 at 0.2-0.5 My, z=0.6 at 1 My, and z=0.8-1.0 at 10-100 My. This implies that valid paleointensities can be obtained from exposed submarine basalt but only if the basalt is younger than a few hundred thousand years. Equally good paleointensities were obtained from strongly magnetized (H-type) basalt and moderately magnetized (L-type) basalt.-from Authors