Physics
Scientific paper
Aug 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976jgr....81.4166d&link_type=abstract
Journal of Geophysical Research, Volume 81, Issue B23, p. 4166-4172
Physics
5
Scientific paper
Paleomagnetic field intensities have been determined by a modified Thellier heating method for five basalts from a collection of 23 rocks drilled at sites 332 and 335 of Deep-Sea Drilling Project leg 37 in the Americas plate just west of the mid-Atlantic ridge at 37°N. Samples from hole 332B were from depths of 220-555 m in oceanic layer 2. Natural remanent magnetizations (NRM's) of these rocks remained fairly stable during alternating field demagnetization to 1000 Oe, and 6 of 7 tested were directionally stable during thermal damagnetization to 250°C, NRM intensities were low in relation to submarine basalts samples elsewhere (0.5 to 4.5×10-3 emu cm-3 and a mean of 2.26×10-3 emu cm-3). Although there is a pronounced negative (linear) anomaly over the site, the NRM's are of mixed polarities. Inclinations are uniformly shallow, -1-34 and are much less than the dipole inclination of 50-55° at 37°N. Because of the low intensities, mixed polarities, and shallow inclinations of NRM, the integrated magnetic signal above the sequence is insufficient to explain the linear magnetic anomaly over site 332. The main anomaly source must be deeper than 600 in layer 2 or, conceivably, in layer 3. Hole 335 samples (about 110-m basement penetration) had NRM's of similar intensity but with uniform negative polarity in accord with the sign of the sea surface anomaly and with the high inclinations (48-57°) expected for 37°N. Rock magnetic studies revealed two types of magnetic beahvior. Type 1 rocks have a single low (150-200°C) Curie point and reversible thermal behavior, but they tend to be magnetically soft and viscous. Type 2 rocks have two Curie points. They are magnetically hard and nonviscous, but they change irreversibly when they are heated (in air or vacuum) above 300°C. Both types are less than ideal material for paleointensity determination, since heatings are limited to the 20-200° range. As a result, only four or five points could be obtained for each Thellier plot, and because less than 20% of the NRM was demagnetized at 200°C in four of the five samples successfully studied, the scatter is greater than it would be in similar studies of continental rocks. The paleofield intensities indicated are 0.1-0.45 Oe at 3.5+/-0.1 m.y. B.P. (site 332) and ?0.3 Oe at 10.5-16.5 m.y. B.P. (site 335). Although these paleointensities are apparently reliable (within broad confidence limits), some may be low in comparison with the present field because of viscous decay of the NRM, which may be undetectable when heatings are limited to 200°C. In support of this ideas there is a correlation in our results between unstable behavior during a 2-month storage test of NRM and very low values in rocks magnetized during a normal polarity epoch (e.g., rocks from the median valley of the mid-Atlantic ridge) and for low apparent paleointensities in rocks away from the ridge magnetized during reversed epochs or events.
Dunlop David J.
Hale Christopher J.
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