32. Paleomagnetism of Basalts and Interlayered Sediments Drilled during Dsdp Leg 49 (n-s Transect of the Northern Mid-atlantic Ridge)

Content Provider	Semantic Scholar
Author	Faller, Alishia M.
Copyright Year	2006
Abstract	We report paleomagnetic results for Leg 49 obtained onboard Glomar Challenger and subsequently in the shore laboratories. The intensity of remanent magnetization of basaltic ocean crust fluctuates with depth at all sites; within-site variation of this property exceeds between-site variation. No significant differences in the remanence intensity of the upper ocean crust occur between magnetic anomalies 13 and 2' or between groups of sites at latitudes 37°N, 45°N, and 63°N. An arithmetic mean intensity of 5.0 × 10~ emu/cm for all Leg 49 basalt specimens is close to previous average values for samples from the North Atlantic Ocean floor. Median destructive fields, weak field susceptibilities, and Konigsberger ratios are comparable with previous results from ocean floor basalts. The Konigsberger ratios show that remanence dominates the magnetization of the upper ocean crust. In seven out of eight holes, the sign of the stable inclination is appropriate to the magnetic anomaly. Values of stable inclination fluctuate with depth. At two sites, abrupt changes in stable inclination together with other paleomagnetic properties make it easy to define distinct magnetic units. At the other sites, it is more difficult to locate boundaries using magnetic evidence alone. Fewer magnetic units than petrographic units are identified in all cases. INTRODUCTION Paleomagnetic measurements on oceanic basement drilling in situ are extremely important in establishing the nature, thickness, and geometry of the magnetized bodies which generate marine magnetic anomalies. Leg 49 drilling took place over a wide geographic area, and the sites were carefully positioned to sample the centers of the sources of well-defined magnetic anomalies. The paleomagnetic results therefore constitute a significant extension to the pre-existing data. The sites sampled form three groups: Holes 407, 408 and 409 on the west flank of the Reykjanes Ridge, Holes 410 and 410A at latitude 45°N, and Holes 411, 412A, and 413 in the FAMOUS area. The immediate objectives of the shipboard studies were to identify variations of the magnetic properties with depth and use them to help distinguish lithologic boundaries. The broader objectives were to compare the magnetic data from different sites and to compare all these data with the results of previous DSDP drilling into basement, with measurements on dredged samples, and with theoretical predictions. These paleomagnetic studies are supported by rock magnetic studies which examine the sources of the remanence and its petrogenetic significance (see Day et al., this volume; Kobayashi et al., this volume). METHODS OF INVESTIGATION Paleomagnetic measurements were normally made on cylindrical cores, 2.54 cm in diameter and between 2.40 and 2.54 cm high, obtained by drilling into the cut surface of the working half of the core. An arrow drawn on the top flat face of each cyUnder indicated the up-hole direction. Samples were taken at a frequency of at least one per 1.5 meter section of core, whenever possible; they were taken only from pieces too long to have overturned in the core barrel. At Holes 411, 412A, and 413, where the recovered basement material was sparse, a few thinner paleomagnetic samples, 1.0 cm in diameter and 2.4 cm high, were used in addition to those of standard size. At Hole 410, nine additional samples of 1.0 cm diameter, 1.0 cm high were taken from small basalt chunks in basalt-limestone breccia. These thinner cores were inserted into Styrofoam plugs for measurement. Shipboard measurements were made using a Digico balanced fluxgate spinner magnetometer, an alternating field (AF) demagnetizer with a two-axis tumbler, and a Schonstedt thermal demagnetizer in which the specimens were heated and then cooled in air. Three specimens known to be magnetically stable were measured on the Digico, which gave results very close to those obtained previously using a Schonstedt spinner magnetometer. Without a fluxmeter, any residual field in the cooling chamber of the thermal demagnetizer could not be measured, but tests on a granitic specimen suggested that such a field did not become important until the remanence intensity had decreased to 5 × 10~ emu/cm. Subsequent measurements were made onshore using an identical Digico spinner, an AF demagnetizer with a two-axis tumbler, and a large thermal demagnetizer, where the field in the specimen chamber is
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