MEASUREMENTS

Magnetic susceptibility () anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM), and the direction of characteristic remanent magnetization (ChRM) were measured for 277 basalt samples from Sites 1183, 1185, 1186, and 1187. A total of 166 samples were taken from pillowed sections; the remaining 111 were obtained from the more massive units. Pillowed units are characterized by the presence of glassy margins, groundmass grain-size variations, and vesicle-rich bands. More massive units are those that do not display any of these characteristics over core lengths >2 m. The lack of complete recovery, however, means that some massive units may be part of a pillowed sequence. In some cases, therefore, massive units may simply represent somewhat thicker pillows, whereas in other cases they may be part of a flow. The pillow and massive unit samples are not evenly distributed between the four sites because Sites 1183 and 1187 are dominated by pillows, whereas Sites 1185 and 1186 are characterized by more massive units. Although samples were obtained on average every 1 to 2 m, the stratigraphic sampling interval at each site is very irregular. Some intervals were intensively sampled (<20 cm), and others were sparsely sampled (>5 m). There are also several large gaps involving >10-m intervals where sampling was not possible due to poor core recovery or the fragmented nature of the core. One complete pillow from Site 1187 was sampled at 10-to 15-cm intervals from the top through the interior to the base of the pillow. Of the >160 samples obtained from pillows, only four samples were collected from regions known to be near their glassy rims. More than 90% of the pillow samples were obtained from points >10 cm from the nearest identifiable margin.

The magnetic measurements were made at three different laboratories: University of California at Santa Cruz, University of Houston, and University of Munich. The susceptibility measurements were made using either a Kappabridge KLY-3 (University of California at Santa Cruz and University of Munich) or an SI-2 by Sapphire Instruments (University of Houston). In all cases the basalts are strongly magnetic and therefore produced reliable estimates of both bulk susceptibility and AMS. Repeat measurements on the same instrument carried out on a number of Site 1185 samples demonstrated remarkable consistency, confirming the reliability of the measurements. Although no individual sample was measured at more than one laboratory, comparison of the results from each laboratory with those obtained on board the JOIDES Resolution demonstrated consistent results, indicating that the individual measurements are directly comparable. Details regarding the measurement of NRM intensities and ChRM directions are provided in Riisager et al. (2003).

Anhysteretic remanent magnetization (ARM) measurements were carried out on 20 samples (10 each from Sites 1183 and 1187). Samples were placed in a steady 0.2-mT field and then demagnetized using a maximum alternating field (AF) of 70 mT. Results of the AF demagnetization of NRM indicate most samples have median destructive fields <25 mT and that <10% of the remanence is carried by minerals with coercivities >70 mT.