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doi:10.2204/iodp.proc.327.102.2011 PaleomagnetismThe objectives of the paleomagnetic studies program during Expedition 327 were to measure paleomagnetic directions for tectonic and polarity studies and to examine magnetic properties on a regional scale by comparing the results with those of Expedition 301. Measurements were made on the archive-half sections of cores. Paleomagnetic instrumentsA 2G Enterprises pass-through cryogenic direct-current superconducting quantum interference device (SQUID) rock magnetometer (model 760R) was used to make paleomagnetic measurements. This magnetometer is equipped with an in-line alternating-field (AF) demagnetizer (2G model 2G600) that allows for demagnetization of samples up to 80 mT. The magnetometer and AF demagnetizer are interfaced with a workstation that is used to collect the data. The magnetization of the core liner sets the resolution of the instrument to ~3 × 10–5 A/m, but all samples had magnetization intensities above this limit. The magnetic susceptibility of core sections was measured with two devices. Whole-round sections were measured on the WRMSL (see “Physical properties”). Section halves were measured on the SHMSL (see “Physical properties”). Paleomagnetic measurementsStandard IODP orientation conventions were applied to the archive halves of the core (+x: vertically upward; +y: horizontally to the right when looking downcore; and +z: downcore) (Fig. F10). The natural remanent magnetization (NRM) of samples was measured initially, followed by the magnetization after progressive AF demagnetization. Measurements were made on oriented pieces and along core sections that were not excessively fractured. Measurements were made using superconducting rock magnetometer (SRM) Discrete and SRM Section software. Hard rocks were demagnetized at 5 mT steps from 0 to 30 mT, with two additional steps at 40 and 50 mT. Hard rock pieces measured in discrete mode were positioned so that a single measurement was made at their center points. Hard rock pieces measured in section mode were sampled at 1 cm intervals. Because of orientation differences in how data were collected with the two applications, the y and z magnetic moments stored in the database of hard rock discrete pieces must be reversed before data generated by the two different measurement modes can be compared. Sediments were demagnetized at 10, 20, 30, and 40 mT steps and measured at 5 cm intervals. Characteristic remanent magnetization directions (i.e., the magnetization believed to be that acquired initially upon cooling of the igneous flows) were determined by examining orthogonal vector plots of demagnetization steps and looking for a consistent direction of decay toward the origin. The mean directions of these vectors were calculated using principal component analysis (Kirschvink, 1980). Geomagnetic polarity timescaleMagnetic polarity results were correlated to the polarity reversal sequence and absolute age using the Berggren et al. (1995) geochronology (Fig. F11). This timescale incorporates the widely used calibration of age and polarity intervals derived by Cande and Kent (1995). |
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