Slab samples were cut in two for preparing polished thin sections and for rock magnetic measurements. We prepared 114 cubic specimens, 0.7-1 cm long, for paleomagnetic measurements of NRM and magnetic susceptibility. These 1- to 3-g specimens were constrained in 1-in Teflon cylinders and fixed with nonmagnetic screws during measurement. NRM intensities were measured with a Natsuhara Giken SMD-88 spinner magnetometer. Values of low-field magnetic susceptibility (k; volume specific) were measured with a Bartington Instruments MS2 system. Volume and density of each specimen were determined in order to calculate volume-specific magnetic parameters. In order to examine the stability of the remanent magnetization, we carried out stepwise thermal demagnetization and alternating-field (AF) demagnetization of the NRM on 16 and 24 selected specimens, respectively. Thermal demagnetization was conducted with a Magnetic Measurements MMTD80 thermal demagnetizer. A total of 25 heating steps from room temperature to 580°C were employed (i.e., 25°, 80°, 120°, 160°, and increments of 20° from 160° to 580°C). After each heating step, a 2G Enterprises superconducting rock magnetometer measured remanent magnetization. For AF demagnetization, specimens were treated with a triaxial AF demagnetizer in-line with the cryogenic magnetometer. AF demagnetization was performed at 26-28 steps to 150 mT, with 5- to 10-mT increments between steps.
Crushed samples of 1-4 mg were used for measurements of magnetic hysteresis parameters including saturation magnetization (Ms), remanent saturation magnetization (Mrs), coercivity (Bc), and remanent coercivity (Bcr). We measured hysteresis parameters using a Princeton Measurements MicroMag 2900 alternating gradient force magnetometer with a maximum field of 1 T. Ms and Bc were determined after correcting the contribution of the paramagnetic component by applying high-field slope correction between 0.7 and 1 T.
Another ~100-mg portion of crushed samples was prepared for high (25°-600°C) and low (5-300 K) temperature measurements. We used a semihorizontal thermomagnetic balance to obtain thermomagnetic curves and to determine the Curie temperatures of magnetic minerals using a graphical method described by Grommé et al. (1969). Most of heating and cooling (8°C/min) runs were performed in air with a constant direct-current (DC) field of 0.75 T, but some were performed under moderate vacuum (~1 Pa) for comparison. Low-temperature magnetic properties were measured on a Quantum Design magnetic property measurement system (MPMS2). Samples were imparted at 5 K with 1-T DC field to acquire a low-temperature isothermal remanent magnetization (low-T IRM) and then thermally demagnetized in zero-field to room temperature.
To observe magnetic minerals and their microtextures, we used reflected-light optical polarizing microscopy and scanning electron microscopy to study polished thin sections of the cored specimens. A JEOL JSM-840A scanning electron microscope (SEM) equipped with a backscattering-electron (BSE) detector and a Link energy dispersive spectrometry (EDS) detector was used for the petrographic study and mineral identification.