One of the major experimental requirements in paleomagnetic research is to isolate the characteristic remanent magnetization (ChRM) by selective removal of secondary magnetization. To investigate the nature of the remanent magnetization of the basement rocks at Kerguelen Plateau, discrete samples were alternating-field (AF) or thermally demagnetized during shipboard study. A total of 169 discrete paleomagnetic samples were used for shore-based rock magnetic studies. The rock magnetic data presented in this paper are from measurements performed at the paleomagnetism laboratories at University of California at Santa Cruz (UCSC), University of Munich in Germany, and the Institute for Rock Magnetism at the University of Minnesota. For rock magnetic characterization, samples were subjected to a wide range of magnetic measurements. These included (1) Curie temperature determinations using both low and high applied fields (0.05 mT and 1 T, respectively); (2) hysteresis loop parameter measurements: saturation magnetization (Js), saturation remanence (Jr), coercivity (Hc), and remanent coercivity (Hcr) as a function of temperature (10-400 K); (3) saturation isothermal remanent magnetization (SIRM) as a function of temperature (10-300 K); (4) alternating-current (AC) susceptibility measurements as a function of field amplitude and field frequency; and (5) Mössbauer spectroscopy. A brief description of each experiment is given below.
The Curie temperature is the temperature below which a magnetic mineral is magnetically ordered. For an example, when a magnetite sample is heated to a temperature of ~580°C (the Curie point for magnetite), it loses all its remanent magnetization. This loss in spontaneous magnetization is due to thermal agitations overcoming the exchange energy. Because the Curie temperature is a sensitive indicator of composition, it is useful in understanding the magnetic mineralogy. In this study, Curie temperatures were determined by measurement of low-field magnetic susceptibility or induced moment vs. temperature (using both the Kappabridge susceptometer at UCSC and the Princeton MicroMag vibrating sample magnetometer at the University of Minnesota). To avoid oxidation that could lead to chemical alteration, we conducted thermomagnetic analyses in an inert helium or argon atmosphere on 105 samples representative of Leg 183 cores. We used a graphic method (Grommé et al., 1969) that uses the intersection of two tangents to the thermomagnetic curve that bounds the Curie temperature to determine the Curie temperature. This method is most straightforward to perform by hand, although it tends to underestimate Curie temperatures with the two other methods presented by Moskowitz (1981) and Tauxe (1998).
To further understand the magnetic properties and estimate the domain structure of magnetic minerals, room-temperature hysteresis loops were measured on representative samples. Hysteresis loop parameters are useful in characterizing the intrinsic magnetic behavior of rocks. Thus, they are helpful in studying the origin of remanence. In this study, hysteresis loops and the associated parameters Jr, Js, Hc, and Hcr were obtained using Princeton Measurements Corporation alternating gradient magnetometers (AGFM) capable of resolving magnetic moments as small as 5 x 10-8 emu (Flanders, 1988). Saturation magnetization (Js) is the largest magnetization a sample can have. The coercivity (Hc) is a measure of magnetic stability. The two ratios, Jr/Js and Hcr/Hc, are commonly used as indicators of domain states and, indirectly, grain size. For magnetite, high values of Jr/Js (>0.5) indicate small (<0.1 µm or so) single-domain (SD) grains, and low values (<0.1) are characteristic of large (>15-20 µm) multidomain grains (MD). The intermediate regions are usually referred to as pseudo single-domain (PSD). Hcr/Hc is a much less reliable parameter, but conventionally SD grains have a value close to 1.1, and MD grains should have values >3-4 (Day et al., 1977; Dunlop, 2002). Hysteresis parameters for 106 representative samples were determined at the University of California, Santa Cruz, the University of Munich in Germany, and the University of Minnesota. For a selected group of samples, we also examined the change of hysteresis loops during warming from 10 to 400 K (at intervals of 10 K).
Low-temperature measurements were made on 31 representative samples to help characterize the magnetic minerals and understand their rock magnetic properties. These measurements were designed to determine the Néel temperature and other critical temperatures of a magnetic substance. The measurements were made from 10 K to room temperature (300 K) on 100- to 300-mg subsamples in a quantum design magnetic property measurement system (MPMS) at the University of Minnesota. Samples were given an SIRM in a steady magnetic field of 2.5 T at 300 K and cooled in a zero field to 20 K; the remanence was measured at 5-K intervals. The sample was then given an SIRM in a field of 2.5 T before being warmed to 300 K in zero field while measuring the remanence value every 5 K in sweep measurement fashion. Unlike high-temperature measurements, there is no risk of oxidizing the sample because it is at low temperature and is not heated.
To investigate the field- and frequency-dependent susceptibility of the Leg 183 cores, AC susceptibility measurements were made on a few selected samples with a LakeShore Model 7130 AC susceptometer and the MPMS at the University of Minnesota. The temperature dependence of the in-phase (m´) and quadrature (m'') mass susceptibility between 15 and 300 K was measured at five frequencies (40, 140, 400, 1000, and 4000 Hz), with AC field amplitudes between 100 and 1000 A/m.
Two samples were selected for Mössbauer effect investigation to provide another means of identifying magnetic carriers in basalts from the Kerguelen Plateau and Broken Ridge. Measurements were taken at room temperature using 57Co in a rhodium source and a constant acceleration spectrometer. A crushed sample (~0.5 g in powder form) was packed into a polythene sample holder with a cross-sectional area of ~1 cm2.