Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?
Authors: Dekkers Mark J. , Rochette Pierre 
Published: February 2008

Goethite was synthesized under controlled field conditions (horizontally directed field of 0.30 mT) at 30° and 55°C with and without the presence of microfiber glass filters. Upon goethite aging from freshly precipitated ferrihydrite an inverse fabric of the magnetic susceptibility develops, more prominently for the 55°C than for the 30°C synthesis. This is compatible with the fact that for well crystalline goethite the minimum susceptibility axis coincides with the long needle axis. Scanning electron microscope (SEM) observation yielded a grain‐size range from ± 0.3 μm to ± 5 μm. For the 55°C synthesis the majority of the grains are at the upper end of this range, whereas for the 30°C synthesis this is less the case. Calculation of the crystallite size from X ray line broadening revealed a range between 20 and 40 nm, indicating that the grains are polycrystalline. The magnetic properties of the resulting grain‐growth chemical remanent magnetization (CRM) which parallels the inducing field, appear to be dependent on the synthesis temperature and the presence of a substrate. Specific values for the CRM range from 1 to 2.5 μA m2/kg in the presence of the microfiber glass filters. Without these filters, values are higher, up to 25 μA m2/kg. Specific values for a thermoremanent magnetization (TRM), also acquired in a 0.30 mT inducing field, are of the same order as the CRM and less than one per mil of the saturation TRM (acquired in a 2 T field) for these samples. All remanences, the 0.30 mT CRM and TRM, as well as the 2 T TRM, showed only marginal decay upon alternating field (AF) demagnetization, as expected for goethite. Thermal decay curves, obtained by routine stepwise thermal demagnetization, appeared to be very similar for all remanence types. For the 30°C synthesis series the maximum blocking temperature of the CRM and the 2 T TRM was 65°C whereas that of the 0.30 mT TRM tends to be slightly higher (∼10°C). For the 55°C synthesis series the maximum blocking temperature of all remanence types was 80–85°C. This indicates a small grain size for the polycrystalline (XRD) synthetic goethites, in agreement with SEM observations. For the 55°C synthesis series without microfiber glass filters the maximum blocking temperature was 95°C, indicating larger goethite grains. The CRM/TRM ratio decreases with increasing grain size as inferred from the maximum blocking temperature. For the 30°C series the average ratio is 1.6 (range 0.9–2.3) and for the 55°C series the average ratio is 0.56 (range 0.44–0.78), whereas a ratio of 0.17 was obtained for the sample synthesized at 55°C without substrate. To test the applicability of the CRM/TRM concept to natural rocks, that is to test whether it was possible to assign a goethite NRM as a CRM with this test, some preliminary results of goethite‐bearing samples from two sites in SE France are presented. Prior to thermal demagnetization, the samples were AF demagnetized up to 100 mT fields to remove a possible low‐coerciyity contribution to the NRM. At one site, BL, in a Jurassic schist, the goethite is formed by recent weathering; the NRM direction corresponds to the present‐day geomagnetic field direction. At the other site, RD, in a Jurassic limestone, a recent NRM component as well as a reversed NRM component were determined, both residing in goethite. Unfortunately, the lack of a fold test hinders establishing a pretilting or posttilting age for the reversed component. The first senario would imply an Eocene age for the reversed component, while the latter would point to a Matuyama age. Also in the pretilting senario, the direction of the reversed component does not correspond with the expected Jurassic direction for stable Europe. Foi BL goetliite, NRM/TRM ratios appear to be up to 1.7, compatible with fine‐grained goethite. For RD goethite, NRM/TRM ratios for die stable goethite range from 0.14 to 0.51, pointing to larger grains or better linked crystallites. A paleomagnetically unstable sample from this site has a much higher NRM/TRM ratio, in agreement with its much finer grain size. The impact of geomagnetic dipole intensity variation on the derived ratios is discussed. Extremely fine‐grained natural goethite shows a remarkable low‐field TRM behavior with a threefold remanence increase after AF demagnetization and storage in a field‐free space for 6 days. The importance of a relatively large number of thermal demagnetization steps below 100°C to detect NRM goethite components is discussed. The observed agreement between the CRM/TRM ratio of experimental data and the NRM/TRM ratio of natural samples is encouraging. This ratio might be shown to be useful for the discrimination between stable and unstable goethite NRMs. More experimental research is necessary before the potential of the CRM/TRM technique can be fully appreciated, also for possible paleointensiy determinations. For example, the possible influence of Al, which may be incorporated in the goetliite lattice, needs to be investigated before the NRM/TRM ratio technique to assess the paleomagnetic stability of goethite can be applied to lateritic environments.