ALBERT

Description: Experiments were performed demonstrating that a chemical remanent magnetization (CRM) can be acquired during the aqueous oxidation of synthetic titanomagnetites (TM40:Fe/2.6/Ti/0.4/O4) by the removal of iron, as it does in the oceanic crust basalts. Results showed that the CRM is acquired along the ambient field direction during oxidation, and that there is no coupling of the CRM and the thermoremanent magnetization. The rate of oxidation was found to be pH-dependent, with the amount of CRM acquired varying inversely with the pH of the oxidizing solution. The reaction produced nascent hydrogen gas. Results support the chemical equation for titanomagnetic oxidation proposed by Worm and Banerjee (1984).

Description: The time-temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists, rock magnetists, and magnetic anomaly modellers. We have investigated this relationship by studying the viscous remanent magnetization (VRM) of coarse-grained multidomain (MD) magnetite. Our experiments covered the temperature range from 22 to 400 C, times from minutes to months, and included both Australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method. VRM acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories. Thermal cycling between room temperature (at which the measurements were made) and the VRM acquisition temperature sharply decreased the temperature dependence of the VRM. Room temperature VRM acquisition accelerates with time when plotted on a semilog plot, whereas at elevated temperature the curves are quasilinear against log(time) for both the natural and synthetic samples. This change in behavior may suggest a variation in the VRM acquisition mechanism as a function of temperature for MD magnetite. The granulites have a nearly linear increase in VRM acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 C, but increase by nearly a factor of 3 by 400 C. The increase in VRM of the glass ceramics between 200 and 400 C is in general qualitative agreement with thermal fluctuation theory. There was no systematic change in the rate of VRM acquisition with grain size for the multidomain magnetites used in this study. Elevated temperature (e.g., 400 C) VRM acquisition by the deep crustal granulites, if extrapolated over the Brunhes chron, would produce a magnetization of several A/m which, if true, is of the order required by models for the source of long-wavelength magnetic anomalies.

Description: Rock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudosingle-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to the thermal demagnetization, with nearly 50 percent of the magnetization remaining after 400 C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanant magnetization along the present-day field is likely to dominate.