Possible mechanisms causing failure of Thellier palaeointensity experiments in some basalts

Abstract

The normally magnetized zone of the Jurassic Lesotho basalts, although providing apparently quite reliable palaeofield directions (Kosterov and Perrin 1996), shows anomalous behaviour when studied in vacuum using the Thellier palaeointensity method: typically the slope of the natural remanent magnetization-thermoremanent magnetization (NRM-TRM) curves is very steep at intermediate temperatures (200 to 400-460°C). In order to elucidate the reasons for such an anomalous behaviour, six representative samples (from a total of 74 studied using this method) were subjected to a variety of analyses. These experiments indicate that the magnetic properties are dominanted by pseudo-single-domain (PSD) magnetite grains some 1 μm in size, resulting from high-temperature oxidation of titanomagnetite. Laboratory heatings in vacuum up to the Curie point do not change significantly the room-temperature hysteresis characteristics or the initial susceptibility k. Similarly, the k(T) curves in vacuum are (with a single exception) rather reproducible. Since the laboratory TRMs yield almost ideal NRM-TRM plots, the anomalous NRM-TRM plot is presumably due to some peculiarity of the natural TRM. The partial TRM (pTRM) acquisition capacity in the moderate temperature range (cooling from 200 to 20°C) is generally very strongly reduced after heating to 270°C, which indicates that some magnetic alteration has already occurred at these temperatures. Hysteresis measurements between room temperature and the Curie temperature Tc show that some small (less than 10%) but significant irreversible changes in hysteresis characteristics also occur during heating. In particular, the coercive force Hc0 at room temperature is typically reduced after heating at a moderate temperature (175°C) but increases after treatments at 475°C and, more pronouncedly, at 580°C. The saturation magnetization Js0 remains unchanged, except for a very small decrease (less than 5%) occurring in some samples after the two latter treatments. These changes are most clearly seen on Hc(T)-Js(T) bilogarithmic plots, which show that the moderate-temperature change in coercivity can extend up to 200-250°C. Thus hysteresis measurements as a function of temperature offer a promising tool for sample pre-selection for Thellier experiments. Alternating-field demagnetization and cycling of pTRMs at liquid-nitrogen temperature suggest that the blocking mechanism is largely multidomain-like near room temperature but becomes less so as the Curie point is approached. The main reason for the failure of the Thellier experiments is the loss of a fraction of the NRM (natural TRM) at temperatures apparently lower than the blocking temperatures in nature. It is suggested that this anomalous behaviour results from the reorganization of the domain structure of the PSD grains during heating. This transformation, which seems to be triggered by the coercivity decrease observed at very moderate temperatures, can reduce the NRM intensity without requiring any correlated pTRM acquisition.