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 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 dominated by pseudo-single-domain (PSD) magnetite grains some 1 mm in size, resulting from high-temperature oxidation of titanomagnetite. Laboratory heatings in vacuum up to the Curie point do not change significantly the roomtemperature 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 T c