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Abstract

The thermal fluctuation field (H-f) is central to thermoremanent acquisition models, which are key to our understanding of the reliability of palaeomagnetic data, however, Hf is poorly quantified for natural systems. We report H-f determinations for a range of basalts, made by measuring rate-dependent hysteresis. The results for the basalts were found to be generally consistent within the space of Hf versus the coercive force H-C, i.e., the "Barbier plot", which is characterized by the empirically derived relationship; log H-f proportional to 1.3 log H-C obtained from measurements on a wide range of different magnetic materials. Although the basalts appear to occupy the correct position within the space of the Barbier plot, the relationship within the sample set, log H-f proportional to 0.54 log H-C, is different to the Barbier relationship. This difference is attributed to the original Barbier relationship being derived from a wide range of different synthetic magnetic materials, and not for variations within one material type, as well as differences in methodology in determining H-f. We consider the relationship between H-C and the activation volume, nu(act) which was found to be H-C proportional to nu(-0.68)(act) for our mineralogically homogeneous samples. This compares favourably with theoretical predictions, and with previous empirical estimates based oil the Barbier plot, which defined the relationship as H-C proportional to nu(-0.73)(act).