AbstractThe thermal fluctuation field (Hf) is central to thermoremanent acquisition models, which are key to our understanding of the reliability of palaeomagnetic data, however,Hfis poorly quantified for natural systems. We reportHfdeterminations 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 ofHfversus the coercive forceHC, i.e., the “Barbier plot”, which is characterized by the empirically derived relationship; logHf∝ 1.3 logHCobtained 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, logHf∝ 0.54 logHC, 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 determiningHf. We consider the relationship betweenHCand the activation volume,υact, which was found to beHC∝ υ ***** for our mineralogically homogeneous samples. This compares favourably with theoretical predictions, and with previous empirical estimates based on the Barbier plot, which defined the relationship asHC∝****.