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, H f 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 H f versus the coercive force H C , i.e., the "Barbier plot", which is characterized by the empirically derived relationship; log H f ∝ 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 ∝ 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, v act , which was found to be H C ∝ v −0.68 act 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 as H C ∝ v −0.73 act .