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Thermal fluctuation fields in basalts

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Abstract

The thermal fluctuation field (Hf) 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 Hf 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 HC, i.e., the “Barbier plot”, which is characterized by the empirically derived relationship; log Hf 1.3 log HC 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 Hf 0.54 log HC, 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 Hf. We consider the relationship between HC and the activation volume, υact, which was found to be HC 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 as HC ****.

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Correspondence to Adrian R. Muxworthy.

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Key words

  • Basalt
  • thermoremanence
  • thermal fluctuations
  • rock magnetism