Thermal fluctuation fields in basalts

Adrian R Muxworthy,Daniel M Michalk,David Heslop

doi:10.1186/bf03352890

Abstract

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∝****.

Highlights

Neel (1950, 1951) introduced the concept of a thermal fluctuation field to describe the influence of thermal fluctuations on a magnetic system
It might be argued that palaeomagnetists are more commonly interested in thermal activation of remanent magnetisation rather than during hysteresis, that is, it might be more appropriate to use viscous decay of remanence curves to determine Hf if the findings are to be applied to palaeomagnetic investigations
Different methods have been employed to determine Hf; the method utilised in this paper determines Hf at higher fields than the viscous decay mechanisms; Hf is determined at different positions on the hysteresis curve, that is, one mechanism is examining the effect of thermal fluctuations on the remanence state the others on an in-field magnetisation

Summary

Introduction

Neel (1950, 1951) introduced the concept of a thermal fluctuation (or viscosity) field to describe the influence of thermal fluctuations on a magnetic system. Thermal fluctuations influence all thermally activated magnetic processes, i.e., for temperatures T > 0 K, becoming increasingly important for magnetisation processes at high or variable temperatures, such as thermoremanence (TRM) acquisition. As such the concept of thermal fluctuation fields are central to many theories of both single-domain (SD) and multidomain (MD) TRM acquisition, on which many palaeomagnetic concepts and methodologies are based, e.g., the Thellier and Thellier (1959) method and its modifications for palaeointensity determination. Wohlfarth’s description is based on the work of Neel (1950, 1951) and Street and co-workers (Street and Woolley, 1949, 1950; Street et al, 1952; Street and Woolley, 1956), and relates Hf to the magnetic viscosity parameter S and to the irreversible suscepti-

Results

Discussion

Conclusion