Thermal fluctuation fields in basalts

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, 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 .

A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 2. Experimental testing

“…Key to the determination of the critical energy barriers is the use of the empirically derived Barbier relationship [ Barbier , 1954; Wohlfarth , 1984] that is used in the proposed Preisach paleointensity method [ Muxworthy and Heslop , 2011] to relate the thermal fluctuation field (and the activation volume) to the coercive force. In the Preisach paleointensity protocol we use the relationship derived specifically from basaltic lavas [ Muxworthy et al , 2009], i.e., log H f ≈ 0.54 log H C – 0.52, where H f is thermal fluctuation field. Clearly, this is a potential source of error associated with the use of this relationship.…”

“…To assess this possible source of error we systematically varied the gradient, i.e., 0.54, and recalculated the linear relationship (in log space) by considering the mean of the experimental data reported by Muxworthy et al [2009]. We allowed the gradient to vary between 0.1 and 1.1 and recalculated the Preisach paleointensity estimates for the all the natural data sets (Figure 12).…”

“…Variation in the gradient of the Barbier relationship, i.e., log H f ≈ 0.54 log H C – 0.52, versus returned Preisach paleointensity estimate for the samples from Hekla, Askja, Paricutin, Vesuvius, Láscar, and Mount St. Helens. The relationship is used in the Preisach paleointensity calculation and was empirically derived for basalts [ Muxworthy et al , 2009]. The error bar is half the CI 95 range.…”

A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 1. Theoretical framework

“…Wohlfarth [1984] did not, however, consider any natural materials in his study, and thus, Muxworthy et al [2009] tested this relationship for a series of basaltic lavas from various locations, obtaining H C / V act −0.68 , which, as shown by Muxworthy et al …”

“…For example, measurement of the time dependency of remanent or induced magnetization, e.g., viscous decay curves [Sholpo, 1967], or by determining the time dependency of hysteresis, e.g., variable field sweep rate hysteresis [Bruno et al, 1990;Muxworthy et al, 2009].…”