On the suitability of the Thellier method of palaeointensity determinations on pseudo-single-domain and multidomain grains

Abstract

Summary We report an experimental and theoretical study of non‐linear Arai–Nagata diagrams for samples containing pseudo‐single‐domain (PSD) and multidomain (MD) magnetite. Our aim is to reveal the physical reasons for the deviation of these plots from ideal straight lines. Contrary to expectations, the concavity of the Arai–Nagata diagrams is not related to the two most noticeable violations of the Thellier laws documented for non‐single‐domain particles: the tail of partial thermoremanence and the dependence of the magnitude of pTRM on the thermal history of the sample. Indeed, the contributions from these two factors mutually cancel each other. Phenomenologically, the non‐linear Arai–Nagata plots occur because samples during low‐temperature stages of the Thellier procedure lose too much remanence and recover too little of it. The excessive loss of the previously imparted total TRM is due at least partly to some partial demagnetization of high‐temperature TRM components and to progressive stabilization of domain structure during the repetitive heatings to moderate temperatures that form the basis of the Thellier procedure. For natural MD samples a linear fit to the low‐temperature data points on the Arai–Nagata plots leads to a palaeointensity overestimated by as much as 60 per cent. The samples containing hydrothermally grown or crushed and sieved MD magnetite provide low‐temperature apparent palaeointensities two to three times larger than the correct value. For small PSD samples the overestimate is less than 10–20 per cent and, in general, PSD samples can be used for the palaeointensity determinations.

Thellier-type paleointensity data from multidomain specimens

“…These so called pTRM tails persist to high unblocking temperatures and can only be demagnetized if the specimen is heated above the Curie temperature (T c ) of the constituent magnetic minerals. The magnitude of pTRM tails increases with increasing grain size (Shcherbakov and Shcherbakova, 2001;Yu and Dunlop, 2003), which represents an increasing violation of Thellier's law of reciprocity.…”

“…As the degree of MD-ness increases, for all protocols, the regions of high proportions of accurate results ( J 70% of results are accurate) shrink. The effect is most pronounced for the Thellier and Aitken protocols, where this manifests as a strong angular dependence, (2011) (data obtained from Levi, 1977;Muxworthy, 1998;Shcherbakov and Shcherbakova, 2001;Krása et al, 2003;Yu and Dunlop, 2003;Xu and Dunlop, 2004). Circles are the data from Muxworthy et al (2014), where grain size is taken to be the cube root of the dot volume (assuming a cylindrical shape).…”

“…Recently, Muxworthy et al (2014) demonstrated that specimens with grain sizes on the order of 0.1-0.25 lm had linear Arai plot segments that recorded an accurate paleointensity (B Lab = B Anc ), although these specimens thermally altered at high temperatures. The Arai plot behavior of MD specimens is due to a balance between excess NRM demagnetization (Shcherbakov and Shcherbakova, 2001) and the fact that a partial TRM (pTRM) acquired by heating to temperature T i and cooling back to room temperature (T 0 ) in an applied field is not fully removed by reheating to T i in zero-field (Bol'shakov and Shcherbakova, 1979). These so called pTRM tails persist to high unblocking temperatures and can only be demagnetized if the specimen is heated above the Curie temperature (T c ) of the constituent magnetic minerals.…”

Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders

“…Non-SD particles can have complicated blocking/unblocking temperature functions spanning the entire temperature range below the Curie temperature (38,39). Furthermore, non-SD can show dependency on the experimental history of the sample (40,41) that leads to complicated behavior during the paleointensity experiment (42). Hence, irreversible changes in the micromagnetic structure of non-SD alters the blocking−unblocking relationships as well as the demagnetization-remagnetization relationships over the entire temperature spectrum below the Curie temperature, and not only the low-temperature portion.…”

Geomagnetic paleointensity at ∼2.41 Ga as recorded by the Widgiemooltha Dike Swarm, Western Australia

“…The Thellier laws hold to a good approximation if the magnetization is carried by single-domain (SD) or smaller pseudosingle-domain (PSD) magnetic grains, but are violated for larger PSD and multidomain (MD) carriers (e.g., Shashkanov and Metallova, 1972;Bolshakov and Shcherbakova, 1979;Shcherbakov and Shcherbakova, 2001). The presence of such "non-ideal" grains in many rocks renders paleointensity determination difficult or sometimes impossible (e.g., Xu and Dunlop, 2004).…”