Paleomagnetic recording fidelity of nonideal magnetic systems

Abstract

A suite of near-identical magnetite nanodot samples produced by electron-beam lithography have been used to test the thermomagnetic recording fidelity of particles in the 74–333 nm size range; the grain size range most commonly found in rocks. In addition to controlled grain size, the samples had identical particle spacings, meaning that intergrain magnetostatic interactions could be controlled. Their magnetic hysteresis parameters were indicative of particles thought not to be ideal magnetic recorders; however, the samples were found to be excellent thermomagnetic recorders of the magnetic field direction. They were also found to be relatively good recorders of the field intensity in a standard paleointensity experiment. The samples' intensities were all within ∼15% of the expected answer and the mean of the samples within 3% of the actual field. These nonideal magnetic systems have been shown to be reliable records of the geomagnetic field in terms of both direction and intensity even though their magnetic hysteresis characteristics indicate less than ideal magnetic grains.Key PointsNonideal magnetic systems accurately record field directionWeak-field remanences more stable than strong-field remanences

Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens

“…Although there is a discrepancy between these two values, which is probably due to imprecise measurements of M s , M r s , etc., both values agree in that the statistical thermodynamic error in this case is negligible. The experiments conducted by Muxworthy et al [] support the conclusion that these samples are accurate recorders for both paleointensity and paleodirections.…”

Thellier-type paleointensity data from multidomain specimens

“…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).…”

“…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). Crosses represent new data measured for this study (described in Section 7.2).…”

Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite

“…Although interaction alone has been shown not to preclude samples from being reliable geomagnetic recorders (Muxworthy et al, 2014), it has been empirically demonstrated that igneous rock samples exhibiting PSD-like behavior in high-field experiments can exhibit MD-like behavior in weak-field, i.e., PI, experiments (Biggin & Thomas, 2003a;Calvo et al, 2002;Shcherbakova et al, 2000). Although interaction alone has been shown not to preclude samples from being reliable geomagnetic recorders (Muxworthy et al, 2014), it has been empirically demonstrated that igneous rock samples exhibiting PSD-like behavior in high-field experiments can exhibit MD-like behavior in weak-field, i.e., PI, experiments (Biggin & Thomas, 2003a;Calvo et al, 2002;Shcherbakova et al, 2000).…”

“…Fabian and Leonhardt (2010) found that intermediate grain sizes were more strongly biased by previous zero-field steps than larger MD and smaller (near-SD) grains were. The behavior of MD grains during demagnetization of the NRM and acquisition of a laboratory TRM has been the focus of many studies (e.g., Paterson et al, 2015;Xu & Dunlop, 2004) yet there are fewer studies focusing on the behavior of interacting grains (Cisowski, 1981;Davis & Evans, 1976;Evans et al, 2006;Muxworthy et al, 2014). The behavior of MD grains during demagnetization of the NRM and acquisition of a laboratory TRM has been the focus of many studies (e.g., Paterson et al, 2015;Xu & Dunlop, 2004) yet there are fewer studies focusing on the behavior of interacting grains (Cisowski, 1981;Davis & Evans, 1976;Evans et al, 2006;Muxworthy et al, 2014).…”