PALEOINTENSITY DETERMINATION FROM IRON, METEORITIC IRON, MAGNETITE, TITANOMAGNETITE, PYRRHOTITE, HEMATITE, TITANOHEMATITE, TROILITE

When rocks cool through the blocking temperatures of their applicable magnetic minerals, the overall magnetic efficiency of the magnetic recording, as quantified by the ratio of the thermoremanent magnetization to saturation remanent magnetization, M_tr/M_rs, follows the theory developed by [1].

Various attempts have derived simplified empirical relationships for estimating the magnetizing field from the magnetic properties of rocks and minerals measured at room temperature. Though this technique is applicable to many magnetic minerals, the numerical proportionality constant is based largely on data from rocks containing magnetite.

In a different contemporaneous approach, [2] considered the magnetization of single magnetic minerals. For many magnetic minerals, the saturation magnetizations differ only by a small factor, explaining the success of the approach used by [3]. However, for hematite, M_s is more than two orders of magnitude smaller than that of magnetite. When accounting for M_s, [2] showed that the magnetic acquisition of all minerals followed a single linear relationship, with an uncertainty of only a factor of 2. If a rock is composed of more than one magnetic species, paleointensity estimation needs to consider the fact that the magnetic efficiency for each species will be different for the same magnetizing field.

We revisit and expand upon many of the concepts developed by [2]. First, we make use of additional data for the acquisition of thermoremanent magnetization, and show how the shape and the rate of cooling of the mineral affects the acquired magnetization. Second, we discuss how our new relationship can be used to determine the paleointensity when a rock cools below the blocking temperature. Third, based on the magnetic properties of troilite, we show that this mineral could potentially play an important role in explaining the magnetization of lunar and extraterrestrial samples where troilite is known to be abundant [4].

[1] Néel, L. (1955), Adv. Phys. 4, 191-243[2] Kletetschka, G.et al. (2004) EPSL, 226, 521-528 [3] Gattacceca, J., and P. Rochette (2004), EPSL, 227 377-393 [4] Rubin (1997) MPS, 32, 231-247