Thermal stability of pTRMs created from different magnetic states

Abstract

The apparent successful application of the Thellier’s method of paleointensity or paleofield direction determinations on multi-domain (MD) and pseudo-single-domain (PSD) grains suggests the validity of the Thellier’s laws of independence and additivity of partial thermoremanent magnetizations (pTRMs). However, it is well known that these laws are violated for PSD and especially for MD samples, leading to a noticeable inequality of blocking ( T b) and unblocking ( T ub) temperatures. Thus, a preliminary determination of domain structure (DS) of a sample is desirable. The thermomagnetic criterion of DS is based on a direct check of the equality of T b and T ub by measuring the parameter A a( T 1, T 2)=intensity of the tail[pTRM a( T 1, T r)] normalized to the original pTRM a( T 1, T r), both measured at T r. Here T r is room temperature and pTRM a( T 1, T r) is a pTRM acquired when the upper temperature T 1 of pTRM acquisition is reached by cooling a sample in zero field from the Curie temperature T C. A detailed study of properties of this pTRM and its tail was performed in our previous paper [J. Geophys. Res. 105 (2000) 767]. However, the application of the thermomagnetic criterion for pre-selection of the rock samples requires the undesirable heating of samples to T C. To avoid the heating, one can try to impart a pTRM AF( T 1, T r) from the AF-demagnetized state and use the tail of this pTRM as a more suitable test of DS. The advantage of this approach is that the measurement of the tail[pTRM AF( T 1, T r)] requires a preliminary heating of a sample to a moderate temperature T 1 only (say, T 1=300°C). It is shown that the tail[pTRM AF(300, T r)] is usually much shorter than that of pTRM a(300, T r) and of smaller magnitudes by a factor of about 3. Thus, this test is suitable mainly for the pre-selection of MD samples having relatively large tails of pTRM AF(300, T r). For PSD grains, the tails of pTRM AF(300, T r) are often too small to distinguish them from the single-domain (SD) ones. A detailed study of properties of different pTRMs created from AF-demagnetized, thermodemagnetized and zero-field cooled states has been undertaken. Physically the dependence of values of both pTRMs and their tails on the initial magnetic state of a sample is explained by the phenomenon of metastability.