Single-domain chemical, thermochemical and thermal remanences in a basaltic rock

Tiny basaltic samples containing finely grained titanomagnetite with Curie temperature less than 100°C were heated in air in weak field (25–100μT) at temperatures between 400°C and 560°C for times as long as 32 hr. Oxyexsolution of titanomagnetite resulted in the crystallization of interacting single domain particles with Curie point close to 540°C and the concomitant development of one of two types chemical remanence, depending upon thermal treatment: isothermal chemical remanence (CRM) or thermochemical remanence (TCRM), the latter acquired under the combined effects of chemical change and temperature decrease. CRM and TCRM acquired under various conditions were subjected to Thellier-type experiments. All these treatments were carried out using a vibrating sample thermomagnetometer allowing the continuous recording of magnetization and a very precise temperature control. All CRM—TRM and TCRM—TRM plots were found to be linear over almost the entire TRM blocking temperature range, whether pTRM checks are positive or not. An apparent strength of the acquisition field of CRM or TCRM could thus be obtained and divided by the actually applied field to obtain a ratio R, which is representative of the CRM/TRM or TCRM/TRM ratios over most of the unblocking/blocking temperature spectrum. For CRM, it is found that R is less than 1 and increases rapidly with acquisition temperature (0.36 ± 0.07 at 400°C, 0.66 ± 0.02 at 450°C, and 0.90 ± 0.02 at 500°C), in qualitative agreement with expressions derived from the theory of non-interacting single domain grains. Thus, very large underestimate of geomagnetic field palaeostrength can occur when a natural CRM is not recognized as such and is believed to be a natural TRM. Palaeointensity data obtained from geological material prone to the development of secondary minerals, as for example baked contacts and volcanic glasses have, therefore, to be considered with caution. In an attempt to mimic deuteric oxyexsolution in cooling magma, TCRM was imparted from 560°C to 400°C at a cooling rate of 0.1°C/min. In contrast to the CRM case, R is found to slightly exceed 1 (1.11 ± 0.05). According to theoretical considerations, this finding suggests that in practice TCRM approximates to TRM.