Thierry Poidras

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

SUMMARY 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.

Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)

SUMMARY We report a survey carried out on basalt flows from Amsterdam Island (Southeastern Indian Ocean) in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the Brunhes epoch, completing this palaeomagnetic record with palaeointensity determinations and radiometric dating. Because the palaeomagnetic sampling was done over a few hours during the resupply of the French scientific base Martin du Viviers by the Marion Dufresne vessel, we could collect only 29 samples from four lava flows. The directional results corroborate the findings by Watkins & Nougier: normal polarity is found for two units and an intermediate direction, with associated virtual geomagnetic poles (VGPs) close to the equator, for the other two units. A notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations. 50 per cent of the samples yields reliable intensity values with high-quality factors. An original element of this study is that we made use of the thermomagnetic criterion PTRM-tail test of Shcherbakova et al. to help in the interpretation of the palaeointensity measurements. Doing this, only the high-temperature intervals, beyond 400 ◦ C, were retained to obtain the most reliable estimate of the strength of the ancient magnetic field. However, not applying the PTRM-tail test does not change the flow-mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 ◦ C. The normal units yield virtual dipole moments (VDM) of 6.2 and 7.7 (10 22 Am 2 ) and the excursional units yield values of 3.7 and 3.4 (10 22 Am 2 ). These results are quite consistent with the other Thellier determinations from Brunhes excursion records, all characterized by a decrease of the VDM as the VGP latitude decreases. 40 Ar/ 39 Ar isotopic age determinations provide an estimate of 26 ± 15 and 18 ± 9 kyr for the transitional lava flows, which could correspond to the Mono Lake excursion. However, the large error bars associated with these ages do not exclude the hypothesis that this event is the Laschamp.

Controlled atmosphere vibrating thermo-magnetometer (CatVTM): a new device to optimize the absolute paleointensity determinations

The laboratory of paleomagnetism of Montpellier (France) has developed a new one-axis vibrating thermal magnetometer dedicated to the study of physical properties of natural rocks remanence. Among its key characteristics, this apparatus allows both measurement of the magnetization moment on the interval from room temperature to 700°C with a precision of 2 × 10-9 A m2 and acquisition of a total or a partial thermo-remanent magnetization using a steady field from -100 up to 100 μT. Another point that is worth noting is that one can apply a controlled atmosphere by means of argon flux to prevent oxidation of the studied sample during heating. We report here a technical description of this new instrument and review some specific applications in absolute paleointensity surveys.

Directional results and absolute archaeointensity determination by the classical Thellier and the multi-specimen DSC protocols for two kilns excavated at Osterietta, Italy

We present a detailed rock-magnetic and archaeomagnetic study of two brick kilns, named OSA and OSB, discovered at the location of Osterietta, in northern Italy. The magnetic properties of representative samples have been investigated to identify the nature of the magnetic carriers, their domain state and thermal stability, and investigate their suitability for archaeomagnetic determinations. Thermally stable, mainly pseudosingle domain (PSD) magnetite is identified as the main magnetic carrier. The full geomagnetic field vector has been determined for the two kilns, including directional and intensity analysis. Archaeointensities have been recovered with both the classical Thellier and the multi-specimen protocols. The multi-specimen procedure was performed with a very fast-heating oven developed at Montpellier, France. A Matlab® code for anisotropy correction during the Thellier experiment is provided. The archaeointensity results obtained from both techniques for the OSA kiln are of high quality and in good mutual agreement. For the OSB kiln, Thellier results are characterized by large standard deviation and the multi-specimen (MSP) technique was not successful. The obtained full geomagnetic field vector (declination, inclination and intensity) has been used for the archaeomagnetic dating of the two structures suggesting that the OSA kiln was for the last time used between 1761?1841 A.D. and the OSB kiln between 1752?1831 A.D., at 95% probability. This study shows that intensity determinations do not restrict the dating results when referring to the last few centuries, as this period is characterized by very small intensity variations.