Magnetic monitoring of low-temperature (95 deg C) transformation in natural clays

In natural clays from Bure and Mont Terri, we carried out heating at 95 deg C in confined natural atmosphere during 21 to 144 days. During heating, a 2 mT vertical magnetic field was applied either with a unique polarity or with magnetic reversals. Magnetic reversal allows to test whether the newly formed magnetic grains results from nucleation/growth process or oxidation of a host mineral. During the experiment, we monitored the chemical remanent magnetization evolution (CRM). Different patterns of the increase of CRM are observed for Mont Terri and Bure clays. In the Mont Terri clays, there is two orders of magnitude between the initial natural remanent magnetization (NRM) and the CRM. Then the CRM steadily increased until the end of experiment at t+22 days. In Bure clays, we observe one order of magnitude between NRM to CRM. Then CRM is continuously increasing until the first magnetic reversal at t+55 days. The flip of magnetization is very sharp for the first reversal at t+60 and more progressive at t+118 for the second reversal. We test the thermal viscosity of the remanent magnetization at the end of experiment. The small drop of CRM at both clays indicates that the largest part of magnetization is of chemical origin. Our results point unambiguously for chemical transformation. The cross analysis of magnetic parameters allows to identify the original and newly magnetic grains. In Mont Terri, magnetite is detected in started and heated materials. These magnetites show however distinct signatures. In Bure clays, a mixture of magnetite and hematite is inferred. Our results suggest therefore that the product during low thermal heating is different in the two clays. In the Bure clays where we applied two magnetic reversals during the experiment, the AF demagnetization shows a particular pattern, indicating that the two magnetic polarities are detected. We infer from this demagnetization pattern and other magnetic parameters different steps of magnetic grains growth during the experiment. Magnetic monitoring is then a powerful tool to track the very low temperature chemical transformation of iron and sulfide oxides in clays that takes place after few days of heating at 95 deg C. Complementary to magnetic monitoring of CRM, a detailed approach between rock magnetism and geochemistry is currently undertaken to better understand the chemical processes involved in theses transformation. (authors)