Laboratory chemical remanent magnetization in a natural claystone : a record of two magnetic polarities

A record of two magnetic polarities during the acquisition by heating of a laboratory chemical remanent magnetization (CRM) in a natural pyrite-rich claystone was investigated. The samples were heated for 22 hr at 250 • C in a controlled magnetic field (1.0 mT) under an argon atmosphere. We interpret the origin of CRM as the surface oxidation of pyrite to magnetite, which in turn is progressively oxidized into haematite. We carried out experiments under a constant-polarity magnetic field and under two opposite polarities. The resulting CRM was measured after cooling in zero field: it is parallel to the applied field and has the direction of the last polarity. Thermal demagnetization under an argon atmosphere isolates an unexpected lowunblocking-temperature component (T UB < 220 • C). This component probably results from thermal alteration of magnetic carriers during subsequent thermal demagnetization. In the onepolarity experiments, thermal demagnetization of CRM above 220 • C isolates a well-defined component parallel to the imparted field direction. In the experiments with two magnetic polarities thermal demagnetization of CRM reveals two components of opposite polarity. The component which is parallel to the direction of the last applied field is well defined, while the other component, which has the polarity of the first applied field, is ill defined. Oppositely directed components are also detected by using alternating field demagnetization