The magnetization of clay-rich rocks in sedimentary basins: low-temperature experimental formation of magnetic carriers in natural samples

Rocks are often magnetically altered during burial, thus providing a useful method to study their thermo-chemical history. In order to improve the understanding of burial diagenetic magnetization, we experimentally synthesized magnetite in natural samples. Lower Jurassic argillites from the Paris Basin were heated to 150 °C, under a confined atmosphere of argon. When a magnetic field was applied during the heating, a chemical remanent magnetization was acquired. Rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite. Using anhysteretic remanent magnetization as a measure of the quantity of magnetite, an increase of up to 150% after heating was observed. Ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3% calcite, and new magnetite is sometimes completely absent in samples containing less than 0.5% calcite. Hysteresis measurements suggest that the new magnetite is in the pseudo-single domain state. Scanning electron microscope studies indicate that the new magnetic carriers are associated with pre-existing pyrite. The magnetic transformations during the experiments involved a fluid which reacted with iron-bearing and sulfur-bearing minerals in ways depending on local redox conditions. This study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins, without any external supply from long-range fluid flow.