A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi formation, Sicily)

The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations. It distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals. As a result, the initial magnetic mineralogy can also be determined. The method is demonstrated with samples of fine-grained marine marls from the Pliocene Trubi formation in Sicily (Italy). Single domain (SD) magnetite is the dominant remanence carrier in this sediment. A first alteration during heating to only 145°C is accompanied by a considerable reduction of coercivities higher than 0.1 T. This alteration is thought to be due to the reduction of stress in superficially maghemitized SD magnetite grains. Between 390 and 480°C grains with magnetite-like properties are produced, most likely by oxidation of pyrite in the sediment. At about 560°C the breakdown of iron-bearing silicates also produces magnetic grains. The magnetic grains formed during both alteration processes have low blocking temperatures. Demagnetization above 510°C is not only due to unblocking of the IRM. Alteration of magnetite to haematite proves to be an important demagnetization mechanism.