New insights into partial oxidation model of magnetites and thermal alteration of magnetic mineralogy of the Chinese loess in air

Abstract

SUMMARY New rock magnetic data including low- and high-temperature thermomagnetic runs and hysteresis loops, as well as X-ray diffraction measurements, are presented from the Chinese loess/palaeosols at JiuZhoutai, on the northwestern margin of the Chinese loess plateau, to provide sound constraints on the partial oxidation model of magnetites and variations in magnetic mineralogy, grain size and oxidation states of loess samples during thermal treatments from room temperature up to 700 °C. The results show that the partially oxidized magnetites in samples are first reduced, probably as a result of the burning of organic matter below 300 °C, and then reoxidized above that. However, even after heating to 700 °C the highly diminished (in size) magnetite core still remains stoichiometric as revealed by the 120 K Verwey transition. We conclude that the diffusion of Fe2+ from the magnetite core to the surrounding maghemite rim is confined to the oxidation front (magnetite-core/maghemite-rim boundary), and has less effect on the stoichiometry of the magnetite core. The new results also reveal that the absolute number of superparamagnetic grains (magnetite/maghemite) remains relatively stable during heating. Furthermore, the left-downward shift in the data points of Day plots (mainly between 25 and 150 °C) is caused by a decrease in the oxidation gradient. In contrast, the right-upward (mainly between 500 and 650 °C) shift of the Day plots corresponds to a much higher degree of oxidation, resulting in the increase of a new oxidation gradient between the magnetite core (or the residual maghemite mantle) and the newly developed haematite outer rim, which is transformed from maghemite by heating. These new results place strong constraints on the fidelity of palaeomagnetic results from the Chinese loess constructed by thermal demagnetization, and provide a more accurate interpretation of the Day plot and low-temperature magnetic behaviours for these partially oxidized magnetites. Furthermore, this study provides robust lines of evidence for the core–rim coupling model between different magnetic phases (magnetite, maghemite and haematite).