GYROMAGNETIC REMANENCE ACQUIRED BY GREIGITE (FE3S4) DURING STATIC 3-AXIS ALTERNATING-FIELD DEMAGNETIZATION

A magnetic study was carried out on lacustrine sediments from the Zoig e basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisitio n is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples . X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning elec tron microscopy (SEM) reveals that the greigite particles are in the g rain size range of 200-300 nm, possibly in the single-domain state. Gr eigite clumps of about 3 mu m size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15 nm, indic ating that the particles seen in the SEM are polycrystalline. GRM inte nsities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar . Variation of the demagnetization sequence confirms that GRM is mainl y produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for d etailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found alo ng the demagnetization axis. Behaviours of acquisition and AF demagnet ization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.