Hu, S.; Appel, E.; Hoffmann, V.; Schmahl, Wolfgang W.; Wang, S.
Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization.
In: Geophysical Journal International, Vol. 134, No. 3: pp. 831-842
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A magnetic study was carried out on lacustrine sediments from the Zoigebasin, Tibetan Plateau, in order to obtain a better understanding ofpalaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisitionis unexpectedly observed during static three-axis alternating field (AF)demagnetization in about 20 per cent of a large number of samples. X-raydiffraction (XRD) analysis on a magnetic extract clearly shows thatgreigite is the dominant magnetic mineral carrier. Scanning electronmicroscopy (SEM) reveals that the greigite particles are in the grainsize range of 200-300 nm, possibly in the single-domain state. Greigiteclumps of about 3 mu m size are sealed by silicates. Fitting of XRDpeaks yields a crystalline coherence length of about 15 nm, indicatingthat the particles seen in the SEM are polycrystalline.GRM intensities of most samples are of the same order as the NRM, whileothers show much stronger GRM although their magnetic properties aresimilar. Variation of the demagnetization sequence confirms that GRM ismainly produced perpendicular to the AF direction. The anisotropydirection can be derived from GRM, but more systematic studies areneeded for detailed conclusions. An attempt to correct for GRM faileddue to high GRM intensities and because smaller GRM acquisition was alsofound along the demagnetization axis. Behaviours of acquisition and AFdemagnetization of GRM are comparable with those of NRM, ARM, IRM,indicating fine grain sizes of remanence carriers.