Are magnetite spherules capable of carrying stable magnetizations?

Details Are magnetite spherules capable of carrying stable magnetizations? Are magnetite spherules capable of carrying stable magnetizations?

Apr 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994georl..21..517x&link_type=abstract

Geophysical Research Letters (ISSN 0094-8276), vol. 21, no. 7, p. 517-520

Physics

9

Grain Size, Magnetite, Magnetization, Paleomagnetism, Remanence, Spherules, Carbonates, Crystal Structure, Electron Microscopy, Limestone, X Ray Diffraction

Scientific paper

To determine if magnetite spherules extracted from ancient carbonate rocks can carry ancient remanent magnetizations, electron microscope observations and magnetizations, electron microscope observations and magnetic data have been obtained for more than 100 individual spherules extracted from the Jurassic Twin Creek limestone, Wyoming, and the Mississippian Leadville limestone, Colorado. Two categories of surface textures (smooth and patterned) correlate with different hysteresis parameters. Spherules with patterned surfaces (Group A) have ratios of saturation remanent magnetization to saturation magnetization (M(sub sr)/M(sub s) less than 0.1 and ratios of remanent coercive force to coercive force (H(sub cr)/H(sub c)) mostly greater than 3. Most spherules with smooth surfaces (Group B) have M(sub sr)/M(sub s) is between 0.1 and 0.3 and H(sub cr)/H(sub c) is less than 3. The patterned surfaces consist of (1) parallel dendritic nets with units approximately 1 micron wide, (2) platelets with widths of 1 to 3 microns, (3) mosaic surface units with diameters between 2 and 5 microns; each of these surface elements consists of a single crystal, as verified by a selected-area electron diffraction patterns showing sharp spots, whereas the spherules as a whole show multicrystalline patterns by X-ray diffraction. The M(sub sr)/M(sub s) and H(sub c) values of group A increase with decrease in grain size; these Group A magnetite spherules are undoubtedly multi-domain in character and are likely to carry stable ancient remanences. The smooth-surfaced Group B spherules are not very abundant (less than 8% of the entire population) and have pseudo-single-domain (PSD) or multi-domain (MD) magnetic properties; they may carry ancient remanences, although we infer that single-domain (SD), non-spherical, submicrometer magnetite is the principal carrier of remanence in most carbonates.

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