ARE MAGNETITE SPHERULES CAPABLE OF CARRYING STABLE MAGNETIZATIONS

To determine if magnetite spherules extracted from ancient carbonate r ocks can carry ancient remanent magnetizations, electron microscope ob servations and magnetic data have been obtained for more than 100 indi vidual spherules extracted from the Jurassic Twin Creek limestone, Wyo ming, and the Mississippian Leadville limestone, Colorado. Two categor ies of surface textures (smooth and patterned) correlate with differen t hysteresis parameters. Spherules with patterned surfaces (Group A) h ave ratios of saturation remanent magnetization to saturation magnetiz ation (M(sr)/M(s)) < 0.1 and ratios of remanent coercive force to coer cive force (H(cr)/H(c)) mostly > 3. Most spherules with smooth surface s (Group B) have 0.1 < M(sr)/M(s) < 0.3 and H(cr)/H(c)< 3. The pattern ed surfaces consist of (1) parallel dendritic nets with units almost-e qual-to 1 mum wide, (2) platelets with widths of 1 to 3 mum, (3) mosai c surface units with diameters between 2 and 5 mum; each of these surf ace elements consists of a single crystal, as verified by selected-are a electron diffraction patterns showing sharp spots, whereas the spher ules as a whole show multicrystalline patterns by X-ray diffraction. T he M(sr)/M(s) and H(c) values of group A increase with decrease in gra in size; these Group A magnetite spherules are undoubtedly multi-domai n in character and are unlikely to carry stable ancient remanences. Th e smooth-surfaced Group B spherules are not very abundant (< 8% of the entire population) and have pseudo-single-domain (PSD) or multi-domai n (MD) magnetic properties; they may carry ancient remanences, althoug h we infer that single-domain (SD), non-spherical, submicrometer magne tite is the principal carrier of remanence in most carbonates.