GEOPHYSICAL RESE•CH LETTERS, VOL. 21, NO. 7, PAGES 517-520, APRIL 1, 1994 Are magnetite spherules capable of magnetizations? carrying stable Weixin Xu, Rob Van der Voo, and Donald R. Peacor Department of Geological Sciences, University of Michigan, AnnArbor Abstract. To determine if magnetite spherules extracted from ancient carbonate rocks can carry ancient remanent magnetizations, electron microscope observations and magnetic data have been obtained for more than100individual spherules extracted from the Jurassic Twin Creek limestone, Wyoming, and the Mississippian Leadville limestone, Colorado. Two categories of surface textures (smooth and patterned) correlatewith different hysteresis parameters. Spherules with patterned surfaces (GroupA) have ratios of saturation remanent magnetization to saturation magnetization (Msr/Ms) < 0.1 andratios of remanent coercive forceto coercive force (Hcr/Hc) mostly) 3. Most spherules with smooth surfaces (GroupB) have 0.1 < Msr/M s < 0.3 and Hcr/Hc< 3. The patterned surfaces consist of (1) parallel dendfitic nets with units = 1 gm wide, (2) platelets with widths of 1 to 3 gm, (3) mosaic surface units with diameters between 2 and 5 gm; each of these surface e!9ments consists of a single crystal, as verified by selected-area electron diffraction patternsshowing sharp spots, whereas the spherules as a whole show multicrystallinepatterns by X-ray diffraction. TheMsr/Ms and Hc values of group A increase with decrease in grainsize;these GroupA magnetite spherules are undoubtedly multi-domain in character and are unlikely to carry stable ancient remanences. The smooth-surfaced Group B spherules arenot very abundant (< 8% of the entirepopulation) andhave pseudo-single-domain (PSD) or multi-domain (MD) magnetic properties; they may carry ancientremanences, although we infer that single-domain (SD), non-spherical, submicrometer magnetite is the principal carrier of remanence in most carbonates. Introduction Magnetite spherules extracted from limestones have been inferred to be authigenic in origin and to carry ancient secondary magnetizations [McCabeet al., 1983; Hortonet al., 1984]. However, spherical magnetite grains have alsobeen found in unremagnetized Paleozoiccarbonates from Indiana and Alabama [Suk et al., 1992]. Halgedahl and Suk [1992] studied the demagnetization characteristics of magnetite spherules vs. whole-rock specimens, and concluded that the spherules contribute very little to natural remanent magnetization (NRM). Moreover, our own preliminary measurements of two magnetite spherules from the Twin Creek limestone yielded Msr/Ms < 0.05 [Xu et al., 1993], values Copyright 1994 by the American Geophysical Union. Paper number 94GL00366 0094-g 534/94/94GL-003 66503.00 whichare characteristic of synthetic MD magnetite [Day et al., 1977]. Thus an increasing body of evidencesuggests that magnetite spherules are not the carrier of pervasive remagnefizations in carbonate rocks. Nevertheless, according to Suk and Halgedahl[1992], somemagnetite spherules can have Msr/Ms valuesgreater than 0.3 and Hcr/Hc valuesless than 3, indicating that some spherules exist that may be capable of carrying a stable magnetization. This impliesthat more than one kind of magnetite spherule may occur in carbonates. In order to clarify which kinds of magnetite spherules, if any, are capable of carrying stable ancient magnetization, we have carried out electron microscope observations and measured magnetic hysteresis parameters of individual magnetite spherules extractedfrom limestones. Samples and Methods The samplesused in this study were collected from the Jurassic Twin Creek Formation, Wyoming, and the Mississippian Leadville limestone, Colorado. Both limestones were deposited in a shallow, stable shelf environment and remagnetized in late Mesozoic and Tertiary time [McWhinnie et al., 1990; Horton et al., 1984]. Magnetic extracts were preparedby dissolving crushed rock samples in dilutebuffered aceticacid [McCabeet al., 1983] and separating the magnetitecomponent of the insolubleresidue with a rare-earth permanent magnet. Single magnetite spheres were selected using an optical microscopeand placed on glass plates. After being gold coated,the sphemles were examined using a scanning electron microscope (SEM). The SEM used is a HitachiS-570, equipped with a Kevex Quantum energy dispe.rsive analysis (EDA) system. The magnetite grains were then enclosedwithin a small drop of nail enamel for convenience in handling during magnetic measurements. Hysteresis propertiesof the single magnetitespherules were measured on an alternatinggradient force magnetometer (AGFM) in the Institute for Rock Magnetism (IRM) at the University of Minnesota. A few grainswere selected on the basisof hysteresis values for observation in a scanning transmission electron microscope (STEM). The STEM used is a Philips CM-12 fitted with a Kevex Quantum EDA system. The crystalline states of whole spheres were studied using spheres attached to glass fibers mounted on a precession single-crystal X-ray machine. Results Magnetitespherules foundin the Twin Creek limestone and Leadville limestone have identical size ranges and surface textures. All were almost perfectly round spheres with diameters ranging from several to 85 I.tm (Figure 1). SEM observations showedthat grains vary in surfacetexture on the 517