Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri

Magnetic carriers in remagnetized early Paleozoic carbonates of the North American midcontinent were studied in extracts and in situ, using rock magnetism and scanning electron microscopy (SEM). SEM observation of extracts and in situ samples shows that the dominant magnetic particles are large irregular grains and framboids (10–20 μm in diameter) and individual spherical and euhedral particles (0.2–1 μm). Comparing the saturation remanence of the bulk rock and extracts indicates that the extracts only account for ∼10% of the saturation remanence of the bulk rock. Most of the remanence carriers were lost during the dissolution and separation procedure, especially the finer‐grained carriers. The framboids and the individual submicron euhedral magnetic particles observed by SEM thus make a fairly minor contribution to the magnetic remanence of the remagnetized carbonates. Hysteresis loops of the extracts are very similar to those reported by Suk et al (1993), with saturation remanence to saturation magnetization (J r /J s ) ratios from 0.1 to 0.13 and remanent to bulk coercivity (H cr /H c ) ratios ∼4. However, the loops for the “nonmagnetic” residue are strongly “wasp‐waisted”, and H cr /H c ratios are very high, similar to whole rock specimens of remagnetized carbonates (Jackson, 1990). This suggests that bulk rock and “nonmagnetic” residue contain very fine (superparamagnetic (SP) and single domain (SD)) magnetic particles which were not recovered by the magnetic separation procedure. Low‐temperature experiments support the conclusion that the principal remanence carriers in the remagnetized carbonates are SD magnetite and that SP magnetite plays an important part in the unusual bulk magnetic properties of these rocks. Owing to their dissolution, the actual natural remanent magnetization bearing particles cannot be observed petrographically.