Scanning and transmission electron microscope observations of magnetite and other iron phases in Ordovician carbonates from east Tennessee

Abstract

Previous paleomagnetic observations for the carbonates of the Lower Ordovician Knox Group have indicated that ancient magnetizations in these rocks are of the same age as the late Paleozoic Alleghenian Orogeny. Rock magnetic properties strongly suggest magnetite as the carrier of the magnetization, but the textural and crystalline characteristics, sizes, morphologies, and mineral associations of these magnetites are poorly known. We have examined magnetic extracts and iron oxides in thin sections with scanning (SEM) and scanning/transmission (STEM) electron microscope techniques to determine whether the observed iron‐oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers. Several different forms of magnetite in limestones and dolomites, which in places are host to Mississippi‐Valley type deposits, are documented by imaging and energy‐dispersive analysis using SEM and STEM, by X ray diffraction and electron diffraction patterns using STEM. The magnetite is either spherical with a dimpled surface or nonspherical in the form of void‐filling single grains or grain aggregates. Most of the iron oxides have the composition of pure end‐member magnetite, but occasional titanomagnetite and hematite, including rare zincian hematite, have been observed (only in limestone). Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium‐feldspar, illite, and quartz. Some iron oxides have, in fact, inclusions of K‐feldspar and quartz. Some of the magnetite (spherical and nonspherical) is polycrystalline; this implies that the larger observed grains may consist of single domains or pseudo‐single domains. This provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single‐domain (but interacting?) subgrains, on the basis of remanent coercivities and blocking temperatures. We interpret the pure end‐member magnetite to be authigenic, having formed at approximately the same time as the K‐feldspars, which in nearby areas have yielded late Paleozoic radioisotopic ages (278–322 Ma). The Knox carbonates therefore are inferred to carry a chemical rémanent magnetization. Iron‐rich clays or original iron‐titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution‐precipitation processes. These processes require rock‐fluid interactions which are thought to be related to migrating connate brines during the Alleghenian Orogeny.