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

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