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

Journal Article · Fri Aug 10 00:00:00 EDT 1990 · Journal of Geophysical Research; (United States)

OSTI ID:5009053

Suk, Dongwoo; Van der Voo, R; Peacor, D R ^[1]

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1. Univ. of Michigan, Ann Arbor (United States)

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. The authors 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 are documented. 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. Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium-feldspar, illite, and quartz. The authors 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 remanent 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.

• OSTI ID: 5009053
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:B8; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
36 MATERIALS SCIENCE
CARBONATE ROCKS
MAGNETIC PROPERTIES
GEOLOGIC FORMATIONS
PALEOMAGNETISM
TENNESSEE
AGE ESTIMATION
CLAYS
CRYSTAL STRUCTURE
DISSOLUTION
DOLOMITE
ELECTRON DIFFRACTION
FELDSPARS
GEOLOGIC HISTORY
HEMATITE
ILLITE
IRON OXIDES
LIMESTONE
MAGNETITE
MAGNETIZATION
MORPHOLOGY
ORIGIN
PALEOZOIC ERA
PRECIPITATION
QUARTZ
SCANNING ELECTRON MICROSCOPY
TEXTURE
TITANIUM OXIDES
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION
CARBONATE MINERALS
CHALCOGENIDES
COHERENT SCATTERING
DEVELOPED COUNTRIES
DIFFRACTION
ELECTRON MICROSCOPY
FEDERAL REGION IV
GEOLOGIC AGES
IRON COMPOUNDS
IRON ORES
MAGNETISM
MICROSCOPY
MINERALS
NORTH AMERICA
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
ROCKS
SCATTERING
SEDIMENTARY ROCKS
SEPARATION PROCESSES
SILICATE MINERALS
TITANIUM COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
USA
580000* - Geosciences
360603 - Materials- Properties