Scanning and transmission electron microscope observations of iron oxides in remagnetized and unremagnetized carbonates.

Abstract

Scanning and transmission electron microscope (SEM/STEM) observations of iron oxides and related minerals in remagnetized and unremagnetized carbonates from several localities in eastern North America have been used to distinguish the differences in magnetic mineralogy of these carbonates in order to better understand the remagnetization processes. SEM/STEM observations of remagnetized carbonates (the Ordovician Knox, Trenton, Black River carbonates and the Devonian Onondaga and Helderberg limestones) provide evidence in favor of chemical remanent magnetization (CRM) rather than of viscous remanent magnetization (VRM). The evidence, such as cogenetic relation between magnetite and authigenic feldspar, inclusions of quartz and dolomite in magnetite, replacement by magnetite of pyrite either in the form of pseudoframboids or non-spherical grains, and the presence of abundant fine grained magnetite of uniform size and composition, indicates collectively that the formation of authigenic magnetite involves a chemical dissolution-crystallization process in the presence of fluids. Thus, the remagnetization in the carbonates is believed to be due to a CRM acquired when new magnetite formed long after the formation of the carbonates. Fluid-flow is thought to be triggered by tectonic activity nearest in time and space but the role and source of fluids may vary from locality to locality. SEM/STEM observations of unremagnetized carbonates (the Silurian Wabash and Mississippian Pride Mountain Formations) reveal several differences between the magnetic mineralogy of unremagnetized carbonates and that of remagnetized carbonates. Replacement of pyrite by hematite, the presence of large euhedral pure-iron oxides of secondary origin, and a rare occurrence of fine grained magnetite in unremagnetized carbonates are unique. The infrequent occurrences of the fine grained magnetite are inferred to be a diagnostic factor to distinguish the unremagnetized from the remagnetized carbonates, although the question of the remanence carriers in unremagnetized limestones remains unresolved. The presence of pure-iron spherical magnetite is no longer a diagnostic factor in the recognition of remagnetized carbonates because the spherical magnetite is found in both remagnetized and unremagnetized rocks.