Our State of Understanding the Mechanisms for Carbonate Remagnetization: Unraveling the Causes and Consequences for Authigenic Fe-Oxide Production

Previous paleomagnetic and rock magnetic studies of carbonate units from around the world and across the geologic timescale have revealed a ubiquity of remagnetizations, and although the broad occurrence of remagnetizations are widely accepted, their mechanism, and in particular their relationship to orogeny and basinal fluids, is not fully understood. The general consensus is that secondary chemical remanent magnetizations (CRM) are the cause of most remagnetizations, and that thermal viscous remanent magnetizations are unlikely given the relatively low burial temperatures determined for most studied rocks. One of the well established consequences of these CRMs is their distinct rock magnetic fingerprints. Of which, hysteresis properties are arguably the most universal, with typical remagnetized carbonates having anomalously high coercivity ratios with respect to given remanence ratios. Such ratios are typically associated with bimodal distributions of vastly different magnetic coercivity phases caused by mixtures of either grain size (i.e., a combination of superparamagnetic (SP) and single-domain (SD) grains), magnetic mineralogy, or particle anisotropy. In remagnetized carbonates these ratios have been mainly attributed to an abundant volume of secondary SP grains, and thus by inference to the growth of new Fe-oxides during remagnetization. The question remains then - what produces such large volumes of authigenic Fe-oxides? Several Fe-oxide producing reactions have been proposed, including: 1) illitization, 2) dedolomitzation, and 3) oxidation of Fe-sulfides. Recent correlation of Ar/Ar ages for smectite-illite transformations with independently determined remagnetization ages from the same rocks suggests that clay reactions are potentially an important source for remanence-carrying authigenic Fe-oxides. However, there is still significant uncertainty as to the mechanism of this and other Fe-oxide producing reactions, and thus more work is needed to better understand the remagnetization process. Such work should ultimately lead to better understanding of rock-fluid interactions, maturation and migration of hydrocarbons and ore-generating fluids, and orogenic evolution.