Anomalous unblocking temperatures, viscosity and frequency-dependent susceptibility in the chemically-remagnetized Trenton limestone

Abstract

Chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine-grained magnetite, which can commonly be recognized by a distinctive set of rock-magnetic attributes. These carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence. Recent work has shown that departures from Néel theory (as applied in the model of Pullaiah et al.) can generally be attributed to VRM carried by multidomain particles, to which the theory does not apply. In the remagnetized carbonates, however, rock-magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single-domain particles. Therefore we explore possible alternative causes for the anomalous unblocking temperatures, including maghemitization and magnetocrystalline rather than shape anisotropy. New blocking–unblocking models for these cases, however, fail to match the observed behavior, leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers. A possible alternative explanation is an increase in coercivity due to maghemitization of single-domain particles after acquisition of the viscous overprint, causing a significant increase in their unblocking temperatures. New frequency-dependent susceptibility measurements at low temperatures allow us to focus on the time–temperature behavior of the particles at the superparamagnetic-stable single-domain threshold, and we find that these do closely follow the predictions of Néel theory, as formulated in the models of Pullaiah et al. and Walton–Middleton–Schmidt.