Self-Reversed Magnetization in Sediments Caused by Greigite Alteration

Abstract

Multipolarity remanence in greigite-bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite-bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single-polarity” to “multi-polarity” samples. The inward alteration of sulfide grains is topotactic and is size-dependent with higher alteration in smaller grains. We propose a multi-phase self-reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self-reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications.