Self-reversal of remanent magnetization in basalts due to partially oxidized titanomagnetites

Abstract

A range of basaltic samples from Olby (France) and Vogelsberg (Germany) displaying the phenomenon of partial and complete self-reversal was studied in order to decipher the physical mechanism responsible for self-reversed magnetic remanence in basalts. Microscopic observations and rock magnetic measurements show titanomagnetite to be the carrier of remanence. Due to low-temperature oxidation the titanomagnetites are present in two magnetic phases forming close side-by-side phase assemblages: The ore grains consist of a non-oxidized magnetically soft titanomagnetite part with Curie temperatures between 140 and 300 °C and a magnetically hard low-temperature oxidized part with Curie temperatures between 410 and 590 °C. Magnetic force microscopy observations present evidence that the oxidation process does not only influence the Curie temperatures, and thus the blocking temperature spectrum, but also the domain configuration. During acquisition of a thermoremanence the two phases are magnetically coupled, leading to a remanent magnetization of the low Curie temperature phase which is antiparallel to the applied external magnetic field. Computational modelling of the remanence acquisition process explains the coupling of the two phases by magnetostatic interaction. As the observed partial oxidation of ore grains is not uncommon in subaerial basalts, partial and complete self-reversal is probably a frequently occurring phenomenon. Nevertheless, it remains unnoticed in most cases as it cannot be detected by the standard stepwise thermal demagnetization technique. Despite the complex remanence properties, our investigations indicate that the upper part of the blocking temperature spectrum still carries reliable palaeodirectional information.