Two types of chemical remanent magnetization during the oxidation of magnetite

Abstract

The chemical remanent magnetization (CRM), M CRM, resulting from oxidation of equidimensional magnetite to a mixture of 90% hematite and 10% cation-deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization, ( M ARM), of the parent magnetite and the field H CRM applied during oxidation. M CRM lies in the plane defined by H CRM and M ARM (which were perpendicular in our experiments) in an intermediate orientation determined by the relative strengths of H CRM (0.05, 0.1 and 0.2 mT) and M ARM (0.002–0.12 × saturation remanence). The CRMs are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization. Similar experiments in which acicular single-domain cation-deficient magnetite was oxidized to maghemite gave quite different results. M CRM was always parallel to M ARM. Its direction was unaffected by field strengths as high as H CRM = 1.5 mT. The essential factors controlling CRM direction seem to be the number and crystallographic structure of oxidation products. For single-phase oxidation without change in crystal lattice, CRM remains parallel to the initial remanence of the parent mineral. For multiphase oxidation to a mixture of rhombohedral and spinel phases, both initial remanence and the field acting during oxidation influence the CRM direction, which therefore has an intermediate orientation that parallels neither M ARM nor H CRM.