Partial Self-reversal of Remanent Magnetization and Anisotropy of Viscous Magnetization in Basalts

Abstract

Summary The Rauher Kulm basalt from Oberpfalz, West Germany contains homogeneous titanomagnetites as its only ore mineral. Samples of this basalt have been progressively oxidized by heating in air at 400°C for prolonged times in the laboratory. During the heatings a CRM was built up in the oxidized (daughter) phase in a field of 0-450e and the samples were then cooled in zero field to room temperature. A TRM was nevertheless acquired by the mother phase due to an internal magnetic field originating in the CRM of the daughter phase. The strength of this interaction has been calculated to be 0.15 Oe and is sufficient to influence the growth and decay of viscous magnetization at room temperature. The magnetic viscosity of the heat treated rock is noticeably stronger than that of the natural rock and is attributed to the existence of very small nuclei of the daughter phase. With progressive heat treatment at temperatures up to about 400 C in the laboratory, and in nature with the passage of geological time, when temperatures of up to 200 C may have been attained, these nuclei grow and in so doing acquire a stable remanence the strength of which is influenced by the interaction field from the TRM of the mother phase. This phenomenon is thought to be of importance in the interpretation of marine magnetic anomalies because basalts from the Ocean floor are in fact slightly oxidized. In particular, the relatively large size of the central anomaly may be explained in this way.