Self-Reversal of Thermo-Remanent Magnetism of Igneous Rocks (III)

Abstract

Besides the dacitic pumice of Mt. Haruna, an igneous rock having the tendency of self-reversal of thermo-remanent magnetization has been found in a pitchstone of Mt. Asio. Although the self-reversal property of the Asio ferromagnetic minerals is weaker than that of the Haruna ferromagnetic minerals, the magnetic and crystallographic properties of the former are fundamentally same as those of the latter.By thermo-magnetic separation, an ensemble of ferromagnetic grains in both the Haruna and the Asio rocks were divided into A, B and AB groups. The results of thermo-magnetic measurement, chemical analysis, and X-ray analysis of these three groups showed that the A-constituent is a titanomagnetite of the crystal structure of inverse spinel type having the Curie-point at 430°-510°C, and the B-constituent is an ilmenite-hematite solid-solution of the rhombohedral crystal structure having the Curie-point at about 230°C, while the AB grains are composed of both A- and the B-constituents.The measurement of thermo-remanent magnetization of a large number of single grain gave the result that only the AB grains can have the reverse thermo-remanent magnetization, the A- and the B- grains getting the ordinary normal thermo-remanent magnetization.