Magnetic glasses in the Azuki and AT volcanic ashes in Japan

Abstract

Magnetic properties of volcanic glass shards have been described on the widespread Azuki and AT ashes in Japan. Thermomagnetic analysis and isothermal remanent magnetization (IRM) acquisition experiments were performed on both bulk samples and separated glass samples of the Azuki volcanic ash. Together with progressive thermal demagnetization of natural remanent magnetization (NRM), the results demonstrate that titanomagnetite contained in volcanic glass shards is the sole contributor to the stable NRM of the Azuki ash. The results of a Lowrie-Fuller test suggest that the remanence carrier of the ash layer is single-domain grains of titanomagnetite, which can be seen as submicron particles using electron microscopy. On the basis of progressive IRM acquisition in samples with various contents of AT volcanic glass, it is also suggested that titanomagnetite in glass shards is a significant magnetic carrier of NRM for the AT ash. Since most of the glass shards are tens of microns in size, they are expected to acquire depositional remanent magnetization (DRM) without experience of succeeding post-depositional process under aqueous conditions. A redeposition experiment with the Azuki ash shows that stable DRM, whose intensity is comparable to that of the NRM of the water-laid Azuki ash, is acquired during several hours.