マグネティックペトロロジー―火山学への適用―

Abstract

Magnetic petrology is the science of magnetic minerals, mainly iron-titanium oxide minerals. It aims to identify the magnetic carriers and to determine the factors that control the occurrence and abundance of these phases by using two traditional fields of study, rock magnetism and petrology. By rock magnetic analyses, we can find the magnetic properties, that give us information about magnetic minerals. By petrological analyses, we can find the occurrence of oxide minerals. By integrating both types of information, the origin of magnetization and the evolution of oxide minerals can be considered. In volcanic materials, there are abundant iron-titanium oxide minerals and they show a wide variation in a single lava dome, a single lava flow and one sequence of tephra. Therefore, by magnetic petrological analyses, we can estimate the genesis and evolution of magma. In this paper, I describe three studies about magnetic petrology. 1) Oxides can be frequently oxidized during initial cooling between the magma reservoir and a deposit. According to the difference of temperature, cooling rate and oxygen fugacity, various oxide minerals are produced in lava dome eruptions. 2) In many volcanic materials, like lava flows or tephra, fine-grained oxides can be crystallized in a matrix during the cooling process. This process is important in considering the genesis of the magma or the materials. 3) Heating experiments indicate that exsolution titanohematite lamellas in titanomagnetite can be an indicator of heating temperature and duration.