Magnetic petrology through Fe-Ti oxide minerals in various modes of eruption: case studies of Unzen and other volcanoes

Volcanic eruptions generate a variety of materials depending on varying magma, degrees of degassing and different cooling process. Therefore, it is important to understand what condition the material is generated from. Iron-titanium oxide minerals are very useful for studying the generation condition. Their initial chemical compositions record the equilibrium state. Successively they are oxidized in deuteric cooling process and transformed into composite multiphase grains whose phases have distinct chemical compositions. In addition, two solid solution series, titanomagnetite and titanohematite, show magnetic properties and acquire thermoremanent magnetization during cooling from above the Curie temperature. Their magnetic properties depend on compositions, grain sizes and amount of oxide minerals. Therefore, if we identify iron-titanium oxides and determine their properties, we can estimate the equilibrium state and the oxidation process of iron-titanium oxides during eruption. Such a method of study has been recently graced with a formal name "magnetic petrology". We have carried out magnetic petrological study in some volcanoes in order to understand a variety of volcanic materials. In order to understand the oxidation state in lava dome, magnetic petrological analyses were carried out on lava samples from the lava domes and block-and-ash-flow deposits of the 1990-1995 eruption of Unzen volcano, Japan. As a result, we found that lava samples were classified into three types with different iron-titanium oxide mineral assemblages and deuteric oxidation in the lava dome separated samples of each type. By field survey of Unzen lava dome, we found that the exogenous dome consists of fresh lava, while the endogenous dome oxidizes and produces oxidized blocks in the surface. This is because the exogenous dome generated much fresh lava due to successive highly supply of new lava, while the endogenous dome was exposed to the air for a long time at high temperature and oxidized because the hot lava slowly intruded within the dome. We will show some evidences in terms of application of our methods in magnetic petrology to other volcanoes including Suwanose volcano in SW Japan and Kilauea volcano in Hawaii.