The Ferromagnetic Minerals in the Aso Pyroclastic Flow Deposits

Examination of the Reliability for Correlating the Different Units by Lattice Parameters

Abstract

Recently, it has been widely recognized that tephras are important key beds for studies of the Quaternary System. In this connection, many methods of correlating tephras and pyroclastic flow deposits have been proposed.
The author has examined the assemblage of the ferromagnetic minerals and their lattice parameter contained in the Aso pyroclastic flow deposits, which are the largest pyroclastics in Central Kyushu. They are classified into 4 compound cooling units. Aso-1, Aso-2, Aso-3 and Aso-4.
Two boreholes were used for studying the vertical variation of the characteristics of ferromagnetic minerals. In addition, surface samples were collected in various locations around Aso caldera. After the magnetic separation of ferromagnetic minerals from samples, the species and the lattice parameter of each sample were examined by X-ray diffraction method.
Titanomagnetite, titanomaghemite and hemoilmenite were identified. The degree of welding and weathering do not seem to affect the characteristics of ferromagnetic minerals. Each unit of the Aso pyroclastic flow deposits is characterized by the assemblage of ferromagnetic minerals and the lattice parameter of titanomagnetite. In particular, Aso-4, which is a largescale erupution of Aso volcano, is characterized as titanomagnetite with the average lattice parameter 8.417Å. Thus ferromagnetic minerals can be used as one correlation criterion.