Keywords:Volcanic fluids, Fluid-Rock interaction, altered minerals
Altered volcanic products from Tokachidake were mineralogically observed to interpret interaction between volcanic fluids and rock, by using XRD, Raman spectroscopy, and SEM-EDS. We collected samples from the 4.7 ka pyroclastic flow deposit (Gfl-0), lower and upper units of the 3.3 ka pyroclastic flow deposit (Gfl-1 and Gfl-2), and the 1926AD eruption deposits consisting of the lower debris avalanche deposit (Unit A), the middle hydrothermal surge deposit (Unit B), and the upper debris avalanche deposit (Unit C). Each product contains unaltered ash grains consisting of primary igneous minerals and volcanic glass, weakly-altered ash grains in which unaltered part coexists with altered minerals, and intensely-altered ash grains consisting only of altered minerals. Individual ash grains have one of three types of altered mineral assemblages: silica mineral (silica type), silica mineral-alunite±kaolin (alunite type), and silica mineral-kaolin (kaolin mineral type). Most ash grains in Gfl-0 have undergone alteration that produces the alunite type. The samples from Gfl-1 contain abundant kaolin mineral type ash, subordinate alunite type ash, and minor unaltered ash grains. Alteration types in the Gfl-2 deposit are similar to those of Gfl-1, but unaltered ash grains are more abundant in Gfl-1. Most of the ash grains in the 1926AD products underwent alteration which produced mainly silica and alunite types. These mineral assemblages in every product indicate only acidic alteration. The presence of unaltered parts in the most abundant weakly-altered ash indicates rock alteration by a brief, incomplete chemical reaction. For such brief and incomplete reaction, the followed two fluid-rock interactions can be available. One is water-rock interaction which acid hydrothermal water reacts with rocks. Another is vapor-rock interaction which volcanic vapor separated from magma reacts with rocks. Thus, the presence of weakly-altered ash suggests that rock alteration occurred by the brief, incomplete fluid-rock interactions was undergoing an acid-hydrothermal system and/or a volcanic vapor-dominated system developed under the crater when a magma intrudes and degasses. This concluded that the conditions of rock alteration at Tokachidake volcano can be controlled by a magma intrusion.