Characteristics of hydrothermally altered minerals in volcanic products at Tokachidake volcano, central Hokkaido, Japan

Abstract

Volcanic products from Tokachidake volcano were observed mineralogically and petrographically, focusing on hydrothermal alteration of ash particles to interpret the pre-eruptive interaction between magma and the subvolcanic hydrothermal fluid. The 4.7 ka Ground Crater pyroclastic flow deposit, the 3.3 ka Ground Crater pyroclastic flow deposits, and the 1926 AD lahar deposits were examined. Each sample consists of ash grains undertaken alteration in various degrees from unaltered to intensely altered. Based on mineral assemblage, the alteration of ash is classified into three types: silica type (silica mineral alone), alunite type (silica mineral-alunite±kaolin), and kaolin type (silica mineral-kaolin). Most ash grains in the 4.7 ka and 1926 AD products belong to either of silica and alunite types. The 3.3 ka products are rich in kaolin-type ash and exhibit a stratigraphic variation in the proportion of unaltered ash: unaltered ash increases with the stratigraphy. All the alteration observed in the ash imply acid hydrothermal alteration, regardless of degrees of alteration. The weakly altered ash grains consist of unaltered volcanic rock and acid alteration parts, indicating that an acid hydrothermal fluid-rock interaction at open-fluid flow system formed their altered rock texture. It shows the reaction process proceeding during discharging the reactant fluid from the inside of the rock. These features reflect an unstable process of a short-lived subvolcanic hydrothermal system accompanying with a magma intrusion to the shallow potion and also with the reaction between the rock and acidic fluid that passed through the rocks.