Boundary Layer Crystallization in a Basaltic Magma Chamber: Evidence from Rishiri Volcano, Northern Japan

Abstract

The evolution of basaltic magma by mushy boundary layer processes in a magma chamber is documented for an alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan, on the basis of zoning patterns of plagioclase phenocrysts and their spatial distribution in the lava. Plagioclase phenocrysts with complex zoning patterns can be divided into four types. Type 1 is characterized by an extremely An-rich core (An71–90), which is commonly corroded and filled with sodic plagioclase. Type 2 has an Ab-rich core surrounded by a calcic mantle characteristic of partial dissolution. Type 3 is characterized by an Ab-rich core that is commonly surrounded by a reversely zoned slightly calcic mantle. Type 4 has an Ab-rich core without a calcic mantle. The An-rich cores of the Type 1 plagioclase are too calcic to have crystallized from a liquid represented by any whole-rock composition at the estimated temperatures of the Kutsugata magma system. The An-rich cores are inferred to have been formed in a mushy boundary layer along the wall of a magma chamber, where the magma was relatively cool and rich in water because of significant olivine fractionation and possibly because of addition of water expelled from the chilled margin and/or groundwater from the surrounding crust. The Ab-rich cores of Types 2–4 plagioclase phenocrysts are interpreted to have crystallized in the main magma body of the reservoir. The Types 2 and 3 Ab-rich cores were brought to the mushy layer by convection. The temporal and spatial variations of zoning patterns of plagioclase phenocrysts in basaltic lava flows give useful information on the evolution of mafic magma chambers.