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 INTRODUCTION in a magma chamber is documented for an alkali basalt (Kutsugata The evolution of magma chambers is of major petrologic lava) from Rishiri Volcano, northern Japan, on the basis of zoning interest and has been studied extensively. Experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava. Plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types. Type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundAn-rich core (An71–90), which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase. Type 2 has an Ab-rich core surrounded by a calcic the chamber walls (e.g. Brandeis & Jaupart, 1986; Turner mantle characteristic of partial dissolution. Type 3 is characterized by et al., 1986; Marsh, 1988; Tait & Jaupart, 1992). Layered igneous complexes (e.g. Morse, 1969; Shirley, 1987; an Ab-rich core that is commonly surrounded by a reversely zoned McBirney, 1996) provide useful information on how slightly calcic mantle. Type 4 has an Ab-rich core without a calcic mafic magma chambers evolve and consolidate, but they mantle. The An-rich cores of the Type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole-rock a chamber, because those intrusions integrate processes composition at the estimated temperatures of the Kutsugata magma that occurred over long solidification times (McBirney & system. The An-rich cores are inferred to have been formed in a Noyes, 1979; Tait, 1988). Volcanic products, such as mushy boundary layer along the wall of a magma chamber, where lava flows and tephra deposits, can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber. There are a few olivine fractionation and possibly because of addition of water studies along this line of approach (Tait, 1988; de Silva, expelled from the chilled margin and/or groundwater from the 1989; Nakada et al., 1994). These studies focused on surrounding crust. The Ab-rich cores of Types 2–4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir. The Types 2 and 3 Ab-rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection. The temporal and spatial variations reservoir. However, these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers. and chemical features between silicic and mafic magma chambers. In this paper, the magmatic evolution of an alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan, is studied. The Kutsugata lava is divided into three lava flows on the basis of whole-rock chemical