Phenocryst Zoning Patterns Developed during Evolution of Magma Mixing: Case Studies from Northern Honshu and Western Java Arcs

Abstract:

Phenocryst textures were observed to investigate elementary steps of magma mixing processes, using the andesite samples from two western Pacific island arcs; northern Honshu, Japan, and West Java, Indonesia. Andesite from Chokai volcano in northern Honshu was extensively investigated to determine endmember compositions from glass inclusion compositions, mineral compositions, and whole-rock compositions. Reversely zoned pyroxene phenocrysts coexist with normally zoned ones. Every reversely zoned pyroxene crystal consists of a magnesium-poor core that is rounded or irregular in shape and a more-magnesian, normally zoned margin. The margin exhibits concentric oscillatory zoning with broad normal zoning, implying overgrowth in a chemically fluctuating mixing magma. The thicknesses of margins vary crystal by crystal, and the chemical compositions of innermost margins are well correlated with the thicknesses, indicating that the commencement timing of overgrowth is different on every crystal. Most of normally zoned pyroxene exhibit identical characteristics with the margins of reversely zoned pyroxenes. Therefore, these normally zoned pyroxene crystals started growing in the chemically fluctuating magma, or during magma mingling, although magnesian cores of some normally zoned pyroxenes might be derived from mafic basalt endmember magma. Plagioclase phenocrysts exhibit very similar characteristics with the pyroxenes, involving coexistence of reversely zoned crystals and normally zoned crystals. Margins of reversely zoned crystals exhibit oscillatory bands with broad normal zoning, which is the identical occurrence with most of normally zoned plagioclase. Every core of reversely zoned plagioclase exhibits a resorption texture including dusty zoning. These crystal textures occur in mixed andesite from all examined volcanoes in two island arcs, implying that the formation of the textures is ubiquitous regardless of distance from volcanic fronts. The textures record the elementary steps of magma mixing involving resorption of felsic magma and crystallization in cooled mafic magma during thermal equilibration after the encounter, and oscillatory zoning produced by chemical fluctuation during traveling in heterogeneous mingling magma.