Evidence for a short-lived stratified magma chamber: petrology of the Z-To5 tephra layer ( c . 5.8 ka) at Zao volcano, NE Japan

Abstract

AbstractVolcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150–1200 °C), with high Mg (Foc. 81) olivine; basaltic andesite (1020–1100 °C), with Mg-rich orthopyroxene (Mg#=c. 78) and clinopyroxene (Mg#=c. 78), lower Mg olivine (Mg#=c. 78), and calcic (Anc. 85) plagioclase; and andesite (900–1000 °C) with Mg-poor orthopyroxene (Mg#=61–66) and clinopyroxene (Mg#=64–68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Foc. 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.