Shift and rotation of composition trends by magma mixing: 1983 eruption at Miyake-jima Volcano, Japan

Pre-eruption processes are investigated for magmas erupted in 1983 from Miyake-jima volcano, which is one of the most active volcanoes in Japan. The whole-rock compositional trends of the eruptive products are principally smooth and linear. Magmas erupted from some fissures have compositions that deviate from the main linear trend. Phenocryst contents of samples displaced from the linear compositional trends are significantly lower than those of samples on the main trends. Anorthite-rich plagioclase phenocrysts, present throughout the 1983 products, are too calcic to have crystallized from the erupted magma composition, and were derived from a basaltic magma through magma mixing. Although the linear whole-rock composition trends favor simple two-component magma mixing, this cannot explain the presence of samples that deviate from the main trend. Instead, the observed composition trends were formed by mixing of a homogeneous basaltic magma with andesitic magmas exhibiting compositional diversity. The original linear composition trends of the andesitic end-member magma were rotated and shifted to the direction of the basaltic end-member magma by magma mixing. The samples out of the main trends represent magmas with less basaltic component than those on the trend. The density and viscosity of the basaltic end-member magma were comparable with those of the andesitic end-member magmas. The basaltic magma, discharged from one magma chamber at ∼2 kbar pressure, was injected into a magma chamber at lower pressure occupied by the chemically zoned andesite magma (∼1 kbar), and possibly as a fountain. To establish the characteristic mixing trend of the 1983 magma, the basaltic component must have been distributed systematically in the zoned andesite magma. A requirement is that the basaltic magma spread laterally and mixed with the andesite magma at various levels of ascent of the fountain in the host andesite magma. Analysis of compositional zoning in titanomagnetite crystals revealed that the eruption of the 1983 magmas was initiated soon after the replenishment of the basaltic magma in the 1 kbar magma chamber.