Pre-eruptive magmatic processes of historical activities at Zao volcano, northeastern Japan: Insights from compositional zoning in orthopyroxene phenocrysts

Abstract

Textures and compositional features of minerals in igneous rocks indicate magmatic processes during their crystallization history. Orthopyroxene, one of the most ubiquitous rock-forming minerals, is an archive of magmatic conditions such as pressure, temperature, and melt composition. We examined orthopyroxene phenocrysts from the Okama pyroclastics (Okp) of Zao volcano, one of the most active volcanoes in northeastern Japan. Orthopyroxene phenocrysts in the Okp samples exhibited a wide range of compositions, disequilibrium textures, and complex zoning patterns, demonstrating multiple injections of mafic magma and mixing with shallow magma reservoirs. Orthopyroxene phenocrysts in the Okp coexist in equilibrium and disequilibrium with the melt in composition, and record a complex history with compositional zoning that reflects crystallization in three magmatic environments (MEs): M1 (Mg# 68–76), M2 (Mg# 62–68), and M3 (Mg# 55–62). The connection pathways of these MEs were identified using the zoning patterns of the orthopyroxene phenocrysts. The connection between M1 and M2 actively interacted during the entire period, resulting in the formation of Okp. The modeling of Fe–Mg interdiffusion in orthopyroxene phenocrysts provides residence time from interactions with different magma environments to the eruption. Orthopyroxene phenocrysts record a wide range of timescales, between a day and 100 years. Mixing between M1 and M2 commonly occurred 3–7 years (from compositional gradients of broad high-Mg bands) and a day–3 years (from compositional gradients of narrow high-Mg bands). This variation in residence time was caused by the injection of mafic magmas (M1) into the shallow magma reservoir (M2 and M3) at different times. Combining compositional zoning analysis and diffusion modeling helps develop a comprehensive model of the plumbing system beneath arc volcanoes.