Long-term magma recharge episodes recorded in plagioclase zoning in the Okama pyroclastics at Zao Volcano, northeastern Japan

Motohiro Sato, Masao Ban, Ryuichi Shinjo, Ke-Han Song, Takashi Yuguchi, Tatsuya Adachi, Takumi Imura

Mafic magma recharge into shallow plumbing systems is one of the most important processes for remobilizing crystal mush-dominated magma reservoirs, eventually triggering eruptions. Previous studies have mainly focused on pre-eruptive recharge episodes, revealing short mixing-to-eruption timescales through diffusion chronometry applied to fast-diffusing elements. However, the existence of complexly zoned crystals, commonly observed in arc intermediate magmas, implies multiple recharge episodes. Long-term variations in the rate of mafic magma supply remain unclear, hindering the interpretation of geophysical monitoring data during unrest. Here, we present plagioclase residence timescales from pyroclastic deposits associated with the historical activity at Zao Volcano, northeastern Japan. Compositional zoning patterns of anorthite content within plagioclase crystals record the long-term evolution of magmatic processes due to the slow diffusivity of CaAl–NaSi interdiffusion. The wide diversity of zoning complexities observed at the thin-section scale indicates the remobilization of mushy reservoirs within spatially restricted regions. The distribution of diffusion timescales derived from CaAl–NaSi and Sr in plagioclase reveals that mafic magmas continuously recharged the shallow reservoirs from approximately 100 years before eruption until shortly before the onset. These long-term mixing episodes highlight the importance of continuous monitoring in active arc volcanoes.