The types and tempos of magmatic processes that preceded historic eruptions at Miyakejima volcano, Japan

Abstract:

For future unrest scenarios at volcanic islands of the Izu-Bonin arc in Japan, it is critical that effective hazard mitigation can be enacted based on a detailed understanding of precursory activity for eruptions in the past. Eruptions in A.D. 1874 and 2000 at Miyakejima volcano present valuable case studies for examining the processes of basaltic-andesite magma recharge in this setting. Within the context of prior petrological investigations and geophysical monitoring data, new results from olivine Fe-Mg diffusion modeling are presented here to constrain the types and tempos of processes that lead to eruptions at this frequently active island arc volcano. The products of eruptions in A.D. 1874 and 2000 have populations of olivine with high-Fo cores and low-Fo rims, which record pre-eruptive magma hybridization events. Olivine chemical zoning patterns were modeled by performing fits to the natural profiles using a finite difference scheme that accounts for the variables that affect diffusivity: temperature; pressure; oxygen fugacity; major element composition; anisotropy. Diffusion timescales of 1–4 days for olivines from the 3 July 1874 eruption are consistent with reports of the onset of effusion from flank fissures shortly after a period of seismic activity. Diffusion timescales of 10–20 days for olivines from the 14 July 2000 eruption are consistent with the timing of dike propagation and related seismic activity from 2 weeks before this explosive eruption at the summit. These eruptions at Miyakejima occurred rapidly after magma hybridization events in the shallow plumbing system (<10 km depth). Considering the regular eruption rate at Miyakejima recently (1940, 1962, 1983, 2000), these results highlight the need to be well-prepared for rapid-onset flank and summit eruptions in the near future.