Petrological Constraints on Trachytic Magma Beneath Ioto, Ogasawara, Japan: Implications for Ongoing Post‐Caldera Volcanism

ABSTRACT

Active volcanism commonly occurs in post‐caldera systems, many of which are characterized by crustal uplift associated with resurgent domes. Although geophysical studies have revealed diverse styles of post‐caldera deformation, petrological constraints on magma storage conditions during ongoing post‐caldera volcanism remain limited. Ioto volcano (Iwo‐jima), Japan, is one of the most rapidly uplifting volcanic systems in the world, with uplift rates reaching ~1 m year ⁻¹ . The first historically observed magmatic eruption occurred in 2022, providing a unique opportunity to investigate the physicochemical conditions of magma associated with ongoing post‐caldera activity. We conducted petrographic and geochemical analyses of pumice erupted during the 2022 eruption. The magma is trachytic in composition and contains ~7 vol.% phenocrysts of plagioclase, clinopyroxene, olivine, and magnetite. Mineral compositions are remarkably homogeneous and crystal clots indicate equilibrium among the major phenocryst phases. Application of clinopyroxene–liquid thermometry and plagioclase–liquid hygrometry yields a magma temperature of 970°C ± 30°C and a dissolved H ₂ O content of 1.4 ± 1.1 wt.%, respectively, indicating that the pre‐eruptive magma was characterized by relatively low dissolved H ₂ O contents. The inferred magma properties correspond to a relatively low magma viscosity of approximately 10 ⁴  Pa s. Assuming H ₂ O‐saturated conditions, the estimated dissolved H ₂ O content is consistent with the funnel‐shaped magma body inferred at depths of a few hundred meters to 2 km from geophysical observations. The relatively low viscosity would facilitate repeated magma intrusion through a shallow cone‐sheet system. We propose that repeated magma intrusion through a shallow cone‐sheet system may contribute to the rapid block‐like uplift of Motoyama. These results indicate that Ioto may represent one possible style of post‐caldera volcanism, in which repeated shallow intrusion of relatively low‐viscosity trachytic magma through a cone‐sheet system may contribute to persistent rapid uplift.