Low-aTiO₂ in rhyolitic magma for the Kinpusan pluton case, central Japan

Abstract

Ti-in-zircon geothermometry is widely applied to silicic magmas. However, reliable temperature estimates depend on the activity of TiO₂ in the melt relative to rutile saturation (aTiO₂), which may change significantly during magma crystallization. We constrain realistic aTiO₂ values for high-silica rhyolitic magmas by integrating Ti concentration measurement in zircon with thermodynamic modeling of the Miocene Kinpusan granite, a shallow intrusion emplaced at ∼ 50 MPa in central Japan. Zircons show uniformly low Ti contents (interquartile range of ∼ 2.0-4.5 µg/g). MAGEMin phase-equilibrium modeling combined with zircon saturation calculations indicates that zircon saturation occurs at <800 °C and melt fractions ≤90 vol%. Within the zircon-melt coexistence interval (∼ 750-800 °C), both thermodynamic predictions and the observed Ti concentrations in zircon require low aTiO₂ values of ∼ 0.1-0.4, consistent with the petrographic absence of rutile and ilmenite. The models also predict a sharp increase in Ti concentration in zircon immediately above the solidus, implying that the highest Ti contents reflect eutectic crystallization. These results also support the idea that aTiO₂ must vary during crystallization and cannot be assumed constant in a rhyolitic magma system.