The evolution of crystal-poor rhyolite in the middle Okinawa Trough and its implications for the state of magma chamber

Abstract

The crystal-poor rhyolitic pumice is the product of extreme magma differentiation and is characterized by highly vesicular, honeycomb-like structure. However, little is known about the evolutionary history and the time of volatile saturation of the pumice-forming melt. A crystal-poor rhyolitic pumice (TVG7-1) has been collected from the Yoron Hole hydrothermal field in the middle Okinawa Trough, a young continental margin back-arc basin in the western Pacific. This study conducted a detailed analysis on the texture and in situ chemical composition of plagioclase, orthopyroxene and Fe-Ti oxides to reveal the evolution process, storage state and eruption triggers of eruptible silicic melt. Texture and zoning of specific minerals and mineral populations suggest a magmatic evolution history of open system. Additionally, the hollow reentrant texture, as well as results from plagioclase-liquid hygrometers, confirm a water-rich melt pocket. The exsolution of volatiles (H₂O, etc.), in this melt pocket, will contribute to local oxidation conditions, which may have been recorded by the concordant behaviour of FeO and An in plagioclase. Then, the overpressure caused by volatile exsolution would destabilize the magma chamber, which will eventually be broken by magma mixing/recharge, and lead to violent melt evacuation. Consequently, crystal size distribution (CSD) provides a new perspective on understanding the kinetic effect of magma mixing/recharge in the Okinawa Trough, especially on the mineral populations. Our study reveals the petrogenesis of crystal-poor rhyolite within the framework of the mush model, refines the magma evolution history and demonstrates that pumice magma reaches volatile saturation before the eruption.