Origin and evolution of silicic magmas in the Okinawa trough: insights from coupled cathodoluminescence, trace element, and oxygen isotope analyses of quartz

The Okinawa Trough, a continental-margin back-arc basin in the northwestern Pacific Ocean, is characterized by numerous hydrothermal fields and spatially consistent submarine volcanoes, indicating the presence of active, shallow magma chambers. The potential for volcanic eruptions, coupled with its proximity to populated areas, underscores the need for comprehensive studies of the underlying magmatic processes. In this study, we perform a detailed investigation of cathodoluminescence imaging, trace elements (Ti), and oxygen isotopes in quartz from two rhyolite samples collected from the middle and southwestern Okinawa Trough (MOT and SWOT). We observe varying proportions and CL patterns of rapid growth features in quartz from SWOT (40%) and MOT (4%), which imply different magmatic processes. Step zoning, involving Ti-poor and Ti-rich overgrowths, is prevalent in both regions and may reflect temperature variations rather than changes in TiO₂ activity or pressure. Oscillatory and step zoning represent the products of near-equilibrium crystallization, and their Ti contents can, therefore, be used to reconstruct the P-T conditions of the magma. Based on the estimated crystallization temperatures of 750 °C and 770 °C for SWOT and MOT quartz, respectively, the depths of the shallow silicic reservoirs for SWOT and MOT are inferred to be ~ 9 km. Jagged, bright cores (with high Ti content and variable δ¹⁸O values) in MOT quartz are interpreted as xenocrysts, providing mineralogical evidence for the assimilation of country rocks. The δ¹⁸O values, corrected for fractional crystallization and representing primary magmatic signatures, suggest that the SWOT magma has undergone greater contamination by upper crustal material (15–25%) than the MOT magma (< 18%). Moreover, our study reveals a slight negative correlation between δ¹⁸O values and grayscale values (or Ti content). This relationship can be readily explained by a near-wall crystallization mechanism, which also accounts for the common occurrence of Ti-poor overgrowths. Additionally, we attempted to constrain the crystal residence time using the diffusion of Ti in quartz, but the uncertainties of available calibrations largely hampered the interpretation. Nonetheless, if long-term near-wall crystallization is accepted, the mush model would imply that the magma chamber is primarily melt-dominated. Collectively, the distinctive textures and chemistry support the robustness of quartz as a reliable tracer for magmatic processes in shallow silicic reservoirs beneath the seafloor.