Using Apatite to Track Volatile Evolution in the Shallow Magma Chamber below the Yonaguni Knoll IV Hydrothermal Field in the Southwestern Okinawa Trough

The Yonaguni Knoll IV is an active seafloor hydrothermal system associated with submarine silicic volcanism located in the "cross back-arc volcanic trail" (CBVT) in the southwestern Okinawa Trough. However, the behavior of volatiles during magmatic differentiation in the shallow silicic magma chamber is unclear. Here, the volatile contents of apatite inclusions trapped in different phenocrysts (orthopyroxene and amphibole) and microphenocrysts in the rhyolite from the Yonaguni Knoll IV hydrothermal field were analyzed by using electron microprobe analysis, which aims to track the behavior of volatiles in the shallow magma chamber. Notably, the 'texturally constrained' apatites showed a decreasing trend of X-Cl/X-OH and X-F/X-Cl ratios. Based on the geochemical analyses in combination with thermodynamic modeling, we found that the studied apatites were consistent with the mode of volatile-undersaturated crystallization. Therefore, volatiles were not saturated in the early stage of magmatic differentiation in the shallow rhyolitic magma chamber, and consequently, the metal elements were retained in the rhyolitic melt and partitioned into crystalline magmatic sulfides. Additionally, previous studies suggested that the shallow rhyolitic magma chamber was long-lived and periodically replenished by mafic magma. The injection of volatile-rich and oxidized subduction-related mafic magmas can supply abundant volatiles and dissolve magmatic sulfide in the shallow magma chamber. These processes are important for the later-stage of volatile exsolution, while the forming metal-rich magmatic fluids contribute to the overlying Yonaguni Knoll IV hydrothermal system.