Pre-eruptive water content and volatile degassing processes in the southern Okinawa Trough magma: Implications for subduction zone water recycling and magmatic contributions to hydrothermal systems

Volatiles can act as a critical link that connects the magmatic and hydrothermal systems in subduction zones. Here, we present the first analyses of volatile (e.g., H2O, CO2, and S) concentrations in melt inclusions and matrix glasses of volcanic rocks from a modern arc front volcano (Irabu Knoll) and two back-arc spreading centers (Yaeyama Graben and Yonaguni Graben) in the southern Okinawa Trough, a nascent back-arc basin that is characterized by widespread volcanic and hydrothermal activity. The pre-eruptive H2O concentrations of magmas (∼2.2 to ∼3.1 wt%) were estimated using a combination of melt inclusion analyses and plagioclase-melt hygrometry. From the back-arc to the arc, the H2O/Ce of lavas increase monotonically with the increasing δ11B values, a good geochemical tracer for subducted serpentinites. By combining our data with those from other subduction zones (Mariana Arc, Mariana Trough, Tonga Arc, Lau Basin) and MORB, we established a robust correlation between H2O/Ce and δ11B, suggesting that subducted serpentinites might have a significant contribution to the water budget of arc magmas. The low CO2 concentrations in our melt inclusions (∼45–225 μg/g, after vapor bubble correction) and matrix glasses (<1 μg/g, below detection limit) as compared to >3000 μg/g in primary arc magmas, suggests that significant CO2 degassing has occurred during magma ascent and/or in the magma reservoir, which can release sufficient CO2 to support the high CO2 flux of the hydrothermal system in the Okinawa Trough. This also implies that hydrothermal CO2 in this area is not likely derived from fluid-rock interactions. In addition, the concentration of S in the magma decreases with the progressive loss of H2O, whereas the concentrations of F and Cl remain constant. Our results thus suggest that magma degassing is a potentially important source of CO2, S, and H2O to the hydrothermal system in the Okinawa Trough.