Magma Evolution Processes in the Southern Okinawa Trough: Insights from Melt Inclusions

The Okinawa Trough is an initial back-arc basin that is influenced by the subduction of the Philippine Sea Plate and develops on the continental crust. The Okinawa Trough is a natural laboratory for the study of basin evolution, magmatism, and crust-mantle processes in the early stage of back-arc spreading. Melt inclusions are small droplets of magma that are captured during the mineral crystallization process and can record the geochemical composition changes during magma evolution. In this study, the geochemical compositions of melt inclusions in host plagioclases of two volcanic rock samples at Station nos. 9-1 and 9-2 from the southern Okinawa Trough are systematically analyzed. Based on previous studies, the origin and evolution of magma and the introduction of subducting materials in the study area are discussed. Results show that melt inclusions are characterized by the relative enrichment of large-ion lithophile elements, depletion of high-field-strength elements, and slight enrichment of rare earth elements. Indeed, the subduction of the Philippine Sea Plate introduced sediment-derived melts and fluids into the magma source area of the southern Okinawa Trough. Subsequently, 4% to 5% partial melting of the hydrated mantle produces basaltic magma. The melt inclusions of andesite and dacite investigated in this study were formed by fractional crystallization of basaltic magma. Finally, the crystallization of plagioclase, pyroxene, and magnetite occurred during the late stage of magma evolution. The temperature-pressure data show that the melt inclusions in plagioclase have two capture periods: one is at temperatures above 1250°C and the other is at temperatures between 1180°C and 1200°C. The capture pressure of the inclusions at temperatures between 1180°C and 1200°C is between 5.6 kPa and 6.1 kPa, corresponding to the depth of 15–17km below the seafloor. The geochemical characteristics of major and trace elements in inclusions show that the samples from two stations ( i.e. , 9-1 and 9-2) have similar or identical magma source areas. However, the crystallization differentiation reflected by inclusions in sample 9-1 is more obvious than that in sample 9-2. The inclusions were captured during magma evolution and were not contaminated by crustal materials.