U-Th/He dating and chemical compositions of apatite in the dacite from the southwestern Okinawa Trough: Implications for petrogenesis

Apatite has recently gained considerable attention, providing new insight into a wide range of geological processes, such as geochronology, magmatism, metasomatism and metallogenesis. Here, we investigate the U-Th/He dates, in situ major- and trace- element concentrations, and Sr-87/Sr-85 ratios of apatites in the dacite from the southwestern Okinawa Trough to constrain its eruption age and the petrogenesis. The U-Th/He dating of apatites indicates that the volcanic eruption that formed the dacite was no older than 0.28 Ma and that the eruption was related to the replenishment of mafic magma. Apatites are present in the dacite as inclusions within pyroxene phenocrysts and are protected them from the external melt environment; therefore, they should retain a reliable record of their original chemical composition. In addition, all the apatite grains are fluorapatites with F contents of 1.58-2.54 wt.% and Cl contents of 0.57-0.88 wt.%. These grains are enriched in REEs (FREE = 3341-5527 ppm) with significant Eu anomalies (Eu/Eu* = 0,20-0.24), low Sr/Y radios (0.20-0.35), high Sr-87/Sr-86 ratios (range from 0.70635 to 0.70697). Based on these characteristics, we propose that the dehydration of subducted slab released fluids into the overlying mantle wedge and triggered mantle melting, which produced primitive mafic magma with high Cl content. Then, the mafic magma ascended into the crust, which has low Cl concentrations and high Sr-87/Sr-86 ratios. The mixing of a mantle-derived basaltic magma and a crustal felsic magma in a shallow magma chamber diluted the Cl content and elevated the Sr-87/Sr-86 ratio, thus producing the parental magma of the dacite. When the magma chamber was replenished by mafic magma at approximately 0.28 Ma, the high temperature from the injection would have caused the felsic melt to move upward and finally erupt.