Mafic Magma Recharge Triggered Eruption of a Long-Lived Shallow Silicic Magma Reservoir Beneath a Submarine Volcano from the Southwestern Okinawa Trough

Abundant submarine volcanoes have been identified in the southwestern Okinawa Trough, yet the duration of shallow magma reservoirs and processes driving their eruptions remains unclear. Here, the texture and in situ element compositions of orthopyroxene and magnetite in the dacite and mafic magmatic enclaves (MMEs) from the FDV-1 submarine volcano were investigated to decipher the duration of pre-eruption shallow silicic magma storage and eruption triggering processes. Both unzoned and zoned orthopyroxene phenocrysts were observed in the host dacite. The rims of zoned and unzoned orthopyroxene have similar compositions with Mg# from 49 to 55, Cr = 2.31-9.26 & mu;g/g, V = 54.10-137 & mu;g/g, and Ni = 16.44-21.19 & mu;g/g, and they are in equilibrium with the rhyolitic matrix, which is consistent with growth in the shallow silicic magma reservoir. In contrast, the high Mg# cores (Mg# = 63-79) in the zoned orthopyroxenes are in equilibrium with basalt, with significant enrichment in Cr (1092-1737 & mu;g/g), Ni (106-181 & mu;g/g), and V (183-295 & mu;g/g), which indicates a recycled (antecryst) origin by mafic magma recharge and magma mixing. Furthermore, the timescales from Fe-Mg interdiffusion in orthopyroxene phenocrysts suggest that the magma mixing is approximately 600 years before eruption. Correspondingly, the shallow silicic magma reservoir beneath the submarine volcano is longevity. The presence of bubble and V-depleted magnetite aggregates in the MMEs suggests that volatile-saturated and oxidized mafic magma recharges into the shallow silicic magma chamber. Moreover, the distinct contents of relatively fast-diffusing Ni in magnetite between the MMEs and host dacite suggest that the mafic magma in-jection into the silicic magma reservoir and formation of the MMEs must have occurred over very short time -scales (< 24 h) before the eruption to inhibit complete re-equilibration. Therefore, this last mafic magma recharge induces destabilization of the long-lived shallow silicic magma reservoir and leads to the dacitic magma eruption.