Fluid-fluxed melting of mantle versus decompression melting of hydrous mantle plume as the cause of intraplate magmatism over a stagnant slab: Implications from Fukue Volcano Group, SW Japan

The Pacific Plate subducting from the Japan Trench has accumulated in the mantle transition zone beneath NE Asia, and intraplate magmatism has been active above the stagnant Pacific slab. To understand the origin of the intraplate magmatism in relation to slab stagnation, a petrological and geochemical study was carried out on basaltic samples from a monogenetic volcano of the Fukue Volcano Group, southwest Japan. The eruption products consist of low-Si and high-Si groups, and the two magmas are hypothesized to originate from different mantle source material based on radiogenic isotopic compositions. The H2O contents of the primary magmas were estimated as ~ 2 wt.% for both the low-Si and high-Si groups. Analyses using multicomponent thermodynamics suggested that the low-Si and high-Si primary magmas were generated at ~ 2.5 GPa and 1345 °C and at ~ 1.8 GPa and 1285 °C, respectively. These results, and the geochemical characteristics of the products, indicated that the low-Si magma was generated in the asthenospheric mantle whereas the high-Si magma was produced by interaction of the low-Si magma with the sub-continental lithospheric mantle. The low mantle potential temperature of ~ 1300 °C and hydrous nature (H2O/Ce = ~ 650) of the low-Si magma suggested that the magma was generated by fluid-fluxed melting of the asthenospheric mantle. Based on these results and those obtained in previous studies, intraplate magmatism over the stagnant Pacific slab can be summarized as having been caused by either melting of the asthenospheric mantle through an influx of fluids derived from the mantle transition zone or decompression melting of a hydrous mantle plume derived from the mantle transition zone. We infer that the fluids for the flux melting have been released from the mantle transition zone where water was locally saturated. Meanwhile, hydrous mantle plumes have been generated at the mantle transition zone where a return flow of sub-lithospheric mantle material entrained beneath the subducting Pacific slab has intruded from below.