[SCG64-P05] Water-saturated solidus and second critical endpoint of peridotite determined by textures and chemistry of the quenched liquid
Keywords:wet solidus, peridotite, second critical endpoint
At 3 GPa, the quenched fluids exhibit fragile fibres at 950 ℃ and spherule–fibre mixtures at > 1000 ℃ (Fig. B&C). While the quenched hydrous melt appears as a felt-like mass or as dendritic crystallites and coexists with the fluids (Fig. C). We interpreted the presence of spherule–fibre mixtures as evidence for aqueous fluid above the solidus and fragile fibres as evidence for aqueous fluid below the solidus. Thus, the occurrence of quenched melt and spherule–fibre mixtures indicate that the wet solidus lies between 950 and 1000 ℃ at 3 GPa and that 3 GPa is lower than the critical pressure. The most important textural difference between the run products at 3 GPa and those at other pressures (4 and 6 GPa) is the presence of aqueous fluids in the former (Fig. B&C) and the absence of which in the latter (Fig. D&E). The spherule–fibre mixtures were not found in the 4 and 6 GPa run products. Liquids quenched from 4 and 6 GPa run products are homogeneous (SCF supercritical fluid), suggesting that Pc is lower than 4 GPa.
Compositions of the liquids were analyzed by averaging broad area using EDS. We find that the liquid compositions become more deficient in silica and richer in olivine components with increasing pressures. In conjunction with previous studies, the compositions of silicate melts or SCFs change regularly with pressure: andesitic at 1 GPa, boninite-like at 3 GPa, picritic at 4 GPa, and kimberlite-like at pressures > 5 GPa. These results propose that ultra-basic magmas can be formed in subduction zones at low temperatures, high pressure, and H2O rich conditions. On the basis of present experiments and detailed observation on the cross section of north Japan based on seismology and petrology, we propose a new magma genesis model in subduction zone (Takahashi & Wang, JpGU2020, S-CG64).
The water-saturated solidus and second critical endpoint of peridotite: Implications for magma genesis within the mantle wedge, Jintuan Wang, Eiichi Takahashi, Xiaolin Xiong, Linli Chen, Li Li, Toshihiro Suzuki and Michael Walter (2020) J. Geophys. Res.: Solid Earth, Vol.125, in press.