Water-saturated solidus and second critical endpoint of peridotite determined from liquid texture and chemistry

The melting of peridotite plays a key role in the chemical differentiation of planetary bodies. Water-saturated ('wet') solidus of mantle peridotite defines initial melting temperature of Earth's mantle under water-saturated conditions and the second critical endpoint (SCEP) marks the end of the wet solidus. However, the location of the wet solidus remains an outstanding issue for over 50 years and the position of the SCEP is hotly debated. Published wet solidus shows a difference of 200 - 600 ^oC at given pressures, meanwhile, reported SCEP ranges from < 4 to > 6 GPa. Using a large-volume multi-anvil apparatus, we investigated the water-saturated melting behavior of a fertile peridotite (at 3 - 6 GPa, 950 - 1200 ^oC ) and obtained well-preserved quenched liquid. Based on the texture and chemistry of the quenched liquid, we successfully determined the wet solidus and the SCEP of peridotite (Fig 1 A).

The quenched fluids exhibit fragile fibres at 950 ^oC and spherule - fibre mixtures at temperatures above 1000 ^oC (Fig 1B). At 3 GPa, the quenched hydrous melt appears as a felt-like mass or as dendritic crystallites and coexist with the quenched fluids (Fig. 1B). We interpreted the presence of spherule - fibre mixtures as an 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 indicates that the wet solidus lies between 950 and 1000 ^oC at 3 GPa and that 3 GPa is lower than the critical pressure (Pc). 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. 1B) and the absence of which in the latter (Figs. 1C-D). T he 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 analysed by EDS. We found that with increasing pressures, the liquid compositions become more deficient in silica and richer in olivine components. The compositions of silicate melts or SCFs change consistently with pressure: andesitic at 1 GPa, boninite-like at 3 GPa, picritic at 4 GPa, and kimberlite-like at pressures > 5 GPa.