Direct observation of complete miscibility the albite-H2O system

There are essentially two main types of mobile phases in the Earth's crust and mantle: silicate melts and hydrous fluids. Near the surface of the Earth, the physical and chemical properties of these phases are fundamentally different, But with increasing pressure, the solubility of water in silicate melts and the solubility of silicate materials in hydrous fluids increase. This has led to the suggestion that, above a certain critical pressure, the compositions of the two phases in mutual equilibrium become indistinguishable(1-5). Here we report the direct visual observation of this phenomenon in the albite-H2O system using an externally heated diamond anvil cell, Our results suggest both that there should be complete miscibility between silicate melts and hydrous fluids in the deeper parts of the upper mantle and that, in the presence of a hydrous fluid, a melting temperature for this part of the mantle can no longer be defined.