There are essentially two main types of mobile phases in the Earth's c
rust and mantle: silicate melts and hydrous fluids. Near the surface o
f 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 mutu
al equilibrium become indistinguishable(1-5). Here we report the direc
t visual observation of this phenomenon in the albite-H2O system using
an externally heated diamond anvil cell, Our results suggest both tha
t there should be complete miscibility between silicate melts and hydr
ous fluids in the deeper parts of the upper mantle and that, in the pr
esence of a hydrous fluid, a melting temperature for this part of the
mantle can no longer be defined.