Problem of H₂O in the Mantle

Abstract

A small amount of H₂O would be present in the upper mantle. In the shallower levels of the upper mantle, H₂O may be contained in the crystal structures of some hydrous minerals such as amphibole and mica. It is most likely, however, that such hydrous minerals break down into anhydrous high-pressure mineral assemblage and H₂O with increasing depth, and that H₂O exists in the interstices between crystals as supercritical fluid with specific volume less than 1.0 in the deeper levels of the upper mantle. The temperature of the beginning of melting of the upper mantle is significantly lowered by the presence of H₂O phase, even if its amount is small.