1 Introduction Recently, nominally anhydrous minerals (NAMs), such as olivine, pyroxene and garnet, have been found to contain trace amounts of hydrogen as OH? in their crystal lattice (Bell and Rossman, 1992; Rossman, 1996; Beran, 1999; Ingrin and Skogby, 2000; Williams and Hemley, 2001; Luth, 2003). Because of the importance of volume and mass in the mantle, NAMs probably represent the largest reservoir of hydrogen (Bell and Rossman, 1992; Ingrin and Skogby, 2000). Although the signi…