SOUND VELOCITIES OF CLINOHUMITE, AND IMPLICATIONS FOR WATER IN EARTHSUPPER-MANTLE

The velocities of longitudinal and shear acoustic waves were measured for clinohumite, a possible host for water in the upper mantle. Brillo uin scattering measurements on a natural crystal under ambient conditi ons indicate that the hydration of olivine to clinohumite lowers most of the single-crystal elastic moduli, as well as the bulk and shear mo duli of a polycrystalline aggregate. However, the velocities of mantle rocks in which all olivine is hydrated to clinohumite are only 1-1.5% lower than those of the equivalent dry rocks under ambient conditions . These results suggest that in shallow, cold portions of the uppermos t mantle (for example the seismic lid), water may be difficult to iden tify seismologically if all of it is bound up in humite-group minerals . Negative velocity anomalies in the shallow mantle may require a diff erent explanation than the presence of hydrous minerals.