Sound velocities of clinohumite, and implications for water in Earth's upper mantle

Abstract

The velocities of longitudinal and shear acoustic waves were measured for clinohumite, a possible host for water in the upper mantle. Brillouin scattering measurements on a natural crystal under ambient conditions indicate that the hydration of olivine to clinohumite lowers most of the single‐crystal elastic moduli, as well as the bulk and shear moduli 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 uppermost mantle (for example the seismic lid), water may be difficult to identify seismologically if all of it is bound up in humite‐group minerals. Negative velocity anomalies in the shallow mantle may require a different explanation than the presence of hydrous minerals.