ELASTIC PROPERTIES OF HYDROGROSSULAR GARNET AND IMPLICATIONS FOR WATER IN THE UPPER-MANTLE

The single-crystal elastic properties of a hydrous silicate garnet, hi bschite (Ca3Al2(SiO4)1.72(H4O4)1.28), were measured using Brillouin sp ectroscopic techniques. The adiabatic bulk modulus of hibschite, K(S) = 99.8 +/- 1.0 GPa, and the shear modulus, mu = 64.3 +/- 0.5 GPa, are 40% lower than the bulk and shear moduli for anhydrous grossular garne t Ca3Al2(SiO4)3. This increased compressibility of hydrogarnet is attr ibuted to increased hydrogen bonding with pressure in the H4O4 tetrahe dron. Density considerations indicate that hydrogarnet is likely to be stable relative to an assemblage with H2O as a separate phase through out the upper mantle and probably the transition zone, Assuming garnet to be the sole repository for mantle water, the seismic wave velociti es of a ''wet'' eclogitic layer are 6-8% lower than those of dry eclog ite. A hydrated eclogitic layer several times thicker than the oceanic crust would probably be required for a water-rich region of the mantl e to be seismologically detectable. Lesser quantities of mantle water than those implied by the above scenario may be invisible to seismic t echniques.