Elastic properties of hydrogrossular garnet and implications for water in the upper mantle

The single‐crystal elastic properties of a hydrous silicate garnet, hibschite (Ca 3 Al 2 (SiO 4 ) 1.72 (H 4 O 4 ) 1.28 ), were measured using Brillouin spectroscopic techniques. The adiabatic bulk modulus of hibschite, K s = 99.8±1.0 GPa, and the shear modulus, μ = 64.3±0.5 GPa, are 40% lower than the bulk and shear moduli for anhydrous grossular garnet Ca 3 Al 2 (SiO 4 ) 3 . This increased compressibility of hydrogarnet is attributed to increased hydrogen bonding with pressure in the H 4 O 4 tetrahedron. Density considerations indicate that hydrogarnet is likely to be stable relative to an assemblage with H 2 O as a separate phase throughout the upper mantle and probably the transition zone. Assuming garnet to be the sole repository for mantle water, the seismic wave velocities of a “wet” eclogitic layer are 6–8% lower than those of dry eclogite. A hydrated eclogitic layer several times thicker than the oceanic crust would probably be required for a water‐rich region of the mantle to be seismologically detectable. Lesser quantities of mantle water than those implied by the above scenario may be invisible to seismic techniques.