Elastic Properties of Hydrogrossular Garnet and Implications for Water in the Upper Mantle

Abstract

The single-crystal elastic properties of a hydrous silicate garnet, hibschite (Ca_3Al_2(SiO_4)_(1.72)(H_4O_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_3Al_2(SiO_4)_3. This increased compressibility of hydrogarnet is attributed to increased hydrogen bonding with pressure in the H_4O_4 tetrahedron. Density considerations indicate that hydrogarnet is likely to be stable relative to an assemblage with H_2O 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.

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