Elastic Properties of Hydrogrossular Garnet and Implications

for Water in the Upper Mantle

BRIDGET O'NEILL

Department of Geology and Geophysics, University of California, Berkeley

JAY D. BASS

Department of Geology, University of Illinois, Urbana

GEORGE R. ROSSMAN

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena
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Abstract

The single-crystal elastic properties of a hydrous silicate garnet, hibschite (Ca₃A1₂(SiO₄)_1.72(H₄O₄)_1.28), were measured using Brillouin spectroscopic techniques. The adiabatic bulk modulus of hibschite, Ks = 99.8+-1.0 GPa and the shear modulus, m = 64.3+-0.5 GPa are 40% lower than the bulk and shear moduli for anhydrous grossular garnet Ca₃Al₂(SiO₄)₃. This increased compressibility of hydrogarnet is attributed to increased hydrogen bonding with pressure in the H₄O₄ tetrahedron. Density considerations indicate that hydrogarnet is likely to be stable relative to an assemblage with H₂O 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.

Journal of Geophysical Research 98, B11, 20,031-20,037