Single-crystal elastic properties of chondrodite: implications for water in the upper mantle

Chondrodite, a member of the humite group of minerals, forms by hydration o f olivine and is stable over a range of temperatures and pressures that inc ludes a portion of the uppermost mantle. We have measured the single crysta l elastic properties of a natural chondrodite specimen at ambient condition s using Brillouin spectroscopy. The isotropic aggregate bulk (K) and shear (mu) moduli calculated from the single-crystal elastic moduli, C-ij, are: K -s=118.4(16) GPa and mu=75.6(7) GPa, A comparison of the structures and ela sticity of olivine and chondrodite indicate that the replacement of O with (OH,F) in M2+O6 octahedra has a small effect on the elasticity of humite-gr oup minerals. The slightly diminished elastic moduli of humite-group minera ls las compared to olivine) are likely caused by a smaller ratio of strong structural elements (SiO4 tetrahedra) to weaker octahedra, and perhaps a mo re flexible geometry of edge-sharing MO4(O,OH,F)(2) octahedra. In contrast to the humite-olivine group minerals, the incorporation of water into garne ts and spineloids leads to a more substantial decrease in the elastic prope rties of these minerals. This contrasting behavior is due to formation of O 4H4 tetrahedra and vacant hydroxyl-bearing octahedra in the garnets and spi neloids, respectively. Therefore, the mechanism of incorporation of H/OH in to mineral phases, not only degree of hydration, should be taken into accou nt when estimating the effect of water on the elastic properties of mineral s. The bulk elastic wave velocities of chondrodite and olivine are very sim ilar. If humite-like incorporation of OH is predominant in the upper mantle , then the reaction of OH with olivine will have a minor or possibly no det ectable effect on seismic velocities. Thus, it may be difficult to distingu ish chondrodite-bearing rocks from "anhydrous" mantle on the basis of seism ically determined velocities for the Earth.