High-temperature thermal expansion and elasticity of calcium-rich garnets

We present new high temperature elasticity data on two grossular garnet specimens. One specimen is single-crystal, of nearly endmember grossular, the other is polycrystalline with about 22% molar andradite. Our data extend the high temperature regime for which any garnet elasticity data are available from 1000 to 1350 K and the compositional range of temperature data to near endmember grossular. We also present new data on the thermal expansivity of calcium-rich garnet. We find virtually no discernable differences in the temperatureT derivatives at ambient conditions of the isotropic bulkK _S and shearμ moduli when comparing our results between these two specimens. These calcium-rich garnets have the lowest values of ¦(?K _S /?T) _P ¦ = (1.47,1.49) x 10^-2GPa/K, and among the highest values of ¦(?μ/?T) _P ¦ = 1.25 x 10^-2GPa/K, when compared with other garnets. Small, but measurable, nonlinear temperature dependences of most of the elastic moduli are observed. Several dimensionless parameters are computed with the new data and used to illustrate the effects of different assumptions on elastic equations of state extra-polated to high temperatures. We discuss how dimensionless parameters and other systematic considerations can be useful in estimating the temperature dependence of some properties of garnet phases for which temperature data are not yet available. While we believe it is premature to quantitatively predict the temperature variation ofK _S andμ for majorite garnets, our results have bearing on the amount of diopside required to explain the shear velocity gradients in Earth's transition zone.