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

Abstract

Abstract. 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 temperature T derivatives at ambient conditions of the isotropic bulk Ks and shear # moduli when comparing our results between these two specimens. These calcium-rich garnets have the lowest values of [(3Ks/OT)eI=(1.47, 1.49) x l0-2GPa/K, and among the highest values of I(0#/0T)e[= 1.25 x 10 -2 GPa/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 extrapolated 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 of Ks and/~ for majorite garnets, our results have bearing on the amount of diopside required to explain the shear velocity gradients in Earth's transition zone.

Measurement of elastic properties of single-crystal CaO up to 1200 K

“…The value of v lies near a constant value of 1.23. This value is close to 1.3 for periclase (Isaak et al 1989a), 1.2 for forsterite (Isaak et al 1989b), 1.4 for corundum (Goto et al 1989) and 1.27 for grossular garnet (Isaak et al 1991), but much smaller than the value (v = 2-3) obtained for the lower mantle from seismic data (Woodhouse and Dziewonski 1987). Recently Agnon and Bukowinski (1990) attributed this discrepancy between laboratory and seismic data on v to the pressure effect which tends to lower the Anderson-Grtineisen parameter 6~ (Sect.…”

“…The change in O is caused by decrease in elastic wave velocities and density with increasing temperature. Such a decrease in O is commonly found in other mantle minerals (Anderson et al 1991 a).…”

Thermal Expansion Study of a Uvarovite Rich Garnet

“…The errors in the thermal expansion coefficient were found to be about 0.5 per cent. Figure 1 shows the observed coefficient of the volume thermal expansion of the uvarovite rich garnet sample in the temperature range 300 K-1073 K, together with the data of Issak et al 7 for grossular and that of Suzuki and Anderson 8 for pyrope rich (72.6 mole%) garnet. It is clear that the observed thermal expansion coefficient of uvarovite is intermediate between those of grassular and pyrope.…”

Analysis of the Pressure Dependence of Thermal Expansivity

“…If we integrate equ. (20), by taking the volume as variable, as suggested by Isaak et al [15,16], we get…”