Thermoelasticity of a mineral composite and a reconsideration of lower mantle properties
Abstract
A re-examination of thermal and elastic properties of a mineral composite, emphasising derivative properties, especially pressure and temperature dependences of incompressibility, resolves residual doubts about the application to the lower mantle of thermodynamic estimates of these properties. The only counter-intuitive conclusion is that (∂ KS/∂ P) S≈4.21 at P=0, which is significantly higher than the values for component minerals. This leads to a revised lower mantle equation-of-state and also to a new one which requires as input only K0'=4.21 and K∞'=1.425: K'= K0'( K∞'/ K0') K∞' P/ K. These equations and three other finite strain theories fit the lower mantle PREM tabulation very well and agree on the extrapolation to P=0: K0=203.1±1.6 GPa, μ0=128.2±1.2 GPa, ρ0=3972.5±4.0 kg m -3. A further six finite strain equations are rejected as incompatible with homogeneity of the lower mantle. Application of the theory to thermal properties gives δS=-(1/ αKS)(∂ KS/∂ T) P decreasing with depth from 2.7 to 1.6 over the lower mantle depth range. Conversely ɛ=-(1/ αμ)(∂ μ/∂ T) P increases with depth from 7.1 to 8.7. Corresponding values of (∂ln VS/∂ln VP) P vary from 1.7 to 2.3. This conclusion is not consistent with a simple thermal interpretation of tomographic observations of lower mantle velocity anomalies. Compositional variations must be invoked.
- Publication:
-
Physics of the Earth and Planetary Interiors
- Pub Date:
- April 1998
- DOI:
- 10.1016/S0031-9201(98)00084-3
- Bibcode:
- 1998PEPI..106..219S