HR: 0800h
AN: MR31A-0121    [Abstracts]
TI: A novel thermodynamic model of Mg₂SiO₄ with a superior representation of experimental data predicts negligible layering in mantle convection
AU: Jacobs, M H
EM: Jacobs@geo.uu.nl
AF: Dept. Theoretical Geophysics, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AU: * de Jong, B H
EM: bernard@geo.uu.nl
AF: Petrology Group, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AU: van den Berg, A P
EM: berg@geo.uu.nl
AF: Dept. Theoretical Geophysics, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AB: We present a new thermodynamic database for Mg₂SiO₄. This novel database has three characteristics (1) thermodynamic properties are anomaly free in the complete temperature-pressure space and experimental data are represented within their experimental uncertainties in accordance with Calphad criteria (2) it discriminates between experimental data (3) it includes thermo-mechanical properties and matches them against tomographic results within experimental uncertainty. Recently [1], we showed that large differences exist between experimental data on ambient volume and between thermal expansivity data for γ-Mg₂SiO₄, possibly related to hydration effects. We demonstrated that a thermodynamic technique based on polynomial parameterizations of 1 bar thermodynamic properties cannot discriminate between the different ambient volume data and thermal expansivity data for γ-Mg₂SiO₄, hampering the accurate prediction of bulk sound velocities in the transition zone to within tomographic accuracy. We therefore developed a computational technique based on an extended form of Kieffer's [2] approach to model the vibrational density of states of a substance, a key property to derive the Helmholtz energy. This canonical thermodynamic framework, which uses input parameters from Raman and infrared spectroscopic data, constrains thermodynamic properties tighter compared to methods based on polynomial parameterizations of thermal expansivity, heat capacity and isothermal bulk modulus. We shall present recent results on the application of this approach to the Mg₂SiO₄ system [3]. We discovered that anharmonicity in Mg₂SiO₄ (α) affects the heat capacity (C_P), and position and slope of the α-β phase boundary. For γ-Mg₂SiO₄ our thermodynamic analysis prefers the ambient volume measured by Inoue et al. [4] and thermal expansivity measured by Suzuki [5]. Our analysis reveals that experimental data for MgO and MgSiO₃ are represented to within experimental uncertainty by assuming that these substances behave quasi-harmonically. The predicted Clapeyron slope of the post- spinel phase boundary is -(2.0±0.5) MPa/K. These results, have been included in a numerical model of convection in the Earth's mantle revealing no layered convection in the transition zone. Our model includes the recently discovered post-perovskite phase (P~125 GPa) based on ab-initio results and V-P-T measurements by Murakami et al. [6]. The convection results indicate that the post- perovskite layer at the bottom of the mantle is a time-dependent phenomenon strongly affected by core temperature of a cooling earth. References [1] M.H.G. Jacobs and B.H.W.S. de Jong, Geochim. Cosmochim. Acta (2005), in press. [2] S.W. Kieffer, Rev. Geophys. Space Physics, 17 (1979) 35-59. [3] M.H.G. Jacobs, B.H.W.S. de Jong and A.P. van den Berg, Calphad (2005), submitted. [4] T. Inoue, Y. Tanimoto, T. Irifune, T. Suzuki, H. Fukui and O. Ohtaka, Phys. Earth Planet. Int. 143-144 (2004) 279-290. [5] I. Suzuki, J. Phys. Earth, 27 (1979) 53-61. [6] M. Murakami, K. Hirose, K. Kawamura, N. Sata and Y. Ohishi, Science, 304 (2004) 855-855
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8125 Evolution of the Earth (0325)
DE: 8162 Rheology: mantle (8033)
DE: 8180 Tomography (6982, 7270)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005