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Lookup NU author(s): Dr Isabella Bovolo
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Previous work has shown that the density of the lower mantle does not provide a strong enough constraint to differentiate between pyrolite and chondritic Earth compositions. Here, a simulated annealing and downhill simplex optimisation algorithm is used to fit possible mineralogical models to the bulk and shear moduli, seismic velocities, and mass of the lower mantle. State-of-the-art atomistic (lattice dynamical) simulations now allow the prediction of the required, but unknown mineral properties at any temperature and pressure. New, accurate inter-atomic potentials for the candidate minerals (Mg,Fe)SiO3 perovskite, (Mg,Fe)O magnesiowüstite and CaSiO3 perovskite are required however. New potentials have been derived using shell- and breathing-shell models by fitting to low pressure elastic properties and structures. The predicted ambient densities, elastic properties and wave velocities are in excellent agreement with published experimental results. Moreover the high-pressure properties agree within 5 % of static first-principles calculations for MgSiO3 (Karki et al 1997), and within 2 % of the experimental results for MgO (Chopelas 1996). Excellent high-temperature predictions of heat capacities, thermal expansivity and other thermodynamic properties are also obtained (within the quasi-harmonic approximation, QHA) for MgSiO3. The temperature dependence of the shear modulus is found to decrease significantly with increasing pressure. This has important consequences for the interpretation of seismic tomography data, as well as for the development of lower mantle models. Mineral properties at lower mantle conditions are calculated with a geotherm of 0.3km/K from 2000 K at 670 km. Optimisation results reveal that a pure-perovskite lower mantle is unrealistic. A composition of 36.5, 52.1, 11.3 and 0.1 wt% SiO2, MgO, FeO and CaO respectively (Mg-rich pyrolite) matches PREM density and bulk modulus best. The wave-velocities cannot be fitted to PREM because the QHA fails to model the temperature dependence of the shear modulus (but models the temperature dependence of the bulk modulus well).
Author(s): Bovolo CI
Publication type: Article
Publication status: Published
Publisher: Department of Earth Sciences: University of Bristol