Thermal expansivity and elastic properties of the lithospheric mantle: Results from mineral physics of composites

The elastic properties and the coefficient of thermal expansion (CTE) of the lithospheric mantle are important parameters that affect the results of lithospheric modelling. However, there is still no consensus on which values are the most appropriate to model the lithosphere, and various average values are used for lithospheres of different age, thermal state, and composition. We present an integrated approach to calculate the elastic properties and the CTE of mantle rocks, based on the mineral physics of composites and considering the spatial heterogeneity of the lithospheric mantle. The method considers the dependence of parameters on pressure and temperature, following a procedure based on an extension of the shear-lag model and thermal expansivity systematics. Representative values are calculated for three lithospheric domains: (a) Archean lithosphere, (b) Phanerozoic continental lithosphere, and (c) oceanic lithosphere. For the case of Archean lithosphere, values of CTE between (3.04 and 3.11) × 10^-5 K^-1 are found to be suitable for modelling, and a constant depth-derivative for P-waves ∂V_p/∂z ∼ 2.30 × 10^-3 s^-1 is estimated. Results for Phanerozoic lithosphere show that no single average value of CTE can be used in modelling. Values range non-linearly between (3.25 and 3.47) ×10^-5 K^-1 at pressures equivalent to depths of 25 and 100 km, respectively. The P-wave velocity variation with depth exhibits a decrease in the range of 25-40 km, followed by almost a constant value of ∼8.08 km s^-1 between 40 and 60 km, and a systematic increase with a depth-derivative ∂V_p/∂z ∼ 1.12 × 10^-3 s^-1 from 60 km downwards. The variation in the CTE is largest in oceanic lithosphere. In young plates (≲20 Ma), values of the CTE range non-linearly from (3.25 to 3.82) × 10^-5 K^-1 at pressures equivalent to depths of 10 and 50 km, respectively. In old oceanic lithosphere (∼100 Ma), the CTE is slightly smaller, showing values in the range of (3.0-3.7) × 10^-5 K^-1 at 10 and 80 km, respectively, giving a typical average value of ∼3.45 × 10^-5 K^-1. P-wave velocity in young oceanic lithosphere decreases from ∼8.14 to 8.0 km s^-1 in the first 30 km, then follows a nearly constant path downwards. In old oceanic lithosphere, on the other hand, a systematic reduction from ∼8.2 to 8.1 km s^-1 in P-wave velocities is predicted as depth increases from 10 to 80 km. The effects of heterogeneities in CTE and elastic parameters are partic ularly noticeable in Archean lithosphere, where difference in predicted elevations and geoid heights can reach values of ≥300 and ∼6 m, respectively, when compared to standard models. These effects are less (≲40 and 0.25 m, respectively) in Phanerozoic continental and oceanic lithospheres. Uncertainties in experimental data and geotherms indicate that compositional effects cannot be completely resolved by seismic tomography in regions with P-wave and S-wave anomalies ≲±1.5 and ±3%, respectively.