Mineralogy and mineral physics of selected lower mantle minerals by atomistic simulations

Atomistic simulations represent a valuable route to insights in phase relations, p-V-T equations of state (EoS) and thermal conductivity of lower mantle minerals in either basaltic or peridotitic compositions. The resulting data provide constraints on seismological properties and mantle convection.

The core-mantle boundary (CMB) region represents the most pronounced thermal boundary layer in the Earth, with temperature decreasing from about 4000 to 2500-3000 K over the lower 200-300 km above the CMB (the D”-zone). The material properties of the laterally and radially heterogeneous and structurally complex D”-zone zone is a main governing factor for global convective dynamics and Earth evolution.

Technological challenges and problems related to experiments at conditions of the D” layer, combined with an easy access to large computational resources and appropriate molecular dynamics software, has made ab initio theoretical studies a very attractive approach during the last ten years. The computing power has recently increased to the point at which studies of phases with simple solid solutions are feasible.

We are currently studying the phase relations and partitioning of two Fe-bearing components, FeSiO3 and FeAlO3, in MgSiO3-based bridgmanite and post-bridgmanite. The Earth’s most abundant mineral, bridgmanite, transforms to the higher pressure form, post-bridgmanite, in the coolest regions of the D” zone.

The investigated thermoelastic properties and thermal conductivity of these phases are fundamentally important for the dynamics of the CMB boundary region.

Similar investigations that may be suitable as MSc-projects include:

1. The partitioning of Al2O3 and MgAlO2.5 in MgSiO3-based bridgmanite and post-bridgmanite.

2. The incorporation and substitution mechanism for Na2O in MgSiO3-based bridgmanite and post-bridgmanite. Reconnaissance experiments have indicated strong and largely enigmatic partitioning of Na2O into post-perovskite.

3. The partitioning and substitution mechanism for Al2O3 in the SiO2-dominated phases in the lowermost mantle. A post-stishovite (-stishovite) silica phase with the CaCl2-structure is stable in basaltic lithologies along the entire lower mantle geotherm to about 2500 km depth and is then replaced by the higher pressure seifertite phase with the PbO2-structure in the lowermost mantle. Reconnaissance experiments have indicated strong partitioning of Al2O3 to seifertite (12.6 wt%) relative to the CaCl2-structured phase (3.4 wt%).

4. The phase relations of the Al-rich phases in basaltic lithologies along selected binary join(s) in the system NaAlSiO4-KAlSiO4-MgAl2O4-CaAl2O4 at lower mantle condition. In the upper part of the lower mantle to about 1200 km depth a hexagonal phase (the new aluminous phase, NAL, space group P63/m) may coexist with an orthorombic Ca-ferrite-structured phase (space group Pbnm). The latter phase persists to the lowermost mantle (D”) where it may be replaced by an orthorombic Ca-titanite-structured phase (space group Cmcm).

All of these projects would involve the computation of thermoelastic properties (bulk and shear moduli, p-V-T equations of state) and thermal conductivity as a function of varying phase compositions.

Tags: Deep Earth materials, Centre for Earth Evolution and Dynamics, CEED
Published Nov. 8, 2016 3:19 PM - Last modified Mar. 7, 2018 11:12 AM

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