Braithwaite, JWH; (2018) Heavy element partitioning and the evolution of the magma ocean. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

Density contrasts between liquids and crystals are vital to the understanding of how the mantle evolved from its molten beginnings to its current day state. As we approach the pressures of the core-mantle boundary, liquid densities tend to those of isochemical crystals, but do not exceed them. Consequently an understanding of how heavy elements, such as iron and calcium, partition between crystal and liquid phases is vital to determining the densities. The methodology is based upon using molecular dynamics driven by density functional theory (DFT). In the first instance the technique of thermodynamic integrations is used to compute free energy differences between iron-bearing and iron-free systems. From this we can determine the effects of pressure, temperature and iron concentration on the partitioning. In all of our results we find that the liquid phase is strongly favoured by iron. For ferropericlase, (Mg,Fe)O, good agreement is seen with cited work at low pressures. The partitioning method is also applied to bridgmanite, (Mg,Fe)SiO3. Here the values determined for the partitioning favour the liquid more strongly than that of main experimental comparison. We attribute this to the presence of Al3+ and ferric (Fe3+) iron. We show evidence for the mantle crystallising from the centre outwards and the possible existence of long lasting melts at the base of the mantle.

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