Abstract
We used density functional theory (DFT) to examine the partitioning of ferrous iron between periclase and bridgmanite under lower mantle conditions. To study the effects of the three major variables — pressure, temperature and concentration — these have been varied from 0 to 150GPa, from 1000 to 4000K and from 0 to 100% total iron content. We find that increasing temperature increases KD, increasing iron concentration decreases KD, while pressure can both increase and decrease KD. We find that KD decreases slowly from about 0.32 to 0.06 with depth under lower mantle conditions. We also find that KD increases sharply to 0.15 in the very lowermost mantle due to the strong temperature increases near the CMB. Spin transitions have a large effect on the activity of ferropericlase which causes KD to vary with pressure in a peak-like fashion. Despite the apparently large changes in KD through the mantle, this actually results in relatively small changes in total iron content in the two phases, with XFefp ranging from about 0.20 to 0.35, before decreasing again to about 0.28 at the CMB, and XFebd has a pretty constant value of about 0.04–0.07 throughout the lower mantle. For the very high Fe concentrations suggested for ULVZs, Fe partitions very strongly into ferropericlase.
Highlights
The presence of iron and its oxidation state has a major effect on the structure and dynamics of minerals in the Earth’s interior
As outlined and using the method in Muir and Brodholt (2015a), we predict the presence of a mixed spin state which contains both high and low spin iron for all iron concentrations at lower mantle pressures and temperatures
Increasing temperature or iron concentration increases the amount of high spin iron, whereas increasing pressure increases the amount of low spin iron in fp
Summary
The presence of iron and its oxidation state has a major effect on the structure and dynamics of minerals in the Earth’s interior. Iron concentrations affect the sharpness of phase transitions (see for example pv to ppv (Catalli et al, 2009)), and the onset and width of spin transitions. (Auzende et al, 2008; Badro et al, 2003; Holmstrom and Stixrude, 2015; Lin et al, 2005; Vilella et al, 2015)). This in turn changes the physical properties of the mantle in those areas (see for example Lin et al (2013)). The partitioning of iron between these phases at pressures $20–110 GPa and at $2000–2600 K has been studied experimentally and with thermodynamic modelling a number of times
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