Effects of spin transition on diffusion of Fe2+in ferropericlase in Earth's lower mantle

Abstract

Knowledge of Fe composition in lower-mantle minerals (primarily perovskite and ferropericlase) is essential to a complete understanding of the Earth's interior. Fe cation diffusion potentially controls many aspects of the distribution of Fe in the Earth's lower mantle, including mixing of chemical heterogeneities, element partitioning, and the extent of core-mantle communications. Fe in ferropericlase has been shown to undergo a spin transition starting at about 40 GPa and exists in a mixture of high-spin and low-spin states over a wide range of pressures. Present experimental data on Fe transport in ferropericlase is limited to pressures below 35 GPa and provides little information on the pressure dependence of the activation volume and none on the impact of the spin transition on diffusion. Therefore, known experimental data on Fe diffusion cannot be reliably extrapolated to predict diffusion throughout the lower mantle. Here, first-principles and statistical modeling are combined to predict diffusion of Fe in ferropericlase over the entire lower mantle, including the effects of the Fe spin transition. A thorough statistical thermodynamic treatment is given to fully incorporate the coexistence of high-and low-spin Fe in the model of overall Fe diffusion in the lower mantle. Pure low-spin Fe diffuses approximately 10 4 times slower than high-spin Fe in ferropericlase but Fe diffusion of the mixed-spin state is only about 10 times slower than that of high-spin Fe. The predicted Fe diffusivities demonstrate that ferropericlase is unlikely to be rate limiting in transporting Fe in deep earth since much slower Fe diffusion in perovskite is predicted.

Effects of the Electronic Spin Transitions of Iron in Lower Mantle Minerals: Implications for Deep Mantle Geophysics and Geochemistry

“…Saha et al . [] simplified the analysis of the rates of the different exchange processes by treating the hopping frequency (except for the exchange between impurity and vacancy) as a function of the exchange rate between vacancy and neighboring Fe and the exchange rate for hopping of Mg in the pure Fe‐free lattice. Another important source of the differences between the two models is the choice of the vacancy concentration: it is fixed to 1 (dilute limit) by Ammann et al .…”

“…[], while Saha et al . [] modeled it based on an estimated extrinsic vacancy concentration corresponding to 100 ppm Al content [ van Orman et al ., ]. Furthermore, Ammann et al .…”

“…[] did not consider mixed high‐spin and low‐spin state conditions, while Saha et al . [] developed a thermodynamic model to describe the behavior of Fe 2+ in an extended mixed‐spin region.…”

Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle

“…For instance, the associated density anomaly can invigorate convection, as demonstrated by geodynamics simulations in a homogeneous mantle (22)(23)(24). The bulk modulus anomaly may decrease creep activation parameters and lower mantle viscosity (10,24,25) promoting mantle homogenization in the spin crossover region (24), and anomalies in elastic coefficients can enhance anisotropy in the lower mantle (16). Less understood are its effects on seismic velocities produced by lateral temperature variations.…”

Computational modelling of inorganic solids

“…The description provides insights into the affinity of elements for different phases during igneous activity. Saha et al 151 …”