Spin transitions and thermodynamics of lower mantle materials

Amy Bengtson, Dane Morgan, Materials Science Program

The lower mantle is approximately 650-2800 km below the Earth’s surface and exists under extremely high pressures and temperatures, making experimental investigation of its properties very challenging.  For studying this region, computational modeling is an extremely powerful and informative tool.  At pressures similar to those in the lower mantle, iron is known to undergo a spin transition in both (Mg_1-x,Fe_x)O ferropericlase and (Mg_1-x,Fe_x)SiO₃ perovskite, the two most prevalent materials in the lower mantle. 

The goal of my research is to use ab initio calculations to study these spin transitions and the different properties of these two phases.  These spin transitions may have an important impact on lower mantle thermodynamics, phase diagrams, and iron partitioning, and we use our calculations on the low-, intermediate-, and high-spin phases to better understand the spin transition in the lower mantle.  The overall impact of this research is to lead to more accurate models of thermal transport, density, rheology, and thermoelasticity within the Earth.

Figure: High- to low-spin transition pressure as a function of Fe composition.  The transition pressure trend is opposite in ferropericlase and perovskite, suggesting the Fe spin transition has a strong structural dependence.

We find that the spin transition as a function of iron composition has a very different trend in the two main lower mantle materials: ferropericlase and perovskite.  Our results show that at high Fe content, Fe in perovskite will be low-spin under lower mantle pressure, but Fe in ferropericlase will remain high-spin.  These calculations agree with experimental trends in composition for ferropericlase, while experimental studies of perovskite are still uncertain due to the more complex structure and possible presence of intermediate spin.

We gratefully acknowledge financial support from the National Science Foundation (NSF), Earth Sciences (EAR) division, award number 0738886. We gratefully acknowledge computing support from the National Science Foundation (NSF) National Center for Supercomputing Applications (NCSA), award number DMR060007.

• Publication

1. Ab initio study of the composition dependence of the pressure-induced spin transition in the (Mg1-x,Fe-x)O system; Persson, K.; Bengtson, A. ; Ceder, G. ; Morgan, D. ,Geophys. Res. Lett. , 2006, 33, 16, L16306.

2. Ab initio study of the composition dependence of the pressure-induced spin crossover in perovskite (Mg_1 − x,Fe_x)SiO₃; Bengtson, A.;  Persson, K; Morgan, D. Earth and Planetary Science Letters,  Volume 265, Issues 3-4, 30 January 2008, Pages 535-545