Grain boundary diffusion of ferropericlase: Implications for the core-mantle interaction

Abstract

Geophysical observations indicate that iron enrichment of various spatial scales may be present in the lowermost mantle. Various mechanisms have been proposed to explain the process of iron infiltration from the core to the mantle, though each with its own inherent limitations. Grain boundary (GB) diffusion significantly outpaces bulk diffusion within crystal interiors, and may facilitate iron transport across the core-mantle boundary (CMB). In this study, we investigate diffusion in two symmetric tilt GBs in ferropericlase using large-scale molecular dynamics simulations. The GB diffusivities of Mg and O of periclase as well as their temperature dependence agree well with previous studies. In addition, we study the GB diffusion of Fe in (Mg,Fe)O GBs for the first time. The results suggest that GB diffusion of Fe is likely to be sluggish near the CMB, and thus may not be an effective mechanism to transport iron from the core to the mantle.

Grain boundary diffusion cannot explain the W isotope heterogeneities of the deep mantle

“…Recently, a theoretical prediction suggests that the GB transport in MgO is considerably less efficient at CMB conditions than has been extrapolated from experimental results at lower pressures and temperatures 23 . Our recent findings also favor sluggish Fe diffusion along ferropericalse GBs, questioning the feasibility of GB diffusion as an effective core-mantle interaction mechanism 24 . Therefore, it is important to further quantify the efficiency of GB diffusion in exchanging W isotopes, especially at the temperature and pressure conditions of the CMB.…”

“…S7), and distribution of elements and vacancies (Fig. S8), to ensure that diffusivity results are converged and robust 24,29 . Fig.…”

“…Additionally, due to the enhancement effect of pressure on activation enthalpy, the GB diffusivities at 140 GPa show a higher temperature dependence than those at 25 GPa, characterized by ∆H = 371 kJ mol -1 for W. The activation enthalpy of GB diffusion for W in ferropericlase is similar to that of Fe (328 kJ mol -1 ) 24 at 140 GPa, which may be related to the similar ionic radii of W 6+ and Fe 2+ (W 6+ : 74 pm; Fe 2+ : 75 pm) 47 . Our diffusivity results are more than four orders of magnitude lower than the extrapolated experimental results (Fig.…”

“…Additionally, the larger volume and thus larger interatomic distances in liquids compared to solids may facilitate W migration, resulting in low ∆H. GBs, as solid-solid interfaces, exhibit characteristics of an amorphous state and should therefore display diffusion behaviors that are intermediate between those of solids and liquids, which is supported by the observations that GB atoms do not have fixed lattice sites, yet there remains a significant steric effect on diffusion 23,24 . Thus, the ∆H for GB diffusion of W should lie between the ∆H for W diffusion in liquids (118 kJ mol -1 , green data in Fig.…”

“…As the temperature increases, the disorder in the GB structure intensifies, and may trigger premelting (i.e., the GB transitions into a liquid-like, thermodynamically stable nanoscale film below melting temperature) 41 , as observed in ferropericlase 24 and many other materials [42][43][44] . This order-disorder transition of GB structures results in the temperature-dependent activation enthalpy, represented by solid-like and high ∆H at low temperature and liquid-like and low ∆H at high temperature, which has been observed in GB diffusion in MgO 23 .…”