Viscosity jump in the lower mantle inferred from melting curves of ferropericlase

Abstract

Convection provides the mechanism behind plate tectonics, which allows oceanic lithosphere to be subducted into the mantle as “slabs” and new rock to be generated by volcanism. Stagnation of subducting slabs and deflection of rising plumes in Earth’s shallow lower mantle have been suggested to result from a viscosity increase at those depths. However, the mechanism for this increase remains elusive. Here, we examine the melting behavior in the MgO–FeO binary system at high pressures using the laser-heated diamond-anvil cell and show that the liquidus and solidus of (MgxFe1−x)O ferropericlase (x = ~0.52–0.98), exhibit a local maximum at ~40 GPa, likely caused by the spin transition of iron. We calculate the relative viscosity profiles of ferropericlase using homologous temperature scaling and find that viscosity increases 10–100 times from ~750 km to ~1000–1250 km, with a smaller decrease at deeper depths, pointing to a single mechanism for slab stagnation and plume deflection.

Implications for the Melting Phase Relations in the MgO‐FeO System at Core‐Mantle Boundary Conditions

“…Although both ideal and regular solution models yield similar best fitting phase loops for the experimental temperature and composition data set reported by Deng and Lee (), the regular solution model is more appropriate to describe the MgO‐FeO binary phase diagram as the fitted parameter, enthalpy of melting of pure FeO (Δ H m,FeO ) given by the ideal solution model (i.e., ~105 kJ/mol at 27 GPa) is not compatible with previous literature values (i.e., ~50 kJ/mol at 27 GPa; Frost et al, ; Komabayashi, ). Therefore, we focus on the thermodynamic properties MgO‐FeO system resolved using the symmetric regular solution model (Deng & Lee, ).…”

“…(Mg,Fe)O ferropericlase is the second most abundant mineral in Earth's lower mantle and therefore plays an essential role in the evolution of the Earth system (Lee et al, ). A good understanding of the physical properties of (Mg,Fe)O ferropericlase at high pressures and temperatures is critical to resolve many important issues in Earth and planetary sciences, such as the rheology of the lower mantle (Deng & Lee, ; Yamazaki & Karato, ), the origin of ultralow velocity zones (ULVZs; Wicks et al, ) and mantle heterogeneity (Dubrovinsky et al, ).…”

“…Accurate measurement of temperature in melting experiments on (Mg,Fe)O ferropericlase at higher pressures using LHDACs is challenging due to large temperature aliasing caused by strong wavelength‐dependent absorption/emission of (Mg,Fe)O (Goncharov et al, ). Deng et al () successfully addressed this issue, which enabled further study of the phase diagram of the MgO‐FeO binary system up to 80 GPa (Deng & Lee, ). With composition and temperature data from the melting experiments, Deng and Lee () used both the ideal solid solution and symmetric regular solution models to fit the data and resolved a full set of thermodynamic properties as a function of pressure.…”

“…Deng et al () successfully addressed this issue, which enabled further study of the phase diagram of the MgO‐FeO binary system up to 80 GPa (Deng & Lee, ). With composition and temperature data from the melting experiments, Deng and Lee () used both the ideal solid solution and symmetric regular solution models to fit the data and resolved a full set of thermodynamic properties as a function of pressure.…”

“…In this study, we explore the thermodynamic parameters resolved by Deng and Lee () and extrapolate them to core‐mantle boundary conditions, based on which the MgO‐FeO binary phase diagram at 136 GPa is constructed and the solidus temperatures of (Mg,Fe)O‐SiO 2 are calculated. We apply the results to two outstanding problems in the enigmatic CMB region.…”