Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

Abstract

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800–1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction3, accelerates plume ascent4 and inhibits chemical mixing5. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection5–7. The high Mg:Si ratio of the upper mantle relative to chondrites8, the anomalous 142Nd:144Nd, 182W:184W and 3He:4He isotopic ratios in hot-spot magmas9,10, the plume deflection4 and slab stagnation in the mid-mantle3 as well as the sparse observations of seismic anisotropy11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.

A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Application to Lower Mantle Viscosity Estimates

“…Our calculation shows that grain growth under the static conditions is too slow to reach sizes (i.e., ∼3 cm) that give high viscosity particularly at the shallow depths (i.e., 1,000–2,000 km) in Mitrovica and Forte (2004). In a recent study (Fei et al., 2023), portions of the lower mantle with high viscosity were considered to have a small fraction of ferropericlase (i.e., f II < 0.05–0.1) that promotes grain growth via reduced grain boundary pinning, which in turn increases mantle viscosity. Based on the relationship $$d\propto {{f}_{\text{II}}}^{-0.5}$$ (Equation 3), at T +200/+200 K for 4.5 billion years and at the shallow depths of 1,000–2,000 km, the final grain size is 6–20 mm, even for a very small ferropericlase fraction of 2 vol%.…”

“…This result does not support our prediction of $${\delta D}_{\text{GB}\_\text{gr}}$$, which should be approximately equal to the diffusivity of the slowest species, that is, $$5\delta {D}_{\text{Si}\_\text{BrGB}}$$ (Equation 6). At present, there is no good explanation for the fact that the diffusivities predicted from the grain growth rates (Fei et al., 2021, 2023) are larger than the measured self‐diffusivities (Dobson et al., 2008; Yamazaki et al., 2000). Parameters for estimating δD comGB and D comLatt over a wide range of pressures and temperatures in the lower mantle are known in more detail from the diffusion studies.…”

“…A pyrolite composition throughout the entire mantle is dominated by bridgmanite and post‐perovskite phases, which account for over 70 vol% in both bridgmanite and post‐perovskite zones in the lower mantle. This distribution predicts well‐connected grains of the primary phase with isolated grains of the secondary phase (Fei et al., 2023). During the phase transition from bridgmanite + ferropericlase to post‐perovskite + ferropericlase at depths of 2,600–2,700 km, multiple grains of post‐perovskite can form from a single grain of bridgmanite phase.…”

“…Grain growth rates controlled by grain-boundary diffusion in bridgmanite + ferropericlase aggregates (Fei et al, 2021(Fei et al, , 2023 were converted to a grain growth factor, k (Equation 1 in Fei et al ( 2021)), which equates to α GB gr γ IPB δD GB gr in Equation 3. With α GB gr = 1.2 × 10 4 /T (m 3 /J) (see Appendix A), we were able to calculate the value of δD GB gr .…”

“…Grain growth rates controlled by grain‐boundary diffusion in bridgmanite + ferropericlase aggregates (Fei et al., 2021, 2023) were converted to a grain growth factor, k (Equation 1 in Fei et al. (2021)), which equates to $${{\alpha }_{\text{GB}\_\text{gr}}{\gamma }_{\text{IPB}}\delta D}_{\text{GB}\_\text{gr}}$$ in Equation 3.…”