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

Hongzhan Fei (), Maxim D. Ballmer, Ulrich Faul, Nicolas Walte, Weiwei Cao and Tomoo Katsura
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Hongzhan Fei: Universität Bayreuth
Maxim D. Ballmer: University College London
Ulrich Faul: Massachusetts Institute of Technology
Nicolas Walte: Technische Universität München
Weiwei Cao: Conditions Extrêmes et Matériaux: Haute Température et Irradiation (CEMHTI)
Tomoo Katsura: Universität Bayreuth

Nature, 2023, vol. 620, issue 7975, 794-799

Abstract: 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.

Date: 2023
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DOI: 10.1038/s41586-023-06215-0

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