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Title: 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 jump 1,2 . The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction 3 , accelerates plume ascent 4 and inhibits chemical mixing 5 . 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 jump 1,2 . 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 convection 5–7 . The high Mg:Si ratio of the upper mantle relative to chondrites 8 , the anomalous 142 Nd: 144 Nd, 182 W: 184 W and 3 He: 4 He isotopic ratios in hot-spot magmas 9,10 , the plume deflection 4 and slab stagnation in the mid-mantle 3 as well as the sparse observations of seismic anisotropy 11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.  more » « less
Award ID(s):
2125895
NSF-PAR ID:
10466377
Author(s) / Creator(s):
; ; ; ; ;
Date Published:
Journal Name:
Nature
Volume:
620
Issue:
7975
ISSN:
0028-0836
Page Range / eLocation ID:
794 to 799
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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