2:15 PM - 2:30 PM
[SIT14-14] Grain-size variation as a potential cause for the mid-mantle viscosity jump
Keywords:lower mantle, viscosity jump, grain growth, diffusion creep
The mid-mantle viscosity jump, an increase of 1-2 orders of magnitude in viscosity at depths of 800-1200 km, is a critical component of lower-mantle dynamics and evolution. This viscosity jump is inferred from geoid inversions, slab stagnation below the 660-km discontinuity, and changes in plume morphology at approximately 1000 km depth. However, the origin of this viscosity jump remains elusive as phase transitions of major lower-mantle minerals do not occur at these depths.
In this study, we propose that grain-size variation in the lower mantle could be responsible for the viscosity jump. This hypothesis is based on the absence of seismic anisotropy in most regions of the lower mantle, suggesting that diffusion creep may dominate. The rate of diffusion creep is inversely proportional to the grain size raised to the power of 2-3. Lithological variation may cause this grain size variation.
To test this hypothesis, we measured the grain-growth kinetics of bridgmanite as a function of the fraction of coexisting ferropericlase. Our results show that while the grain-growth kinetics is almost independent of the ferropericlase fraction down to 20 vol%, it increases rapidly with decreasing ferropericlase fraction at lower fractions. This suggests that bridgmanite grain sizes in pure-bridgmanite rock should be 2-3 orders of magnitude larger than those coexisting with 20 vol% ferropericlase over a timescale of 0.1 to 4.5 Gyr.
If diffusion creep is dominant, pure-bridgmanite rock would exhibit 4-9 orders of magnitude lower flow rates than pyrolite. Considering the contribution of climb-controlled dislocation creep, we estimate that pure-bridgmanite rock should be 1-2.5 orders of magnitude more viscous than pyrolite under lower-mantle stress conditions of 0.1-0.5 MPa.
Therefore, the mid-mantle viscosity jump could be explained by the grain-size contrast between the bridgmanite-enriched rock, which was formed in early Earth history by magma ocean solidification and has been preserved in the deep lower mantle due to its high viscosity, and the overlying pyrolitic rock.
In this study, we propose that grain-size variation in the lower mantle could be responsible for the viscosity jump. This hypothesis is based on the absence of seismic anisotropy in most regions of the lower mantle, suggesting that diffusion creep may dominate. The rate of diffusion creep is inversely proportional to the grain size raised to the power of 2-3. Lithological variation may cause this grain size variation.
To test this hypothesis, we measured the grain-growth kinetics of bridgmanite as a function of the fraction of coexisting ferropericlase. Our results show that while the grain-growth kinetics is almost independent of the ferropericlase fraction down to 20 vol%, it increases rapidly with decreasing ferropericlase fraction at lower fractions. This suggests that bridgmanite grain sizes in pure-bridgmanite rock should be 2-3 orders of magnitude larger than those coexisting with 20 vol% ferropericlase over a timescale of 0.1 to 4.5 Gyr.
If diffusion creep is dominant, pure-bridgmanite rock would exhibit 4-9 orders of magnitude lower flow rates than pyrolite. Considering the contribution of climb-controlled dislocation creep, we estimate that pure-bridgmanite rock should be 1-2.5 orders of magnitude more viscous than pyrolite under lower-mantle stress conditions of 0.1-0.5 MPa.
Therefore, the mid-mantle viscosity jump could be explained by the grain-size contrast between the bridgmanite-enriched rock, which was formed in early Earth history by magma ocean solidification and has been preserved in the deep lower mantle due to its high viscosity, and the overlying pyrolitic rock.