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The large low-shear velocity provinces, LLSVPs, are the most prominent feature of the Earth's lowermost mantle. They are thought to result primarily from high-temperature anomalies, but possibly also from compositional heterogeneity. As such, their origin is unknown. Recently, Talavera-Soza et al. (2025) demonstrated a negative velocity anomaly but weak shear wave attenuation in LLSVPs, suggesting a much larger grain size in LLSVPs than in the surrounding high-velocity regions.
To simulate the grain sizes inside and outside LLSVPs, we have investigated the grain growth kinetics of bridgmanite coexisting with ferropericlase as a function of pressure from 27 to 50 GPa at temperatures of 1800 and 2200 K, and obtained the grain growth exponent of 5.2(4) and the activation energy and volume of 260(20) kJ/mol and 8.3(4) cm3/mol, respectively. The small activation energy but large activation volume cause the grain growth rate to decrease rapidly with increasing pressure.
The high-velocity regions outside the LLSVP are considered to be slab graveyards. Assuming that the subducted slab has temperatures 500 K lower than the surrounding mantle and that slabs reach the CMB within 240 Myr, the grain size in subducted slabs is essentially the same, i.e. 30 μm, throughout the lower mantle. In contrast, even assuming that LLSVPs have temperatures 500 K higher than the surrounding mantle and remain on the core-mantle boundary (CMB) for 4.6 Gyr, their grain size should be less than 10 μm if they had formed on the CMB. To interpret the larger grain size in LLSVPs than in the surrounding high-velocity regions, the LLSVP should have formed in shallower regions, where bridgmanite grains grew rapidly. Otherwise, grains in LLSVPs should have precipitated from the magma ocean, causing the large grain size.
This study also explains the stagnation of slabs subducted in the mid-mantle. The grain size of bridgmanite in subducted slabs is smaller than that in the ambient mantle at depths up to 1100 km. Therefore, the subducted slabs are softer than the ambient mantle in this region, which may result in mid-mantle slab stagnation. |