The slowing of seismic waves as they pass through parts of Earth's deep mantle beneath the Pacific Ocean and African continent has been attributed to the existence of hot and chemically distinct piles of ancient mantle. Numerical modelling combined with thermodynamic calculations suggest that temperature contrasts alone can explain this seismic deceleration.
To analyse the nature of the mantle anomalies that cause passing seismic waves to decelerate, Rhodri Davies at Imperial College London, UK, and colleagues used a numerical model to simulate the accumulation of distinct piles of mantle over hundreds of millions of years as a result of mantle circulation and plate tectonic motions. Their model simulated the formation of piles that are both warmer and chemically distinct from the surrounding mantle or piles that differ only in temperature. Comparing thermodynamic calculations of seismic wave velocities in the simulated piles with observed wave speeds, they found that temperature variations alone can explain the slow velocities, and chemical variations are not required.
The researchers suggest that although considerable chemical heterogeneity must exist in Earth's mantle, its significance for mantle dynamics could be far less than previously thought.