Yale geophysicists may have unraveled one of the great, unsolved mysteries about the Earth’s interior — why the mantle appears both well mixed and unmixed at the same time.
The Earth’s mantle is the 1,800-mile thick layer of rock between the crust and core. Although it is solid rock, the mantle flows quite vigorously due to convection — heated material rises and cold material sinks.
“With all of this convective stirring, one would expect the mantle to be well mixed, however lavas collected over the two main types of mantle upwellings that breach the surface have distinct chemical signatures,” David Bercovici and Shun-ichiro Karato, professors of geology and geophysics at Yale, said in their study published September 4 in the journal, Nature.
Heavy elements, such as uranium and thorium, are depleted in lavas from mid-ocean ridges where tectonic plates pull apart. In contrast, there seems to be enrichment of these elements in lavas from ocean-islands like Hawaii, generated by hot jets of material rising from deep in the mantle.
This finding suggests that these two types of upwellings come from different layers in the mantle that remain unmixed. However, seismological images of the mantle suggest that cold downwellings, known as subducting slabs, sink across the entire mantle. This action is expected to stir the mantle and destroy any layering.
“These pieces of contradictory evidence have been a thorn in the side of the Earth science community for decades,” Bercovici said. “Many theories have been put forward, most on how to keep the mantle layered or poorly mixed even in the face of all this convective stirring.”
Bercovici and Karato proposed an alternative hypothesis. Rather than being divided into unmixed layers, the mantle melts slightly, deep within. In this process, it is filtered and cleaned. The theory proposes that as most mantle material slowly upwells through a region called the “transition zone” at several hundred kilometers depth, it is hydrated; transition-zone minerals are able to retain relatively large quantities of water.
As this material passes out of the transition zone, it can hold less water. A modest or small amount of water for the material in the transition zone causes the same material above the transition zone to be water-saturated.
Since water-saturated minerals have a lowered melting point, and the upwelling mantle materials are hot, they partially melt. The slight melt readily absorbs and extracts heavy elements from the remaining solid minerals. The resulting “cleaned and filtered” solid continues to rise slowly and generates the depleted lavas of mid-ocean ridges. The melt remains behind like a dirty filter, pooled above the transition zone, until it is dragged down by cold, subducting slabs and the “dirt” is stirred into the deeper mantle.
In contrast, when upwelling plume material rises from this deeper enriched mantle, it does not undergo cleaning. It is too hot, and moves too fast to absorb much water in the transition zone. Since it is not water saturated when leaving the transition zone, it does not melt, and remains unfiltered, delivering enriched “dirty” mantle to the surface at ocean islands.
Bercovici and Karato’s new theory attempts to explain the various contradictory observations about mantle composition by providing a new and different mechanism: mantle filtering. The model will require a significant rethinking of mantle convection, and makes testable predictions that will keep geoscientists busy for years to come.
