Crustal and mantle cycles

Abstract

As Earth cooled through the eclogite transition thick-crust tectonics was replaced by delamination, plate tectonics and deep irreversible subduction. Oceanic crust (OC) is not a conveyor belt; it is replaced by material displaced out of the transition zone (TZ) by slabs. Delaminated crust is hotter and may be part of a short, shallow "subterranean" cycle ("eclogite engine"). Deep subduction is intermittent and started late in Earth history; the amount of subducted OC has therefore been overestimated. It can easily be stored in the TZ. MORB is colder and denser at depth than LCC eclogite. Today the mantle is digesting OC–irreversible plate tectonics–but is regurgitating former lower continental crust (LCC) and restites, which are buoyant at ambient temperatures.

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