Global material circulation in our planet from the surface to the bottom of the mantle is controlled by a combination of plate, plume, and "anti-plate" tectonics, where fluids and melts play an active role. Whereas crustal fluids are dominated by CO 2 and H 2 O, with subordinate CH 4 and N 2 , the volatiles in the lower mantle are speculated to be dominantly CO 2 . The process of Archean subduction aided in the sequestration of CO 2 from the ocean-atmosphere system, with the ultimate probable destination being the mantle. When a rising "superplume" hits the tectosphere, the predicted carbonated continental keel would become enriched in CO 2 . During the Phanerozoic, the carbonated upper mantle was drastically reduced in size, as speculated from the scarcity of dry, "ultrahot" orogens. Free fluid circulation within Earth is present only in restricted zones, mostly along the plate boundaries and intracontinental rifts, and particularly along subduction zones, where the fluid is mostly water dominated. The propagating water front in the deep mantle in modern Earth may correspond to the apparent increase in pressure through geologic time, which might be one of the reasons for a general lack of ultrahigh-pressure metamorphic belts in the Archean and their prevalence in the Phanerozoic orogenic belts.
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