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The Recycling of Lower Crust: The Role of Metamorphic Phase Changes
Comparing early Earth tectonics, the nature of tectonic hazards, and the habitability of other planets to modern Earth depends on accurately characterizing tectonic processes. However, processes affecting the mass transfer of Earth’s crust, especially at the crust-mantle interface far from subduction zones, often remain poorly understood because of the scarcity of relevant samples in the rock record. Thanks to a unique dataset—crustal xenoliths from the Pamir—the descent of the lower crust into the convecting mantle is captured in xenolith pressure-temperature (P-T) paths. The crustal xenolith P-T paths and phase changes are compared to predictions from numerical simulations of the recycling of the lower crust to eliminate mass transfer mechanisms that contradict the rock record. Specifically, previously invoked mechanisms for the loss of the lower crust due to drip instabilities predict P-T paths that are too cold compared to the xenolith dataset.
The approach of including phase changes to examine tectonic feedbacks is then expanded to subduction zones to test how thermal fields are stabilized. Whereas phase changes are not sufficient to stabilize decoupling between the downgoing plate and the mantle wedge, preliminary evidence suggests that incorporating the effects of phase changes on viscosities may stabilize thermal fields.
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