Oceanic lithosphere sinks, stagnates, and is deflected sub-horizontally beneath western Pacific island arcs, requiring buoyancy in the slab that is inconsistent with a thermal origin. The transformation of pyroxene to majoritic garnet occurs by extremely slow diffusion, and pyroxene is therefore unlikely to transform at equilibrium pressures and temperatures in the cold interior of slabs. We present high-resolution numerical simulations showing that when slow diffusion inhibits the dissolution of pyroxene into garnet, the slab becomes buoyant relative to the ambient mantle and stagnates, whereas when the phase transformations occur in equilibrium, there is no effect on the slab. We test the model by comparing slab temperature and geometry and find that sub-horizontal slabs are more likely colder than average, consistent with our numerical simulations.

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