Cratonization and a journey of healing: From weakness to strength

The mid-Archean to mid-Proterozoic appears to have favored the formation of cratons, stable crustal shields underlain by unusually thick and cold mantle roots. Most of these cratons and their mantle roots have remained stable and intact since their inception despite continued tectonism along their margins. The source of this longevity is widely thought to be due to cratonic mantle's intrinsically high viscosity imparted by dehydration during the melt-extraction events that produced the depleted peridotite building blocks of cratonic mantle. However, many observations (e.g., protoliths and metasomatic products) suggest that cratons may have formed in or been modified by subduction-like processes, where water or hydrous melts associated with underthrust slabs should have been abundant. Here, we propose that cratonic strength may be controlled more by grain size than by hydrogen content of nominally anhydrous minerals. We show that strength can be achieved by grain coarsening, a process that would be enhanced by the passage of fluids, possibly reconciling the apparent paradox of a strong but wet and metasomatized cratonic mantle. Cratons were not born strong. Instead, they were born weak, becoming stronger with the passage of time.