P-T Paths, H2O recycling, and depth of crystallisation for crustal melts

The P-T (pressure-temperature) paths taken by the high-grade metamorphic rocks during orogenesis govern which melting and crystallisation reactions are encountered and hence the location and amount of melt. Small differences in rock fertility, water amount and migmatite deformability influence the amount and distribution of anatectic partial melt on an outcrop scale. Layer-scale migration of H₂O in response to gradients in μH₂O controls when melting in one layer occurs while partial melt in a nearby layer crystallises. Suprasolidus decompression - dehydration reactions (SDDR) can occur patchily at mid-crustal depths generating feldspathic segregations with alumino-silicates, and releasing H₂O. Quite different mineralogies and textures are diagnostic of the possible crystallisation reactions at different crustal depths. Local mineralogical variations in anatectites can reveal whether melting occurred in response to decompression through a melting reaction or to access of H₂O. This is particularly important to help decide whether anatexis required a localised heat supply or an influx of H₂O.