Melting the myth: Prograde garnet dissolves during early crustal melting

Abstract Garnet is widely thought to increase in stability and volume during prograde metamorphism and partial melting. Yet, whether early-formed subsolidus garnet persists or breaks down when melting begins remains an open question. Here, we integrate 3-D X-ray tomography with high-resolution major- and trace-element mapping of centrally sectioned garnets across a metapelitic metamorphic sequence to track their response from the subsolidus-suprasolidus transition to melting temperatures up to ~770 °C. Contrary to experimental and phase equilibrium predictions, garnet undergoes extensive dissolution at the onset of partial melting, losing >40% of its volume. Melt percolation creates internal cavity networks within garnet, connecting crystal interiors to the reactive matrix, markedly shortening intracrystalline diffusion pathways at the melt-crystal interface. This process leads to the consumption of garnet and results in major- and trace-element redistribution at temperatures too low for intracrystalline diffusion in larger grains. Our findings reconcile the long-standing discrepancy between predicted progressive garnet growth above the solidus and the scarcity of melt inclusions in garnet rims in migmatites and granulites. As partial melting begins, subsolidus garnet reacts and becomes a permeable heavy rare earth elements and yttrium (HREE-Y) reservoir in the residual crust.