Field measurement of slow metamorphic reaction rates at temperatures of 500 degrees to 600 degrees C

High-temperature metamorphic reaction rates were measured using strontium i sotopic ratios of garnet and whole rock from a field site near Simplon Pass , Switzerland. For metamorphic conditions of cooling from 612 degrees +/- 1 7 degrees C to 505 degrees +/- 15 degrees C at pressures up to 9.1 kilobars , the inferred bulk fluid-rock exchange rate is 1.3(-0.4)(+1.1) x 10(-7) gr ams of solid reacted per gram of solid per year, several orders of magnitud e lower than laboratory-based estimates. The inferred reaction rate suggest s that mineral chemistry may lag the evolving conditions in Earth's crust d uring mountain building.