Field Measurement of Slow Metamorphic Reaction Rates at Temperatures of 500° to 600°C

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Abstract
High-temperature metamorphic reaction rates were measured using strontium isotopic ratios of garnet and whole rock from a field site near Simplon Pass, Switzerland. For metamorphic conditions of cooling from 612° ± 17°C to 505° ± 15°C at pressures up to 9.1 kilobars, the inferred bulk fluid-rock exchange rate is 1.3 −0.4 +1.1 × 10 −7 grams of solid reacted per gram of solid per year, several orders of magnitude lower than laboratory-based estimates. The inferred reaction rate suggests that mineral chemistry may lag the evolving conditions in Earth's crust during mountain building.