Measurement of the radial variation of the 87 Sr/ 86 Sr ratio in a single crystal from a metamorphic rock can be used to determine the crystal's growth rate. That variation records the accumulation of 87 Sr from radioactive decay of 87 Rb in the rock matrix from which the crystal grew. This method can be used to study the rates of petrological processes associated with mountain building. It is applied to garnet crystals in rocks from southeast Vermont that were metamorphosed about 380 million years ago. The average growth rate measured for three garnets is 1.4 -0.45 +0.92 millimeters per million years and the average time interval of growth is 10.5 ± 4.2 million years. Garnet and its mineral inclusions provide a sequential record of temperature change, strain, and chemical reactions during metamorphism; therefore, the technique offers the potential for determination of the rates of those processes as well. The growth interval and observed amount of rotation recorded by inclusion trails in one garnet indicate that the mean shear strain rate during garnet growth was 2.4 -0.7 +1.6 × 10 -14 per second.

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Editorial responsibility: 1. Patchett in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that these P-T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4_+~ mm/year and 2 _+ 1 mm/year, respectively.