Rates of Tectonometamorphic Processes from Rubidium and Strontium Isotopes in Garnet

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 $\pm $ 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}\times $ 10$^{-14}$ per second.