The decay of 87 Rb to 87 Sr results in increasing 87 Sr/ 86 Sr in the rock matrix, which is recorded by garnet as it grows. We have shown that variations of 87 Sr/86 Sr within single crystals of garnet can be measured to high enough precision to determine rates of growth of the garnets. The time resolution by this method was of about 2.5 million years due to the relatively low Rb/Sr of the schist matrix from which the garnets grew. With renewal we plan to extend the use of the method and continue to make improvements in sampling and measurement techniques. We should be able to improve the time resolution by at least a factor of 10 by focusing on rocks with higher Rb/Sr ratios and by improving our understanding of Sr isotope redistribution during metamorphism. Our observations have resulted in the first measured rate of a prograde tectonometamorphic process. We now want to extend application of our method to geological problems of regional extent in western New England and to test some samples from the eastern and central Alps toward a similar goal.
