THE EFFECT OF POSTGROWTH THERMAL EVENTS ON GROWTH-ZONED GARNET - IMPLICATIONS FOR METAMORPHIC P-T HISTORY CALCULATIONS

We present a simple diffusion model for investigation of the effect of a post-growth thermal history on growth-zoned minerals. The model treats the mineral as a chemically closed system. Our treatment incorporates the effects of any thermal history into one essential parameter. We then demonstrate that the usability of zoned minerals in metamorphic P-T history calculations depends on the mineral grain size and the post-growth thermal history. Using garnet as an example, we show that while large garnets (>1 mm) are safe to use, garnets smaller than that that have experienced temperatures characteristic of upper greenschist or lower amphibolite facies metamorphism will yield seriously misleading information. The deviation of the calculated growth history from the true growth history can be greater than 100° C and is the greatest for the early part of the garnet history. © 1990 Springer-Verlag.