DO COOLING PATHS DERIVED FROM MICA RB-SR DATA REFLECT TRUE COOLING PATHS

A closed-system Sr isotope exchange model involving isotopic equilibra tion among minerals by volume diffusion within grains and grain-bounda ry diffusion between grains is used to predict mica Rb-Sr ages for slo wly cooled rocks containing biotite, muscovite, and feldspar. Mica age s depend on the rock mode and usually differ from ages predicted by Do dson's model. Cooling rates derived from model ages by using the empir ical difference in closure temperature between muscovite and biotite o f 200 degrees C are similar to the cooling-rate input to the model for feldspar-rich rocks, but are higher for feldspar-poor rocks. In bioti te-rich rocks, biotite ages older than muscovite ages are predicted. M ica Rb-Sr ages may give erroneous cooling paths, and variations in coo ling path between adjacent areas could be an effect of sampling rocks with different modes.