THE RELATIVE DIFFUSION OF PB, ND, SR AND O IN GARNET

Garnet and coexisting minerals from a granulite-facies coronite from Gaupas in the Bergen Arcs, western Norway have been analysed for U-Pb, Sm-Nd, Rb-Sr, major element cation and oxygen isotope compositions. Garnet, plagioclase and clinopyroxene separated from the corona structure give precise and concordant U-Pb ages that relate directly to mineral crystallisation, with a best estimate given by the U-278-Pb-206 age of 1151.2 +/- 4.7 Ma (2 sigma). In contrast, Sm-147-Nd-143 and Rb-87-Sr-87 ages for the same minerals are around 900 Ma, and indicate that the final exchange of these elements in the corona occurred some 250 Ma after U and Pb. Analysis on different scales across the corona structure indicates that the exchange of Sm-Nd ceased at different times, depending on the nature of the coexisting phase and on the sampled grain size. Measured oxygen isotope profiles in the same minerals show them to be homogeneous, except for a small systematic decrease in delta(18)O garnet adjacent to plagioclase which can be attributed to diffusional exchange during cooling. These results, taken with those of other terrains and experimental data, enable a revised estimate of the diffusion rate of Nd in garnet and the measured age differences across the corona imply a simple cooling rate of ca. 4.0 degrees C Ma(-1). These results illustrate the utility of U-Pb, Sm-Nd and Rb-Sr isotopic systems for determining both the timing of peak metamorphic mineral growth and the high-temperature cooling history.