SIMULATED GARNET-CLINOPYROXENE GEOTHERMOMETRY OF ECLOGITES

Abstract

The Fe-Mg thermometer widely used to infer the “equilibration” temperature of garnet-clinopyroxene assemblages of eclogites records the progressive blocking of diffusion-limited exchanges between coexisting mineral phases. It is argued that equilibrium is achieved through the fast grain boundary model in which Fe and Mg circulate in an interstitial medium fast enough for the rate-limiting step to be the volume diffusion in each mineral phase. A semi-analytic solution is found and the influence of the cooling history, grain size distribution, and rock composition on the temperature at which the Fe-Mg exchange between garnet and clinopyroxene is frozen in are quantitatively evaluated. In particular, the model simulates the temperatures that would be obtained from the concentration of Fe and Mg in the rim of adjacent garnet and clinopyroxene crystals such as those commonly obtained by electron probe. For eclogites, the simulations show that correct peak temperatures are retrieved as long as the temperature of the metamorphic climax does not exceed 650 °C. At higher climax temperatures, rim-rim temperatures underestimate the peak temperature and cluster around 650 °C. Fast cooling, however, strongly limits diffusive equilibration. In crustal-type eclogites, cooling rates of a few degrees per million years at 700 °C and of a few tens of degrees at 800 °C preserve the record of the peak temperature. It is shown that these results are largely independent of the chemical composition of the garnet and clinopyroxene. On the contrary, the mineral grain size and the respective proportions of garnet and clinopyroxene have a major effect on retrogressive diffusion. For eclogites formed at a temperature of 800 °C and under conditions of slow cooling, the record of the climax temperature is preserved by crystal rims as long as the volume of clinopyroxene is smaller than that of garnet and crystal size is at least millimetric. The choice of a particular set of experimental data for the diffusion coefficients is found to be immaterial. We suggest that the temperature of the metamorphic climax of eclogites is best estimated from the composition of clinopyroxene rims combined with that of the inner edge of the diffusion boundary layer fringing the coexisting garnet grains.