Calibration and testing of an empirical chloritoid-chlorite Mg-Fe exchangethermometer and thermodynamic data for daphnite

A compilation of literature data on the Fe-Mg composition of coexisting chl orite and chloritoid from metapelites metamorphosed at various P-T conditio ns shows that the logarithm of the Fe-Mg partitioning (lnK(D)) varies linea rly with the inverse of temperature, from about 2.4 at 300 degrees C to abo ut 1.3 at 600 degrees C. In contrast, no trend was observed with pressure, and the molar volumes of Mg- and Fe-chlorite end-members suggest that the p ressure dependence of lnK(D) is not significant. Therefore, the chloritoid- chlorite Mg-Fe exchange reaction is a potential thermometer and has been em pirically calibrated using the analyses of 112 chloritoid-chlorite pairs fr om 28 different localities. Temperatures estimated using the Chl-Cld thermo meter were checked against independent estimates for 20 samples not involve d in the calibration (Beni Mzala window, Morocco), and the results are in f air agreement with independent temperature estimates. However, the analytic al uncertainties and errors are too large to obtain reliable temperature es timates for extremely Mg-rich or Fe-rich compositions. The Chl-Cld thermome ter is unreliable at XMg-(CLD)<0.2 and XMg-(CLD)>0.8 at 700 degrees C, and XMg-(CLD)<0.1 and XMg-(CLD)>0.9 at 300 degrees C. Using the results of the empirical calibration, we calculated new thermodynamic data for daphnite. I mplementing these data, it becomes possible to estimate T and P conditions of metamorphism for the invariant chlorite-chloritoid-quartz-aluminosilicat e assemblage that is widespread in low-grade metapelites. These estimates a ppear to be relevant only in the stability field of kyanite, whereas the un certainties on the calculated pressure conditions are very large in the sta bility field of kaolinite and pyrophyllite.