Estimating P-T conditions of garnet growth with isochemical phase-diagram sections and the problem of effective bulk-composition

Isochemical pressure-temperature phase-diagram sections portray the theoretical equilibrium distribution of mineral assemblages and mineral compositions for a given bulk-composition. To utilize such isochernical sections to derive the P-T conditions of garnet growth requires knowledge of the effective bulk-composition, defined as the rock composition available to the reacting assemblage of phases. Isochemical sections for a Canadian Cordillera garnet - muscovite - biotite - kyanite - plagioclase quartz - rutile - ilmenite - graphite pelite from the kyanite zone at Mica Dam, British Columbia, were calculated in the system MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 using bulk compositions derived by X-ray-fluorescence (XRF) analysis and quantitative X-ray mapping with an electron-probe micro-analyzer (EPMA). Isopleths of measured compositions of the core of the garnet are plotted on isochernical sections to estimate P-T conditions of core growth. Comparison of measured compositions of the core with those predicted from the phase-diagram sections for the XRF-derived composition indicates that initial growth of garnet in this sample occurred at approximately 7.7 kilobars and 555 degrees C, and multi-equilibrium garnet-rim thermobarometry yields 7.2 kilobars, 680 degrees C. Therefore, there is no discernable difference in pressure of formation between core and rim. Recent literature suggests that XRF-derived compositions are not suitable for estimating effective bulk-compositions, but we stress that they may be appropriate in certain situations. Coarse-grained samples require X-ray mapping over prohibitively large areas to obtain effective bulk-compositions. In this study, we dealt with a medium- to coarse-grained pelite. XRF-derived compositions are likely to be suitable in this situation. The benefits of EPMA compositions include the ability to exclude specific phases from the composition, or even exclude portions of compositionally zoned phases to model the evolution of the effective bulk-composition during growth of zoned phases. Garnet is the only phase in this sample that exhibits significant chemical zonation. Quantitative X-ray maps were used to calculate an integrated composition of garnet that was subsequently subtracted from the XRF-derived bulk-composition to model the effect of bulk-rock fractionation during garnet growth. An isochernical section calculated with this fractionated composition predicts that garnet will react into the assemblage at P-T conditions similar to that calculated by garnet-rim thermobarometry, suggesting that the method used to model fractionation is appropriate.