USING CALCULATED CHEMICAL POTENTIAL RELATIONSHIPS TO ACCOUNT FOR REPLACEMENT OF KYANITE BY SAPPHIRINE-BEARING SYMPLECTITES

Calculated mineral equilibria using chemical potential relationships are used to account for the formation of sapphirine–bearing symplectites replacing kyanite. In granulite facies gneisses from the Athabasca granulite terrane in northern Saskatchewan, Canada, metamorphic conditions of >14 kbar and 800°C are established for the high-P garnet-kyanite assemblage using constraints from P–T pseudosections. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near-isothermal decompression to <10 kbar. The chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients, with kyanite persisting as a stable phase while being consumed by symplectite from its edge. In this local equilibrium model, the mineral and compositional spatial relationships are shown to correspond to paths in µ(Na₂O)–µ(CaO)–µ(K₂O)–µ(FeO)–µ(MgO) in the model chemical system, NCKFMAS, with SiO₂ and Al₂O₃ taken to be completely immobile. The values of µ(Na₂O) and µ(CaO) are constrained by fixing P–T conditions and choosing appropriate µ(Na₂O) and µ(CaO) values that correspond to the observed plagioclase compositions. µ(FeO)–µ(MgO) diagrams then show the corresponding spatial relationships with the kyanite and the symplectite phases. These results show that the replacement of kyanite by sapphirine–plagioclase and spinel–plagioclase appears to be metastable with respect to replacement by corundum–plagioclase, presumably reflecting the difficulty of nucleating corundum. Replacement by corundum–plagioclase does also occur, overprinting pre-existing symplectite and also kyanite. The resulting diagrams account for the spatial relationships and compositions observed and are predicted to occur over both a wide range of P–T conditions (5–11 kbar, 650–850 °C), and plagioclase compositions (X_An = 0.5–0.9). The wide range of P–T conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the P–T conditions of retrograde metamorphism. Further work is being carried out to apply this approach to understanding the development of sapphirine-bearing symplectites that occur with cordierite instead of plagioclase.