Thermodynamic modelling of the reaction muscovite plus cordierite -> Al2SiO5 plus biotite plus quartz plus H2O: constraints from natural assemblages and implications for the metapelitic petrogenetic grid

The reaction muscovite + cordierite --> biotite + Al2SiO5 + quartz + H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd + And +/- Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P-T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles. is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe-Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet-absent andalusite + cordierite-bearing schists and hornfelses. The revised topology of reaction I allows for improved analysis of P-T paths from mineral assemblage sequences and micro textures in the same rocks.