The influence of cordierite on melting and mineral-melt equilibria in ultra-high-temperature metamorphism

Abstract

Experimentally constrained calibrations of the incorporation of H₂ O and CO₂ into cordierite as functions of P–T-a_H2O-a_CO2 are integrated with KFMASH grids which define mineral-melt equilibria in pelites. This is used to explore the impact of the volatile content and composition of cordierite on anatexis and melt-related processes in high-temperature (HT) and ultra-high-temperature (UHT) metamorphism. The strongly temperature-sensitive H₂O content of cordierite coexisting with dehydration melts (0·4–1·6 wt.%) causes a 10–25% relative decrease in the amount of melt produced from pelites compared with models which treat cordierite as anhydrous.

KFMASH melting grids quantified for a_H2O demonstrate consistency between the measured H₂O contents in cordierite from granulite-migmatite terrains and mineral equilibria. These indicate anatexis with a_H2O in the range 0·26–0·16 at 6–8 kbar and 870–930°C. The pressure-stability of cordierite+garnet with respect to orthopyroxene+sillimanite+quartz in KFMASH is strongly influenced by cordierite H₂O content, which decreases from 1·1 to 0·5 wt.% along the melting reaction Grt+Crd_H+Kfs=Opx+Sil+Qz+L. The lower-T invariant point involving biotite (8·8 kbar/900°C) that terminates this reaction has a_H2O of 0·16±0·03, whereas the higher-T terminating invariant point involving osumilite (7·9 kbar/940°C) occurs at a_H2O 0·08±0·02. Osumilite-bearing assemblages in UHT terrains imply a_H2O of <0·08, and at 950–1000°C and 8–9 kbar calculated a_H2O is only 0·04–0·02. Cordierites stable in osumilite-bearing assemblages or with sapphirine+quartz have maximum predicted H₂O contents of ca. 0·2 wt.%, consistent with H₂O measured in cordierites from two sapphirine-bearing UHT samples from the Napier Complex.

The addition of CO₂to the H₂O-undersaturated (dehydration-melting) system marginally decreases the temperature of melting because of the stabilisation of cordierite, the solid product of the peritectic melting reactions. The preferential incorporation of CO₂ enhances the stability of cordierite, even at fixed a_H2O, and causes the stability fields of Grt+Crd+Sil+Kfs+Qz+L and Grt+Opx+Crd+Kfs+Qz+L to expand to higher pressure, and to both higher and lower temperatures. The minimum solubility of H₂O in granitic melt is independent of the CO₂ content of cordierite, and the distribution of H₂O between melt and cordierite is similar at a given melt H₂O-content to the H₂O-only system. This enhanced stability of CO₂-bearing cordierite leads to a reduced stability range for osumilite-bearing assemblages to temperatures of ca. 950–975°C or greater. Cordierites in the Napier Complex UHT gneisses contain 0·5 and 1·05 wt.% CO₂, consistent with a role for CO₂ in stabilising cordierite with respect to osumilite in these unusual sapphirine-bearing granul