Progress relating to calculation of partial melting equilibria for metapelites

Abstract

Improved activity-composition relationships for biotite, garnet and silicate liquid are used to construct updated P-T grids and pseudosections for high-grade metapelites. The biotite model involves Ti charge-balanced by hydrogen deprotonation on the hydroxyl site, following the substitution, RM12+ + 2OHHD4+ = TiM14+ + 2OHD2-, where HD represents the hydroxyl site. Relative to equivalent biotite-breakdown melting reactions in P-T grids in K2 O-FeO-MgO-Al2O3- SiO2-H2O (KFMASH), those in K2O-FeO- MgO-Al2O2-SiO2-H2 O-TiO2-O2 (KFMASHTO) occur at temperatures close to 50 °C higher. A further consequence of the updated activity models is that spinel-bearing equilibria occur to higher temperature an..