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 , where HD represents the hydroxyl site. Relative to equivalent biotite‐breakdown melting reactions in P – T grids in K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (KFMASH), those in K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (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 and higher pressure. In…