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 contrast, the addition of Na 2 O and CaO to KFMASH to make the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3–SiO 2–H 2 O (NCKFMASH) system lowers key biotite‐breakdown melting reactions in P–T space relative to KFMASH. Combination of the KFMASHTO and NCKFMASH systems to make Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3–SiO 2–H 2 O–TiO 2–O 2 (NCKFMASHTO) results in key biotite‐breakdown melting reactions occurring at temperatures intermediate between those in KFMASHTO and those in NCKFMASH. Given such differences, the choice of model system will be critical to inferred P–T conditions in the application of mineral equilibria modelling to rocks. Further, pseudosections constructed in KFMASH, NCKFMASH and NCKFMASHTO for several representative rock compositions show substantial differences not only in the P–T conditions of key metamorphic assemblages but also overall topology, with the calculations in NCKFMASHTO more reliably reflecting equilibria in rocks. Application of mineral equilibria modelling to rocks should be undertaken in the most comprehensive system possible, if reliable quantitative P–T information is to be derived.