IMPACT OF WATER-FLUXED MELTING ON MELT COMPOSITION: EXPERIMENTS VS. PHASE EQUILIBRIA MODELING

The nature of fluid regime during high–temperature metamorphism and crustal anatexis is a matter of current debate. The growing list of occurrences where H₂O-fluxed melting is postulated has raised the need of finding geochemical criteria to identify this process. Results of melting experiments have been often used as a benchmark to which natural occurrences (compositions of granitoid rocks) have been compared to infer the nature of fluid regime. Previous experimental studies suggested that trondhjemitic crustal melts are the products of H₂O-fluxed melting (e.g., Patiño Douce & Harris, 1998).

Here a comparative study of phase equilibria modeling and melting experiments is presented. Modelling results do not support the existence of a clear link between fluid regime and melt composition. In particular, the formation of trondhjemitic melts via H₂O-fluxed melting reactions does not seem a general rule under equilibrium conditions. The potential causes of this discrepancy (e.g., persistence of metastable phases during experiments, limitations of models for minerals and melt, disequilibrium melting reactions under H₂O-present conditions) will be evaluated and discussed. The nanogranitoid compositional dataset will be interrogated to determine the possible role of water-fluxed melting in the formation of peculiar melt compositions.