Fluorine and chlorine behaviour during progressive dehydration melting: Consequences for granite geochemistry and metallogeny

Dehydration melting of biotite is the main control on crustal differentiation in the mid to lower continental crust because this reaction produces the most voluminous and most mobile granitic melts. Biotite breaks down over a broad temperature interval, so the partitioning behaviour of elements between biotite and melt is likely to vary. It has been hypothesized that fluorine may stabilize biotite to higher melting temperatures because biotite is typically F‐rich in ultra‐high temperature (UHT) metamorphic rocks. If true, F would be an important influence on crustal differentiation because not only would it broaden the temperature range of melting but also elevated F concentration decreases melt viscosity. Furthermore, ligand partitioning between biotite and melt may be an important influence on the metallogeny of magmas. This study used electron microprobe analysis of biotite in rocks from the Ballachulish and Rogaland metamorphic aureoles to investigate the concentration of F and Cl in biotite heated to 600–1,000°C. Results show a broad increase in biotite F content (up to 5.04% F) with temperature until 850–920°C, beyond which F content decreases (<2.5% F). Chlorine concentrations in biotite are consistently lower (<1% Cl), and show a progressive decrease after the onset of partial melting. It was found that Mg content, and the processes that control Mg distribution, are most strongly correlated with F and Cl concentration in biotite. Calculations based on these results indicate that F‐enriched biotite could be a significant source of F for continental crust‐derived melts. Generation of a hot, F‐rich melt at UHT conditions could be important for transporting lower crustal metals to the upper crust.