Melting of fluorine-rich biotite as a mechanism for generating lithium-rich granites

Granitic melts derived from anatexis of metasedimentary rocks, particularly biotite-dehydration reactions, are important lithium (Li) sources. Petrogenetic models depend on mineral-melt lithium partitioning, yet published partition coefficients ( D_Li^mineral/melt D Li mineral/melt ) vary by over an order of magnitude, and are commonly used as static values. Here we use thermodynamic modelling coupled with relevant published D_Li^mineral/melt D Li mineral/melt ranges, including a dynamic composition- and temperature-dependent D_Li^biotite/melt D Li biotite/melt , to quantify viable enrichment during partial melting and fractional crystallisation. Using the lithium-rich Cornubian granite batholith, we demonstrate the sensitivity of results to D_Li^mineral/melt D Li mineral/melt choices for modally-dominant lithium-poor phases (e.g. quartz), as well as phases traditionally thought to dominate lithium budgets (e.g. biotite). While economic lithium enrichment can result from extreme fractionation with selective D_Li^mineral/melt D Li mineral/melt , we suggest that dehydration melting of fluorinated biotite is the most viable petrogenetic model. The latter reconciles the common observations of fluorite in lithium-granites and their late-orogenic occurrence, and provides a mechanism for extensive fractionation.