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

Abstract 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 ( $${D}_{{{{\rm{Li}}}}}^{{{{\rm{mineral/melt}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Li</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mineral/melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ) vary by over an order of magnitude, and are commonly used as static values. Here we use thermodynamic modelling coupled with relevant published $${D}_{{{{\rm{Li}}}}}^{{{{\rm{mineral/melt}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Li</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mineral/melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ranges, including a dynamic composition- and temperature-dependent $${D}_{{{{\rm{Li}}}}}^{{{{\rm{biotite/melt}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Li</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>biotite/melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , to quantify viable enrichment during partial melting and fractional crystallisation. Using the lithium-rich Cornubian granite batholith, we demonstrate the sensitivity of results to $${D}_{{{{\rm{Li}}}}}^{{{{\rm{mineral/melt}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Li</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mineral/melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> 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 $${D}_{{{{\rm{Li}}}}}^{{{{\rm{mineral/melt}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Li</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mineral/melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , 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.