Apatite composition and phase equilibrium modelling unravel F and Cl behavior during crustal anatexis in the Himalayan orogen

Abstract

Fluorine (F) and chlorine (Cl) are crucial in the evolution of magmatic–hydrothermal systems, but their behavior during crustal anatexis is not well understood. To address this, we combined electron probe micro-analysis of apatite in anatectic rocks and phase equilibrium modelling for partial melting of potential source rock in this study. Apatites in two types of rocks formed under different anatectic pressure–temperature–fluid conditions from the Cona area in the eastern Himalaya have contrasting major-minor element compositions. Apatites in Type I rocks, associated with fluid-present (hydration) melting under low pressures and temperatures, have systematically lower FeO, MnO, Na₂O, and F contents, lower F/Cl ratios, and higher Cl contents than those in Type II rocks, related to fluid-absent (dehydration) melting under high pressures and temperatures. Using measured apatite volatile contents and theoretically constrained exchange coefficients, we estimated F and Cl contents in coexisting melts. The results show that Type I melts consistently exhibit lower F/Cl ratios than Type II melts, which can be ascribed to the differences in anatectic conditions rather than melt evolution. Forward phase equilibrium modelling and element partitioning further show that F and Cl partition preferentially into melts, depleting the solid restites. Lower pressures and fluid addition decrease F/Cl ratios in melts, as observed in the Type I rocks. Given that rare metals (e.g., Li, Be, Ta, W, Sn) are also primarily hosted by muscovite and biotite, similar to F and Cl, in the metapelite, we also modelled their behavior during partial melting. The results indicate that high-pressure dehydration melting enriches F, Cl, and rare metals in melts, offering favorable conditions for rare metal mineralization. These findings not only improve our understanding of F and Cl behavior during crustal anatexis but also shed new light on the role of anatexis in rare metal enrichment.