AN EXPERIMENTAL-STUDY OF PARTITIONING OF FLUORINE BETWEEN K-RICHTERITE, APATITE, PHLOGOPITE, AND MELT AT 20 KBAR

Phase relations have been determined at 20 kbar and primarily under suprasolidus conditions in the Fe-Ti-free F-bearing K-richterite - phlogopite and K-richterite - apatite systems in order to assess the partitioning of F among phlogopite, K-richterite, apatite, and melt under upper-mantle conditions. Both systems are pseudoternary because they contain forsterite, enstatite and a diopside-rich clinopyroxene from the breakdown of the mica and K-richterite. The F-bearing K-richterite systems have lower minimum melting temperatures than the F-bearing phlogopite - apatite system at the same pressure. However in the systems studied, F in phlogopite appears the most effective component in altering minimum liquid compositions whereas comparison between the present study and previous systems suggests that the presence of P2O5 during melting may result in more K-enriched melts. Variations in the compositions of the F-bearing phases are primarily controlled by the bulk compositions of the end-member minerals and by temperature, although buffering by non-F bearing minerals (e.g. clinopyroxene) may be effective. Distribution coefficients (as wt% ratios) between F-bearing minerals and coexisting liquids have been determined as functions of bulk composition and temperature for products of experiments. Distribution coefficients between K-richterite - liquid, apatite - liquid, and phlogopite - liquid are greater than or equal to 1 to slightly <1 for most bulk compositions, indicating that F is generally a compatible element. This conclusion is in agreement with the sequence of F distribution for similar phases in ultrapotassic rocks. These results preclude F-bearing mineral reservoirs in the mantle, at depths corresponding to 20 kbar, being capable of producing F-enrichment in ultrapotassic magmas, or being effective in redox melting processes.