FLUORINE PARTITIONING BETWEEN QUADRILATERAL CLINOPYROXENES AND MELT

Concentrations of fluorine and chlorine were measured by ion-probe in quenched melts (glasses) and coexisting clinopyroxene, orthopyroxene, olivine, and plagioclase in run products of experiments previously used to measure sulfur partitioning between these phases. The partitioning of F between clinopyroxene and silicate melt was determined over a range of pressure, temperature, and melt compositions ranging from basaltic to dacitic (49 to 66 wt.% SiO₂) at 0.8 to 1.2 GPa and 1000 to 1240 °C, at hydrous and anhydrous conditions in 13 experiments. Additionally, we determined the crystal-melt partitioning of F for 4 experiments with plagioclase, 2 with orthopyroxene, and 1 with olivine. Although Cl was also measured in the experiments, the concentrations in the crystals are close to measurements in the nominally Cl-free fused silica glass Herasil® and therefore may be at background concentration levels. The partition coefficients of fluorine between clinopyroxene and melt varied from ~0.09 to 0.29 and were linearly dependent upon the concentration of aluminum in the octahedral M1 site of clinopyroxene. Similar relationships were seen in previous studies of the fluorine partition coefficient between clinopyroxene and melt, but no universal relationship that explains all measurements was discovered. These differences are attributed to differing analytical protocols used in the various studies. Nevertheless, we conclude that the self-consistency of each study indicates that F partition coefficients determined using one protocol can be applied to minerals or glasses analyzed using the same protocol to better understand the storage and transport of fluorine in magmatic systems.