Fluorine partitioning between nominally anhydrous minerals (cpx, ol) and latite and minette liquids

Fluorine is a volatile constituent of magmas and hydrous mantle minerals. Fluorine’s ionic radius is similar to that of OH, but its lower diffusivity renders it less subject to disturbance. Recent studies by Hervig and Bell (2005 Fall AGU) and Guggino et al. (2007 Fall AGU) demonstrated that trace F, like H, is incorporated into nominally anhydrous minerals (NAMs), thus providing a means to estimate pre-eruptive F contents of degassed magmas and, by comparison, to better understand degassing behavior of H. In this study, we set out to determine the partitioning behavior of F in clinopyroxenes and olivines and to compare F partitioning with H partitioning. Fluorine abundances were measured on phases (cpx, ol, glass) from pre-existing experimental run products comprising latite (Esperanca and Holloway, 1986) and minette (Esperanca and Holloway, 1987) compositions. Fluorine ion intensities in the crystals and glass were measured by secondary ion mass spectrometry (SIMS) using a primary beam of 16O- ions and detection of negative secondary ions. Interfering 18OH- was separated from 19F- using high mass resolution. Partition coefficients were determined using raw ion ratios (19F-/18O-)min/(19F-/18O-)liq. For reduced latite experiments, LIW (T: 1026-1150C; P: 6-17 kbar; fO2 = IWG), DFcpx-melt ranges from 0.12 to 0.33. For oxidized latite experiments, LOA (T: 1040-1150C; P: 6.5-15 kbar; fO2 = NNO; aH2O = 0.24-0.39), DFcpx-melt ranges from 0.10 to 0.43. The minette samples show significantly lower D values than the latites. For reduced minette experiments, MIW (T: 1070-1175C; P: 10-20 kbar; fO2 = IWG), DFcpx-melt ranges from 0.045 to 0.098, and DFoliv-melt ranges from 0.0097 to 0.049. Oxidized minette experiments, MOA (T: 1125-1150C; P: 10 kbar; fO2 = NNO; aH2O = 0.22) show an average DFcpx-melt = 0.091±0.0013 and an average DFoliv-melt = 0.042±0.0012. We found that, similar to the partitioning behavior of H in clinopyroxenes (e.g., Tenner et al. 2009), DFcpx-melt shows a positive correlation with Al2O3. In addition, DFcpx-melt shows a strong negative correlation with mg#, indicating that the F partitioning into clinopyroxene increases during fractional crystallization.