FLUORINE DISTRIBUTION DURING PARTIAL MELTING OF QUARTZ + MUSCOVITE

Muscovite + quartz dehydration melting experiments offer insight into anatexis and the generation of peraluminous melts in the lower and middle crust. Powdered muscovite + quartz mixtures (grains < 22 µm) were annealed first at 750ºC and then heated to 850ºC under 1GPa of isostatic pressure. Both starting minerals were from natural sources: oscillator-grade quartz from Mt. Ida, AR, and a single crystal of muscovite, from Renfrew, ON, K_1.2 Mg_0.2 Fe_0.6 Al_5.1 Si_6.5 O₂₂ OH_3.5 F_0.5.

Quenched materials contained glass (quenched melt) adjacent to muscovite and quartz, along with newly precipitated K-feldpsar, biotite, and corundum. Electron microprobe analysis (WDS) reveals that the glass is strongly peraluminous and has a composition of SiO₂ 70.84, Al₂O₃ 13.80, FeO 1.38, MnO 0.05, Na₂O 0.14, K₂O 3.10, F 1.04 wt.%. The feldspar is An₉₂, and the biotite is K_1.7 Mg_0.9 Fe_2.5 Al_4.0 Si_5.9 O₂₂ OH_2.9 F_1.06. Based on their texture, residual muscovite crystals appear to be relicts of partial melting and not new growth. Although otherwise chemically unchanged, they are homogenously reequilibrated with respect to F and have concentrations similar to biotite (0.8 per 22 O).

The results show that the distribution of fluorine in biotite and muscovite relative to melt is 2.3 and 2.2, respectively. This is similar to partitioning in micas at lower temperatures in melts from wet muscovite + quartz + albite and in biotites of like Mg/Mg+Fe+Mn ratios. More importantly, the results indicate that micas quickly reequilibrate with respect to F at these high temperatures. As a consequence, mica grains rapidly buffer the F content of partial melts.