Stability of F-Ti-phlogopite in the system phlogopite-sillimanite-quartz: an experimental study of dehydration melting in H2O-saturated and undersaturated conditions

Melt generation during granulite-grade metamorphism is believed to be controlled by the stability temperatures of biotite, whose breakdown provides H20 and controls fluid-absent melting in the lower crust. In a simple KMASH system, the restite minerals crystallising due to incongruent melting of phlogopite depend upon the bulk composition. In an alumina-poor and silica-rich portion of the system (Phi + Qtz), enstatite appears with the melt, while in an alumina-rich system (Phl+ Sil + Qtz) cordierite appears first instead of enstatite. Since the temperature of biotite stability is believed to be strongly controlled by its F and Ti content, it will have significant effect on the fluid-absent melting reactions during granulite-grade metamorphism of mica-containing granites as well as pelitic rocks in the deeper crust. To tmderstand such effects in an aluminous portion of the KMASH system, experiments were performed (between 850 and 1100~ and at 7, 10 and 12 kbar) with bulk composition containing 2Phl 6Sil - 9Qtz, where natural phlogopite with F/(F+OH) = 0.39 and Mg/(Mg+Fe) = 0.96 was used. In runs with this charge and containing 5 wt.% of excess water, cordierite appeared around 920~ at 7 kbar and 990~ at 12 kbar, and it disappeared at about 1080~ with the appearance of 221 sapphirine. In fluid-absent runs, these boundaries marginally shift to higher temperatures (30-50~ The enstatite which was distinctly absent in H20-saturated runs, crystallises in the high-temperature sapphirine field with up to 12 wt.% A1203 in H20-undersaturated runs. The enstatite formation with cordierite is perhaps inhibited due to the A1 consumption by cordierite and instability of Al-free enstatite at temperatures of cordierite stability. Re-equilibrated phlogopite persists in both the cordierite and sapphirine fields. The temperatures of the beginning of phlogopite breakdown are about 100-140~ above those reported for reaction Phi + Qtz ~ En + Sa + L (Vielzeuf and Clemens, 1992) with F and Ti-free phlogopite, but are ~50-100~ lower than the temperatures reported (Tareen et al., 1995; Dooley and Patino Douce, 1996) for the same reaction containing F- and Ti- bearing phlogopite. The combined effect of the F and Ti content in phlogopite on its stability temperatures in the KMASH system has been found to be additive in relation to those containing only F or Ti. H20-saturated runs produced per-aluminous melts with ~ 27 wt.% A1203 in the cordierite field and ,~ 23% A1203 in the sapphirine field. The H20-tmdersaturated runs produced melts rich in K20 (~ 10 wt.%), SiO2 (72.5 wt.%) and relatively poor A1203 (12 wt.%).