PHASE-RELATIONSHIPS IN THE SYSTEM KALSIO4-MG2SIO4-SIO2-F2O-1 AND THE STABILITY OF F-PHLOGOPITE AT ATMOSPHERIC-PRESSURE, 10 AND 18 KBAR

The liquidus surface of the system quartz-kaisilite-forsterite (SiO2-K AL-SiO4-Mg2SiO4) with 3 and 6 wt % F has been investigated at 1 arm pr essure, and the locations of the most important invariant points have been determined at 10 and 18 kbar. Two different fluorine contents, 3 and 6 wt %, were used for this study and fluorine was added by direct exchange for oxygen (F2O-1). Melting relations in this system differ g reatly from those of the F-free system due to the presence of a large liquidus field for fluorphlogopite (Phl), the size of which correlates with the amount of F in the system. The locations of most invariant p oints are very similar for these different F-contents except for the r eaction points Fo + L <----> En + Phl and En + L <----> Qz + Phl. For both F-contents there are two eutectic points: one on the silica-side at a melt composition of Qz(67)Ks(32)Fo(1) coexists with San, Qz and P hl at 850 degrees C, and one on the kalsilite-side of the join San-Fo, where a melt of composition Qz(12)Ks(75)Fo(13) coexists with Ks, Lc a nd Phl at 1225 degrees C. The composition of the first eutectic point moves towards lower SiO2- contents at increasing pressures (Qz(59)Ks(7 5)Fo(1) at 18 kbar), whereas the second disappears at 10 kbar for 6 wt % F and 18 kbar for 3 wt % F due to the absence of Lc as a stable liq uidus phase at high pressures. We observe a new eutectic point San + K s + Phl + L (Qz(31)Ks(66)Fo(3) at 10 kbar) for which the location is d etermined approximately at 18 kbar. The eutectic point San + Qz + Phl + L and the reaction point Qz + Phl <----> En + L (for which equilibri um temperatures lie between 1150 degrees C at atmospheric pressure and 1250 degrees C at 18 kbar) represent model assemblages for melting pr ocesses in crustal rocks. The location of the reaction point Qz + Phl <----> En + L varies from Qz(72)Ks(26)F(o2) at 1 bar through Qz(68)Ks( 28)Fo(4) at 10kbar to Qz(67)Ks(30)Fo(3) at 18 kbar; with 3 wt % F, and from Qz(76)Ks(20)Fo(4) at 1 bar through Qz(71)Ks(26)Fo(3) at 10kbar t o Qz(67)Ks(30)Fo(3) at 18 kbar with 6 wt % F. Liquids which coexist wi th mafic minerals such as En, Fo and Phi are important for understandi ng the genesis of potassium rich rocks by partial melting of mantle ro cks and their subsequent evolution by fractionation. The peritectic po int Fo + En + Phl + L shows large variations in equilibrium liquid com position and temperature for the two different F-contents and pressure s (Qz(19)Ks(37)Fo(14) at 1250 degrees C and atmospheric pressure, Qz(1 8)Ks(46)Fo(26) at 1390 degrees C and 18 kbar for 3 wt % F; Qz(42)Ks(19 )Fo(39) al 1295 degrees C and atmospheric pressure, Qz(22)Ks(30)Fo(48) at > 1390 degrees C at 18 kbar for 6 wt % F). Melts produced at this point have lower SiO2-contents and evolve to more SiO2-poor compositio ns by fractional crystallization than in the H2O-bearing system at com parable pressures. F-Phl varies considerably in chemical composition a nd the Si-content of Phl correlates positively with the Si-content of the melt.