FLUORINE PARTITIONING BETWEEN GRANITIC MELT AND TITANITE

The need to determine the fluorine (F) content in magmas is necessary because F affects the viscosity, density and polymerization of the magma. Magmas tend to outgas F, so it is slowly lost over time; thus plutonic rocks contain less F than their magmas did. By experimentation initial F contents can be determined. Preliminary cold-seal experiments, run at 650-800C, indicated the distribution of F between titanite and a granitic melt (DFTtn/Gl) range from 1.1-1.8. However, those data are suspect due to small crystal sizes (<5 microns). For more accurate data than previous experiments, the goal of the current experiments is to grow larger titanite crystals. Growing larger titanite crystals is being attempted by over-saturating the runs with respect to silica and water in order to drive the following equation to the left, Ttn + 2F=Fluid + Qtz + Rtl. In previous runs the granitic powders were not Si-saturated. Each run was brought up to 900C for an initial week-long melting stage, then dropped to temperatures ranging from 800-700C for 3-4 weeks, then quenched for analysis of the glass and crystals produced. In each run the initial F content ranged from 0.3-1.3%, and an ASI (Alumina Saturation Index: mol. Al2O3/(CaO+Na2O+K2O)) ranged from 0.93-1.22. By varying the ASI and F content, the goal is to determine the effect of alumina and F content on the partition coefficient of F into titanite. Studying the variation in ASI is necessary due to a coupled substitution of F-Al into the crystal structure. This substitution occurs as a result of the decomposition of SiO4 with increasing temperature, and the substitution controls the amount of Al, and thus F, in titanite.