FLUORITE SOLUBILITY AND DIFFUSIVITY IN HAPLOGRANITIC MELT: PRELIMINARY RESULTS

While fluorite is thought to be a key phase component in the evolution of a number of granitic systems, its solubility and the diffusivity of its components in felsic melts are unknown. In order to investigate these properties, we constructed several cold-seal hydrothermal experiments which precipitated fluorite from water-saturated haplogranitic melts over a range of alkalinity ([Na+K]/Al=0.8 - 2.00). Solubility experiments were conducted at P=100 MPa, T=700, 800, 900, and 1000 degrees C, for 7 to 240 hours, using finely ground haplogranitic glasses mixed with fluorite powder. The diffusivity experiment, conducted at P=100 MPa, T=900 degrees C, for 7 to 216 hours, surrounded a single fluorite crystal with haplogranitic glass.

Electron microprobe analysis (EMPA) of the solubility products (glass + crystals) documented increasing fluorite saturation with melt alkalinity. At 900 degrees C, this relationship is described by C(Fl)=0.69*A+0.97, where A=[Na+K]/Al. Additionally, because of F loss during pre-run fusion at atmospheric pressure, Ca was enriched in run products. These small amounts (2.4-2.8 wt%) of dissolved Ca reduced fluorite solubility. While extensive feldspar crystallization in the lower-T products (700, 800 degrees C) precluded examination by EMPA, analysis of the higher-T runs (900, 1000 degrees C) yielded fluorite solubilities from 1.5wt% (1.1mol%) to 3.1wt% (2.3mol%).

In the diffusivity experiment, EMPA measurement of glass adjacent to the fluorite crystal revealed concentration gradients in Ca and F. These gradients are comparable to theoretical calculations of Ca and F diffusivities (D{Ca} and D{F}, respectively). Thus, these two parameters were calculated through an error-function (C=Co*erfc(x/(4Dt)^1/2)) fit of the concentration gradient at T=900 degrees C: D{Ca}=3.17*10^{-9}cm^{2}/sec and D{F}=1.10*10^{-8}cm^{2}/sec.