FLUORINE IN GARNET: MINERAL/MELT PARTITIONING AND STRUCTURAL ARRANGEMENT

Fluorine lowers the solidus temperature of hydrous granitic melt, shifts the eutectic to higher normative albite, and reduces the viscosity of melt, thereby increasing the diffusivities of other elements. Pegmatites represent highly evolved granitic liquids and are subdivided into two families based on geochemical characteristics (e.g. NYF: Nb, Y, F; LCT: Li, Cs, Ta). One distinction between these two families of pegmatites is the common occurrence of fluorite and topaz in NYF pegmatites, both of which are mostly absent in the LCT types. Garnet is a common accessory mineral in NYF and LCT pegmatites and, where analysed, weight percent levels of F are incorporated in garnet. However, the concentrations of F in garnet vary in an unsystematic way within and between pegmatites. Therefore, as a potential monitor of F concentrations in evolved granitic liquids, the partitioning of F between garnet and granitic melt has been assessed by cold-seal experimental techniques and electron microprobe analysis.

Garnet crystals were synthesized in hydrous granitic melt from a mixture of minerals and reagents at 800°, 200 MPa, and an f(O₂) near NNO-0.5 log units. The compositions of new garnets synthesized at 800°C range from Sps₆₈Alm₀₃Prp_10­ to Sps₈₆Alm₂₀Prp₁₂. Concentrations of F in garnet range from 0.13% to 0.80 wt.%. The corresponding average F content of glass is 2.52 wt.% (1σ=0.10), yielding crystal/melt partition coefficients, K_D(grt/melt), in the range of 0.05 to 0.32.

The results show that the F content of garnet is correlated with deficiency at the T-site and X_Alm (almandine component). The log molar solubility of F in garnet and the garnet/melt partition coefficient, K_D(F)_grt-melt, increase linearly with decreasing X_Alm (r²=0.88). Regression of published garnet EMPA data in conjunction with EMPA data from this experimental study exhibit a strong correlation (r²=0.87) between deficiency at the T-site, calculated as Si deficiency, and the molar concentration of F in garnet. Garnets from pegmatites are likely to record the concentrations of F in pegmatite-forming liquids, but using that information is not straightforward because garnet compositions evolve away from almandine toward spessartine in a single dike. It may be possible, however, to differentiate NYF and LCT pegmatites based in part upon the F content of garnet.