Experiments on the Saturation of Fluorite in Magmatic Systems: Implications for Maximum F Concentration and Fluorine-Cation Bonding in Silicate Melt

The effects of melt composition, temperature and pressure on the solubility of fluorite (CaF 2 ), i.e., fluorine concentration in silicate melts in equilibrium with fluorite, are summarized in this paper. The authors present a statistic study based on experimental data in literature and propose a predictive model to estimate F concentration in melt at the saturation of fluorite ( C F in melt Fl-sat ). The modeling indicates that the compositional effect of melt cations on the variation in C F in melt Fl-sat can be expressed quantitatively as one parameter FSI (fluorite saturation index): FSI=(3Al NM +Fe 2+ +6Mg+Ca+1.5Na-K)/(Si+Ti+Al NF +Fe 3+ ), in which all cations are in mole, and Al NF and Al NM are Al as network-forming and network-modifying cations, respectively. The dependence of C F in melt Fl-sat on FSI is regressed as: C F in melt Fl-sat =1.130−2.014·exp (1 000/ T )+2.747·exp ( P/T )+0.111· C melt H2O +17.641·FSI, in which T is temperature in Kelvin, P is pressure in MPa, C melt H2O is melt H 2 O content in wt.%, and C F in melt Fl-sat is in wt.% (normalized to anhydrous basis). The reference dataset used to establish the expression for conditions within 540–1 010 °C, 50–500 MPa, 0–7 wt.% melt H 2 O content, 0.4 to 1.7 for A/CNK, 0.3 wt.%–7.0 wt.% for C F in melt Fl-sat . The discrepancy of C F in melt Fl-sat between calculated and measured values is less than ±0.62 wt.% with a confidence interval of 95%. The expression of FSI and its effect on C F in melt Fl-sat indicate that fluorine incorporation in silicate melts is largely controlled by bonding with network-modifying cations, favorably with Mg, Al NM , Na, Ca and Fe 2+ in a decreasing order. The proposed model for predicting C F in melt Fl-sat is also supported by our new experiments saturated with magmatic fluorite performed at 100–200 MPa and 800–900 °C. The modeling of magma fractional crystallization emphasizes that the saturation of fluorite is dependent on both the compositions of primary magmas and their initial F contents.