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

Abstract

The effects of melt composition, temperature and pressure on the solubility of fluorite (CaF₂), 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+Fe²⁺+6Mg+Ca+1.5Na-K)/(Si+Ti+Al^NF+Fe³⁺), 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 H₂O 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 H₂O 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 Fe²⁺ 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.

Keywords

fluorine / fluorite solubility / silicate melt / experimental petrology

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