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… Click to show full abstract
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.
               
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