Fluorine partitioning between titanite and silicate melt and its dependence on melt composition: experiments at 50–200 MPa and 875–925 °C

We present experimental results constraining the partitioning of fluorine (F) between titanite and metaluminous silicate melt under H₂O-saturated conditions at 50–200 MPa, 875–925 °C. Resulting experimental melts are metaluminous with ASI values [aluminium saturation index, calculated as molar Al₂O₃/(CaO+Na₂O+K₂O)] falling in the range 0.68–0.97. Titanite crystals and coexisting melts have F contents that vary from 0.2 to 2.6 wt% and from 0.2 to 3.2 wt%, respectively. The calculated proportion of the F–O substitution is within 0.02–0.28 per formula unit, which is predominantly compensated by the substitution of Al and Fe³⁺ for Ti, as well as a subordinate amount of Ti–Mg²⁺ substitution. The mass-ratio partition coefficient $DTtn−meltF$ is within 0.5–1.2, and there is little influence of pressure under the conditions investigated. However, the potential effect of temperature needs further investigation. $DTtn−meltF$ is positively correlated with melt ASI, and can be predicted using the following relation: $DTtn−meltF=2.26∗ASI−1.05.$

This equation is valid only for metaluminous melts (ASI < 1). The effect of melt ASI on $DTtn−meltF$ is related to structural properties of F-bearing melts, which can incorporate higher F concentrations at lower ASI. Assuming that the high-Ti titanite investigated here is subject to ideal mixing, we calculated a Margules parameter (⁠$WmeltF$⁠) of 48.6 ± 4.3 kJ/mol for F in silicate melt with a symmetric binary mixing model. The high value of $WmeltF$ confirms that O–F mixing in silicate melt is far from ideal. The reliability of using of $DTtn−meltF$ to calculate melt F contents has been checked by investigating plutonic rocks of the Liujiawa pluton in the Dabie orogen of eastern China. Our new estimates of melt F content are consistent with those derived from amphibole, biotite and apatite compositions.