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

Abstract

We present experimental results constraining the partitioning of fluorine (F) between titanite and metaluminous silicate melt under H₂ O-saturated conditions at 50–200 MPa, 875–925°C. Resulting experimental melts are metaluminous with ASI values [aluminium saturation index, calculated as molar Al₂ O₃ /(CaO + Na₂ O + K₂ O)] 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 D ^F _{Ttn melt} 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. D ^F _{Ttn melt} is positively correlated with melt ASI, and can be predicted using the following relation: D ^F _{Ttn melt} = 2.26 ASI 1.05. This equation is valid only for metaluminous melts (ASI D F_{Ttn melt} 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 (W ^F _melt) of 48.6 ± 4.3 kJ/mol for F in silicate melt with a symmetric binary mixing model. The high value of W ^F _melt confirms that O– F mixing in silicate melt is far from ideal. The reliability of using of D ^F _{Ttn melt} 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.