The effect of fluorine on clinopyroxene/melt trace-element partitioning

Abstract

The impact of fluorine on phase relations and clinopyroxene–melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the REE and HFSE during crystallisation of alkaline and peralkaline magmas. Clinopyroxene was grown from three $\hbox {H}_2\hbox {O}$-saturated synthetic glasses of tephriphonolite-to-phonolite composition in a rapid quench internally heated pressure vessel at 650–800 $^\circ$C and 200 MPa, with $\Delta$log $f\hbox {O}_{{2}}$ fixed to ca. FMQ + 1. The fluorine content in the charges was varied and produced quenched melts from fluorine-free to 1.6 wt.% F. The experiments yield an assemblage of melt, fluid, sodic clinopyroxene, biotite, magnetite, ± K-feldspar, ± titanite, ± fluorite, and ± hiortdahlite (a Na-Ca-Ti-F sorosilicate). Addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene. Relative to fluorine-free compositions, experiments with 1.6 wt.% fluorine in the melt show a strong decrease in clinopyroxene–melt partition coefficients for the trivalent REE La–Dy and Y with a lesser decrease for the $D_{\mathrm{HREE}}$ and ${D_{\mathrm{HFSE}}}^{4+}$. The diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase, consistent with the formation of REE-F complexes and with modifications to the medium-range structural environment around $\hbox {HFSE}^{4+}$ ions in the melt. An increase in the fluorine content of the melt will thus make the REE and $\hbox {HFSE}^{4+}$ progressively less compatible and, therefore, available for residual enrichment.