An experimental study on the influence of fluorine and chlorine on phase relations in peralkaline phonolitic melts

Fluorine and chlorine affect phase stabilities in magmatic rocks. We present phase equilibrium experiments investigating a peralkaline and iron-rich phonolitic composition with variable F and Cl contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilimaussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000–650 °C and low oxygen fugacity adjusted with graphite-lined gold capsules in an internally heated argon pressure vessel and rapid quench cold seal pressure vessels. To cover this large T interval, we applied a two-step fractional crystallization strategy where glasses representing residual melt compositions at 800 °C were synthesized as starting material for consecutive experiments at lower T. In these experiments, oxidized starting glasses allocate oxygen by reduction of ferric iron and up to 1.2 wt% dissolved OH form through reaction with hydrogen provided by the pressure medium (H2O) in initially dry experiments. For OH determination, hydrated super-liquidus experiments in Au capsules were performed to calibrate the extinction coefficient for the fundamental OH stretching vibration using infrared spectroscopy (e 3,415 = 48 ± 3 L mol−1 cm−1). Observed mineral phases in our experiments are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (Na2Fe5TiSi6O20), alkali feldspar and nepheline (±native iron) coexisting with residual melt. Above 1.5 wt% Fmelt concentrations, fluorite (CaF2) and hiortdahlite (Ca6Zr2Si4O16F4) are stable in favor of Ca-rich clinopyroxene. Sodalite (Na8Al6Si6O24Cl2) and eudialyte (Na15Ca6Fe3Zr3Si26O73(OH)3Cl2) form at Clmelt concentrations of 0.2–0.5 wt% (depending on T) and ZrO2 melt concentrations >0.7 wt% are additionally needed to stabilize eudialyte and hiortdahlite. Therefore, F and Cl may become compatible in such systems and have the potential to influence F/Cl melt ratios in evolving magmas.