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

作者：Giehl Christopher^*; Marks Michael A W; Nowak Marcus

来源：Contributions to Mineralogy and Petrology, 2014, 167(3): 977.

DOI：10.1007/s00410-014-0977-7

• Phase equilibrium experiment
• Liquid line of descent
• Halogens
• Phonolite
• Ilimaussaq
• Agpaitic
• Eudialyte
• Infrared spectroscopy
• Extinction coefficient

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 Ili ' maussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000-650 degrees 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 twostep fractional crystallization strategy where glasses representing residual melt compositions at 800 degrees 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 (epsilon(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% F-melt concentrations, fluorite (CaF2) and hiortdahlite (Ca6Zr2Si4O16F4) are stable in favor of Carich clinopyroxene. Sodalite (Na8Al6Si6O24Cl2) and eudialyte (Na15Ca6Fe3Zr3Si26O73(OH)(3)Cl-2) 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.

• 出版日期2014-3