Compositional dependence of the rheology of halogen (F, Cl) bearing aluminosilicate melts

The rheology of three melt compositions containing different amounts of fluorine (F) and chlorine (Cl) or both was investigated with micropenetration and parallel-plate techniques. The heat capacity and configurational entropy of the melts were also determined. The observed viscosity range is between 105.5 and 1013 Pa s. The melts were produced in a 1 atm furnace at temperatures between 1523 and 1923 K using oxide and carbonate compounds. The halogens were incorporated using halides and halogen-bearing ammonium compounds. The first composition is a peraluminous Na2O–CaO–Al2O3–SiO2 melt (ANCS) with an apparent NBO/T of ~− 0.08, the second composition is a peralkaline Na2O–CaO–Al2O3–SiO2 melt (NACS) (NBO/T = ~ 0.24), which is an analogue for phonolites, and the third is an aluminium-free Na2O–CaO–SiO2 melt (NCS) (NBO/T of ~ 0.68). Five halogen-bearing ANCS melts with up to 1.10 mol% Cl and 18.25 mol% F, 6 halogen-bearing NACS melts with up to 1.38 mol% Cl and 2.58 mol% F and 3 halogen-bearing NCS melts with up to 2.15 mol% Cl and 2.04 mol% F were investigated in this study.Fluorine was found to decrease the viscosity for all compositions, but not with equal strength. Interpolated to 1 mol%, F decreases the viscosity by 0.31 ± 0.08 log units in the peraluminous melt (ANCS), 0.57 ± 0.11 log units in the peralkaline melt (NACS) and 0.47 ± 0.14 log units in the NCS melt.The effect of Cl on rheology depends on the melt composition. Interpolated to 1 mol%, Cl decreases the viscosity by 0.57 ± 0.13 log units in the peraluminous melt, but increases viscosity by 0.87 ± 0.10 log units in the peralkaline melt. There is no measurable effect on viscosity due to the addition of chlorine to the aluminium-free melt.In the peraluminous melts the effects of F and Cl add almost linearly to decrease viscosity. In the peralkaline melts, Cl increases the viscosity while F decreases it, if both are present, the effects appear to balance each other; resulting in no change in viscosity for the addition of equal amounts (in mol%) of fluorine and chlorine. The results were obtained from samples with higher and more varied halogen contents than in the most previous studies, therefore they hold true for a wide range of concentrations of both elements and should be taken into account for modelling magmatic processes.

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► In peralkaline Na–Ca–Al–Si–O melts Cl increases while F decreases viscosity.
► The effects of F and Cl together are to balance each other in the peralkaline melts.
► This composition is a model for phonolitic melts.
► In peraluminous Na–Ca–Al–Si–O melts F and Cl alone and combined decrease viscosity.
► In Na–Ca–Si–O melts Cl has no significant effect and F decreases viscosity.