Joint 52nd Northeastern Annual Section / 51st North-Central Annual Section Meeting - 2017

Paper No. 7-7
Presentation Time: 8:00 AM-12:00 PM


BRUNO, Madeline1, ROBERT, Geneviève1 and SMITH, Rebecca2, (1)Department of Geology, Bates College, Carnegie Science Hall, Lewiston, ME 04240, (2)Department of Geology, Bates College, Carnegie Science Hall, Lewiston, ME 04240; Department of Geosciences, University of Massachusetts Amherst, Morrill Science Center, Amherst, MA 01003,

We studied the effect of up to 4 wt.% nominal fluorine on the viscosity and density of (Na,K)AlSiO4 melts with Na:K ratios of 100:0 (Ne100), 75:25 (Ne75Kls25), 62.5:37.5 (Ne62.3Kls37.5), 50:50 (Ne50Kls50), 37.5:62.5 (Ne37.5Kls62.5), and 25:75 (Ne25Kls75). All fluorine contents reported are nominal. These melts are analogs for natural alkali-rich magmas and all have nominal volatile-free NBO/T values of 0. We measured viscosity by parallel-plate viscometry at temperatures between 680-945°C, representing a viscosity range of ~109-12 Pa s. We report T12, the temperature at which a melt has a viscosity of 1012 Pa s, to compare the effect of F on viscosity between the different melt compositions. The T12 value was obtained by fitting the viscosity data for each melt to a TVF equation. We measured density using the Archimedean method with ethanol as the immersion fluid.

At constant temperature, the addition of fluorine reduces the viscosity of all melts studied. For 2 wt.% F, the reduction in T12 is non-linear but generally increases with decreasing Na:K ratios. The addition of 2 wt.% dissolved F results in a T12 reduction of ~55°C for Ne100, ~125°C for Ne75Kls25, ~140°C for Ne50Kls50, and ~88°C for Ne25Kls75. At 2 wt.% F, the effect of Na:K ratio on T12 for fluorine-bearing melts mirrors what is observed for F-free melts, with the greatest reduction in T12 occurring at intermediate Na:K ratios and K-rich melts having a higher T12 than Na-rich melts. We tested additional F contents of 1, 3, and 4 wt.% for the Ne50Kls50 melt only, and observed a greater reduction in T12 at greater F content. F reduces the density in all glasses studied relative to F-free glasses, but the effect is small, on the order of 2-3 kg/m3 for 2 wt.% nominal dissolved F.

The effect of F on T12 reduction in (Na,K)AlSiO4 melts is smaller than that observed for melts with higher SiO2 content in the same (Na,K)AlSiO4-SiO2 system, specifically along the jadeite-leucite and albite-orthoclase joins. Indeed, the addition of 2 wt.% F to Ne100 melts (SiO2 ~48%) results in a reduction in T12 of only ~55°C, whereas it results in a reduction of ~130°C for both jadeite (SiO2~67%) and albite (SiO2 ~75%) melts.

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