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3.5 i. Melt viscosities along the jadeite-leucite join (K.-U. Hess and D.B. Dingwell)

The system kalsilite (KAlSiO4) - nepheline (NaAlSiO4) - quartz (SiO2) contains the petrologically important normative components nepheline, kalsilite, leucite, jadeite, albite, orthoclase and quartz. The bulk of the chemical composition of three major igneous series (rhyolites, trachytes and phonolites) can be described within this system. In strong contrast to the major experimental effort expended in the haplogranitic subsystem, the properties of silica undersaturated, truly alkaline magmas have been only scantily investigated. Here we present experimental determinations of viscosities for silica-undersaturated melts in this system.

The data were obtained for seven melt compositions along the jadeite (NaAlSi2O6) leucite (KAlSi2O6) join. The melts were synthesized by very high temperature fusion and cooling in air. The 38 viscosity determinations were performed using micropenetration techniques in the viscosity range of log10 (Pa s) from 9.5 to 11.5. Viscosity exhibits a strongly nonlinear variation with composition, remaining approximately constant or decreasing slightly from jadeite to the 50:50 intermediate composition and increasing sharply from 50:50 to leucite. The viscosity of leucite melt is 3.5 log units higher than that of jadeite melt at 950°C.

Comparison of these results with viscosities for melts along other silica isopleths in the kalsilite (KAlSiO4) - nepheline (NaAlSiO4) - quartz (SiO2) system reveals that the viscosity along the nepheline-quartz bounding binary system reaches a minimum below the albite isopleth (75 mol% silica) and possibly even below the jadeite isopleth (67 mol% silica). In contrast, along the kalsilite-quartz binary system the viscosity of leucite melt (67 mol% silica) is significantly higher than that at 75, 82, 85 or 87 mol% silica. Thus a minimum may lie near orthoclase (75 mol% silica). Remarkably, this implies that glass transition temperatures in strongly leucite normative melts are much higher than suspected based on Na-rich melts.

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