• Preliminary phase equilibria of the nepheline-diopside system under variable pressures (up to 28 kb) and temperatures

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    • Keywords

       

      Nepheline; diopside; phase equilibria; melilite

    • Abstract

       

      The nepheline-diopside join defines the ultra-alkaline portion of the basalt tetrahedron and the bulk composition of nephelinitic rocks lie in this join. Schairer and others established that under atmospheric pressure, the join cuts through the primary phase volumes of oliviness, carnegieitess and nepheliness. Melilite coexists with nepheliness, oliviness and diopsidess below 1160±10°C and olivine reacts out at low temperature.

      Experimental studies on seven compositions show the presence of a pseudoeutectic at Ne70Di30 and 1420°C, where diopsidess, nepheliness and liquid are in equilibrium. Olivine and melilite do not appear in the system and the assemblage below 1225±20°C is diopsidess+nepheliness.

      Four compositions studied at 1000°C show the appearance of the assemblage diopsidess+nepheliness+melilite at 15kb, whereas diopsidess and nepheliness are the stable phases at 20 and 25kb. The appearance of melilite is therefore restricted to a pressure of 18±3kb. Diopsidess and nepheliness coexist without olivine and melilite in albite-nepheline-diopside and sanidine-nepheline-diopside system. However, this study shows that feldspar-free nephelinitic rocks, which are devoid of melilite, may have crystallized under mantle conditions, whereas their melilite-bearing counterparts equilibrated at a shallower depth within the crust.

    • Author Affiliations

       

      Puniti Singh1 Rajeev Chopra2 Alok K Gupta1

      1. Department of Earth and Planetary Sciences, University of Allahabad, Allahabad - 211 002, India
      2. High Pressure Technology Division, National Physical Laboratory, New Delhi - 110 012, India
    • Dates

       
  • Journal of Earth System Science | News

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