Experimental Study of the Composition Join NaAlSiO_4-CaCO_3-H_2O and the Genesis of Alkalic Rock--Carbonatite Complexes

Abstract

Phase equilibrium studies have been carried out on the composition join NaAlSiO_4-CaCO_3-H_2O with 25 wt per cent H_2O at 1 kb pressure in the temperature range 600–960 °C. Liquid, in equilibrium with crystalline phases and a sodic, aqueous vapor phase persists across the join down to temperatures of about 600 °C. Fractional crystallization of a carbonated nepheline-rich liquid in equilibrium with vapor is capable of generating successively the crystalline assemblages (1) nepheline, (2) melilite+nepheline, (3) hydroxyhaüyne+melilite, (4) cancrinite+melilite, and (5) calcite+cancrinite+melilite. Late-stage liquid fractions are rich in CaCO3, whereas the vapor phase is enriched in Na. The experimental assemblages are strikingly similar to rocks in alkalic rock-carbonatite complexes in general and in the Oka, Quebec, complex in particular. The succession of assemblages at Oka and at other melilite rock-bearing complexes may be interpreted as the products of fractionation of a carbonated nephelinite magma by analogy with the experimental results. The sodium-bearing vapor phase of the experiments may be analogous to the fenitizing agent of some carbonatite complexes.