Experimental Study of the Composition Join NaAlSiO4-CaCO3-H2O and the Genesis of Alkalic Rock--Carbonatite Complexes |
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Authors: | WATKINSON, DAVID H. WYLLIE, PETER J. |
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Affiliation: | Department of Geology, Carleton University Ottawa, Canada Department of Geophysical Sciences, University of Chicago Chicago, U.S.A. |
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Abstract: | Phase equilibrium studies have been carried out on the compositionjoin NaAlSiO4-CaCO3-H2O with 25 wt per cent H2O at 1 kb pressurein the temperature range 600960 °C. Liquid, in equilibriumwith crystalline phases and a sodic, aqueous vapor phase persistsacross the join down to temperatures of about 600 °C. Fractionalcrystallization of a carbonated nepheline-rich liquid in equilibriumwith vapor is capable of generating successively the crystallineassemblages (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 vaporphase is enriched in Na. The experimental assemblages are strikinglysimilar to rocks in alkalic rock-carbonatite complexes in generaland in the Oka, Quebec, complex in particular. The successionof assemblages at Oka and at other melilite rock-bearing complexesmay be interpreted as the products of fractionation of a carbonatednephelinite magma by analogy with the experimental results.The sodium-bearing vapor phase of the experiments may be analogousto the fenitizing agent of some carbonatite complexes. |
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