Liquid immiscibility in the join NaAlSi_3O_8-Na_2CO_3-H_2O and its bearing on the genesis of carbonatites

Abstract

Phase relationships in the join NaAlSi_3O_8-Na_2CO_3-H_2O through the quinany system Na_2O-Al_2O_3-SiO_2-CO_2-H_2O were studied experimentally at 1 kb as part of a series of studies aimed at elucidating the relationships between alkaline igneous rocks and their associated carbonatites. The phases encountered are albite, cancrinite, sodium carbonate, a silicate-rich liquid, an Na_2CO_3-rich liquid, and vapor. A liquid miscibility gap between the two liquid phases is intersected by this join over a wide range of compositions at temperatures in excess of 725°C; the compositions of the liquids approach each other with increasing H_2O content at constant temperature. The minimum temperature of the vapor-saturated liquidus decreases continuously with increasing H_2O content; it lies at 865°C for a composition of 81 wt percent NaAlSi_3O_8, 19 wt percent Na_2CO_3 with no H_2O present, and at 645°C for an anhydrous composition of 80 wt percent NaAlSi_3O_8, 20 wt percent Na_2CO_3 with 50 wt percent H_2O present. The minimum temperature on the solidus decreases from 685°C with 5 wt percent H_2O present to 590°C with 75 percent H_20 present for an anhydrous composition 75 wt percent NaAlSi_3O_8 + 25 wt percent Na_2CO_3. Albite is the main silicate phase al low H_2O contents, while cancrinite is the main silicate phase at high H_2O contents. The three fluid phases which coexist in this simplified system are: (1) an undersaturated alkaline silicate liquid, (2) an alkaline carbonate liquid containing only a small amount of dissolved silicate, and (3) a vapor phase with a composition varying between H_2O and CO_2, and containing Na_2O and SiO_2 in solution. These fluid phases can be compared with, respectively, (1) nepheline or ijolite magmas, (2) carbonatite melts, and (3) fenitizing solutions, which together form complexes of alkaline igneous rocks and associated carbonatites. It is proposed that processes of fractional crystallization in a carbonated alkalic magma, combined with vapor transport, can result in the formation of these three coexisting fluid phases.

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