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

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

Melting Relationships in the System NaAlSi_3O_8-NaF-H_2O to 4 Kilobars Pressure

The phase relationships in the systems NaF-H_2O and NaAlSi_3O_8-NaF-H_2O were determined between 600° and 900° C. at pressures up to 4 kb., and those in the anhydrous system NaAlSi_3O_8-NaF were determined at 1 atm. The phases encountered were albite, villiaumite (NaF), liquid, and vapor. The liquid quenches to a glass containing skeletal NaF crystals. Primary villiaumite is readily distinguished from "quench" NaF. Albite crystals coexisting with liquid are several times larger than subsolidus crystals. The binary reaction villiaumite + vapor ↔ liquid occurs at 860° ± 7° C. at 1 kb., and the binary reaction albite + villiaumite ↔ liquid occurs at 860° ± 5° C. at 1 atm. The beginning of melting in the ternary system is the reaction albite + villiaumite + vapor ↔ liquid; this occurs at 753° ± 5° C. at 0.5 kb., at 688° ± 5° C. at 1 kb., at 640° ± 5° C. at 2 kb., at 630° ± 5° C. at 3 kb., and at 600° ± 5° C. at 4 kb. The composition of the univariant liquid in the system NaF-H_2O at 1 kb. is approximately 80 wt. per cent NaF and 20 wt. per cent H_2O; the composition of the univariant liquid in the anhydrous system is approximately 84 wt. per cent NaAlSi_3O_8 and 16 wt. per cent NaF. The composition of the univariant liquid in the ternary system varies with pressure. At 1 kb. the composition expressed in terms of the anhydrous components is approximately 86 wt. per cent NaAlSi_3O_8 and 14 wt. per cent NaF, and the water content is about 30 wt. per cent; at 4 kb. it is approximately 75 wt. per cent NaAlSi_3O_8 and 25 wt. per cent NaF, and with a water content of about 45 wt. per cent. Critical conditions are probably reached at a pressure not far above 4 kb. The solubility of the solids in the vapor phase at 4 kb. is about 40 wt. per cent. These results indicate that small variations in NaF content of a silicate magma can produce large variations in the water content of residual magmas and large variations in the amount of water required to saturate the magma. NaF causes residual magmas to persist to significantly lower temperatures than the final consolidation temperature if H_2O were the only dissolved volatile component

Liquid immiscibility in the system Na_2O-Al_2O_3-SiO_2-CO_2 at pressures to 1 kilobar

A liquid miscibility occurs within the Quaternary system, Na_2O-Al_2O_3-SiO_2-CO_2 between Na_2 CO_3-rich liquids and NaAlSi_3O_8-rich liquids. The vapor-saturated liquidus surface does not intersect the miscibility gap at low pressures, but with increasing pressure it undergoes two discontinuous shifts toward the CO_2 apex of the tetrahedron, produced by carbonation reactions in the system Na_2O-SiO_2-CO_2. These position changes cause the vapor-saturated surface to intersect the miscibility gap. At 1 kilobar pressure on the join NaAlSi_3O_8-Na_2CO_3-CO_2, the two-liquid field occurs at temperatures above 870 degrees C

Liquid immiscibility in the join NaAlSi_3O_8-CaAl_2Si_2O_8-Na_2CO_3-H_2O

The phase relationships in the six component system NaAlSi_3O_8- CaAl_2Si_2O_8-Na_2CO_3-H_2O were determined between 650° and 950°C and 1 kb pressure along the joins Ab_(80)An_(20)-Na_2CO_3 and Ab_(50)An_(50)-Na_2CO_3, both with 10 percent H_2O present. The phase relationships are complex. Crystalline phases encountered were plagioclase, nepheline, noselite, cancrinite, wollastonite, sodium carbonate, and sodium calcium carbonate. The solidus was intersected at minimally 695°C. The most important feature is the presence of a two-liquid field above 750°C, separating a carbonate-poor silicate liquid and a silicate-poor carbonate liquid. Electron microprobe analyses of the quenched silicate liquid and atomic absorption data for the quenched carbonate liquid show that the silicate liquid is peralkaline and strongly undersaturated in silica and that the coexisting carbonate liquid is strongly enriched in calcium. A third fluid phase present with these two liquids is an aqueous vapor phase enriched with sodium silicate and CO_2. It is concluded that in this system three fluid phases coexist that correlate well with the natural occurring undersaturated alkalic urtite-ijolite-melteigite rock series, the carbonatites, and the associated metasomatic fenites of the carbonatite-alkalic rock complexes

Melting Relationships in the System NaAlSi_3O_8-NaCl-H_2O at One Kilobar Pressure, with Petrological Applications

Phase relationships in the system NaAlSi_3O_8-NaCl-H_2O appear to be ternary between 850° C and 950° C at 1 kbar pressure. A two-phase field containing NaCl-rich liquid and H_2O-rich vapor extends from the system NaCl-H_2O into the ternary system, although this field was not intersected by the joins investigated. In the system NaAlSi_3O_8-H_2O a wide miscibility gap occurs between liquid and vapor; in the system NaAlS_2O_8-NaCl a wide miscibility gap extends between silicate-rich liquid and NaCl-rich liquid at temperatures above 1,100° C at 1 bar pressure and also by inference at 1 kbar pressure. Both miscibility gaps are connected through the ternary system, separating a silicate liquid from a fluid phase with composition close to the join NaCl-H_2O and containing a small, unknown proportion of dissolved silicates. A temperature minimum is present on the ternary liquidus at 873 ± 5° C, representing the reaction: Albite+Fluid (H_2O-Nacl) ⇔ Liquid_(Albite) (1) The liquid composition, in terms of the anhydrous components, is approximately 99.7 wt percent NaAlSi_3O_8, 0.3 wt percent NaCl; its H_2O content is about 10 wt percent. It is expected that at lower pressures two additional reactions will occur: Albite+Vapor=Liquid_(Albite)+Liquid_(Nacl) (2) and Albite+Liquid_(Nacl)+Vapor; ⇔ Liquid_(Albite). (3) The results contrast with those in the system NaAlSi_3O_8-NaF-H_2O and confirm previous conclusions from the systems NaAlSi_3O_8-HCL-H_2O and NaAlSi_3O_8-HF-H_2O. Whereas fluoride (or NaF) tends to remain in the liquid (magma), chlorine (or NaCl) passes preferentially into the vapor or fluid phase. The solubility of H2O in a silicate melt increases when a small quantity of chlorine is present. Coexistence of H_2O-rich liquid inclusions and NaCl-rich liquid inclusions in crystalline phases in igneous rocks may indicate low-pressure conditions (<1 kbar) during capture of the inclusions