Liquid Immiscibility in the Join NaAlSiO4—NaAlSi3O8—CaCO3 at 1 GPa: Implications for Crustal Carbonatites

The synthetic system Na2O–CaO–Al2O3–SiO2–CO2 has been widely used as a model to show possible relationships among alkalic silicate magmas, calciocarbonatites, and natrocarbonatites. The determined immiscibility between silicate- and carbonate-rich liquids has been strongly advocated to explain the formation of natural carbonatite magmas. Phase fields intersected at 1.0 GPa by the composition joins NaAlSiO3O8–CaCO3 (Ab–CC, published) and NaAlSiO4(Ne)90Ab10–CC (new), along with measured immiscible liquid compositions, provide pseudoternary phase relationships for the composition triangles Ab–CC–Na2CO3(NC) and Ne90Ab10–CC–NC. Interpolation between these, and extrapolation within the CO2-saturated tetrahedron Al2O3–SiO2–CaO–Na2O, provides pseudoquaternary phase relationships defining the volume for the miscibility gap and the surface for the silicate–carbonate liquidus field boundary. The miscibility gap extends between 10 and 70 wt % CaCO3 on the triangle Ne–Ab–CC at 1.0 GPa; it does not extend to the Na2O-free side of the tetrahedron. The liquidus minerals in equilibrium with both silicate- and carbonate-rich consolute liquids are nepheline, plagioclase, melitite, and wollastonite; with increasing Si/Al the liquidus for calcite reaches the miscibility gap. We use these phase relationships to: (1) illustrate possible paths of crystallization of initial CO2-bearing silicate haplomagmas, (2) place limits on the compositions of immiscible carbonatite magmas which can be derived from silicate parent magmas, and (3) illustrate paths of crystallization of carbonatite magmas. Cooling silicate–CO2 liquids may reach the miscibility gap, or the silicate–calcite liquidus field boundary, or terminate at a eutectic precipitating silicates and giving off CO2. Silicate–CO2 liquids can exsolve liquids ranging from CaCO3–rich to alkalic carbonate compositions. There is no basis in phase relationships for the occurrence of calciocarbonatite magmas with ∼99 wt % CaCO3; carbonate liquids derived by immiscibility from a silicate–CO2 parent (at crustal pressures) contain a maximum of 80 wt % CaCO3. There are two relevant paths for a silicate liquid which exsolves carbonate-rich liquid (along with silicate mineral precipitates): (1) the assemblage is joined by calcite, or (2) the assemblage persists without carbonate precipitation until all silicate liquid is used up. The phase diagrams indicate that high-temperature immiscible carbonate-rich liquids must be physically separated fr