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, c alciocarbonatites, and natrocarbonatites. The determined immiscibility between silicate- and carbonate-rich liquids has been strongly advoca ted to explain the formation of natural carbonatite magmas. Phase fiel ds intersected at 1.0 GPa by the composition joins NaAlSiO3O8-CaCO3 (A b-CC, published) and NaAlSiO4(Ne)(90)Ab(10)-CC (new), along with measu red immiscible liquid compositions, provide pseudoternary phase relati onships for the composition triangles Ab-CC-Na2CO3(NC) and Ne(90)Ab(10 )-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 th e surface for the silicate-carbonate liquidus field boundary. The misc ibility 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 tetrah edron. The liquidus minerals in equilibrium with both silicate- and ca rbonate-rich consolute liquids are nepheline, plagioclase, melilite, a nd wollastonite; with increasing Si/Al the liquidus for calcite reache s the miscibility gap. We use these phase relationships to: (1) illust rate possible paths of crystallization of initial CO2-bearing silicate haplomagmas, (2) place limits on the compositions of immiscible carbo natite magmas which can be derived from silicate parent magmas, and (3 ) illustrate paths of crystallization of carbonatite magmas. Cooling s ilicate-CO2 liquids may reach the miscibility gap, or the silicate-cal cite liquidus field boundary, or terminate at a eutectic precipitating silicates and giving off CO2. Silicate-CO2 liquids can exsolve liquid s ranging from CaCO3-rich to alkalic carbonate compositions. There is no basis in phaser relationships for the occurrence of calciocarbonati te magmas with similar to 99 wt % CaCO3., carbonate liquids derived by immiscibility from a silicate-CO2 parent (at crustal pressures) conta in a maximum of 80 wt % CaCO3. There are two relevant paths for a sili cate 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 sil icate liquid is used up. The phase diagrams indicate that high-tempera ture immiscible carbonate-rich liquids must be physically separated fr om parent silicate liquid before they can precipitate carbonate-rich m ineral assemblages. Path (1) then corresponds to the silicate-calcite liquidus field boundary, and a stage is reached where the carbonate-ri ch liquids will precipitate large amounts of calcite and fractionate t oward alkali carbonates (not necessarily matching natrocarbonatite com positions). In path (2) the high-temperature immiscible carbonate liqu id precipitates only silicates through a temperature interval until it reaches the silicate-carbonate liquidus field boundary, where it may precipitate calcite or nyerereite or gregoryite. Sovites are readily e xplained as cumulates, with residual alkali-rich melts causing fenitiz ation. We can see no way in phase diagrams for vapor loss to remove al kalis and change immiscible natrocarbonatite liquids to CaCO3-rich liq uids; adjustments to vapor loss would be made not by change in liquid composition but by precipitation of calcite and silicate minerals. The processes illustrated in this model system are applicable to a wide r ange of magmatic conditions, and they complement and facilitate interp retation of phase relationships in the single paths represented by eac h whole-rock phase equilibrium study.