7.3 Solid Solutions in Ternary Systems
In the Binary Solid Solution chapter, it was shown that the compositions of a solid solution mineral and a magma in equilibrium both depend on temperature. Figure 7.04 shows this for the melting of plagioclase, a diagram that should be familiar to most readers. To refresh your memory on how the compositions of plagioclase and liquid change with temperature, click on Figure 7.04 and use the "Show Phase %" button to see how the compositions change.
Figure 7.05 is a ternary liquidus diagram for the chemical system (NaAlSi3O8)-(CaMgSi2O8)-(Ca-Al2Si2O8) (Ab-Di-An). The composition axis of Figure 7.04 is the bottom edge of the Figure 7.05 composition triangle. There are two liquidus (saturation) surfaces for the magmas of Figure 7.05: one for diopside (Di) and one for plagioclase (Pl). These two liquidus surfaces meet in a cotectic line. Unlike cotectic lines for simple ternary eutectic systems, the composition of one of the phases, plagioclase, changes as the liquid composition changes along the cotectic. The equilibrium cotectic compositions of the plagioclase are not given by the liquidus surface, but they can be estimated using Figure 7.04, giving results similar to those determined experimentally (see Kushiro, 1973). Click on Figure 7.05 for an enlarged, animated version and use the interactive tools to follow the cooling and crystallization of a CaMgSi2O8-rich liquid as described in the caption to the large version of Figure 7.05.
Several aspects of crystallization in this ternary system are similar to those observed for the binary Ab-An system of Figure 7.04. Plagioclase crystals that form on cooling are more Ca-rich than the magma and their growth causes the magma to become more Na-rich. Plagioclase crystals formed at higher temperatures are not in equilibirium with the Na-enriched magmas at lower temperatures. Instead of reacting with the magma, plagioclase crystals have Ca-rich cores and Na-rich rims. At equilibrium, the temperature and composition of the last drop of liquid to crystallize depends on the bulk composition. If the plagioclase crystals are zoned, effectively making the bulk composition more Na-rich, the last drop of liquid will crystallize lower temperatures than predicted by the original bulk composition.
Figure 7.04. Ab-An Diagram. Binary liquidus diagram for the system (NaAlSi3O8)-(Ca-Al2Si2O8). Click on the diagram to see a larger, animated version with the phase compositions shown after clicking the "Show Phase %" button.
Figure 7.05. Ab-Di-An Diagram. Ternary liquidus diagram for the system (NaAlSi3O8)-(CaMgSi2O8)-(Ca-Al2Si2O8). Click on the diagram to see a larger version with the ternary lever rule animated by the "Show Phase %" button.
