Figure 6.06 shows the CaMgSi2O6-Mg2SiO4-CaAl2Si2O8 temperature-contoured liquidus diagram, with liquidus surfaces for diopside (Di), forsterite (Fo), and anorthite (An). The three liquidus surfaces are labeled to identify the saturating mineral. The intersections of the three surfaces define three cotectic lines, along each of which the magma is saturated with two minerals. Each of the cotectic lines originates at a binary eutectic along the two-component sides of the ternary diagram. To see this, open Figure 6.05 and use the slider. The three cotectic lines meet at a ternary eutectic point, the lowest temperature at which a liquid can exist at equilibrium.
We can use the Di-Fo-An liquidus diagram to follow the equilibrium crystallization of magmas with bulk compositions that plot within the ternary diagram. Click on the radio buttons beside Figure 6.06 to see the equilibrium assemblage at various temperatures as one bulk composition crystallizes. Or click on the diagram to open a larger version and select the temperatures there.
Figure 6.06. CaMgSi2O6-Mg2SiO4-CaAl2Si2O8 Diagram. Ternary liquidus diagram for the system Diopside(Di)-Forsterite(Fo)-Anorthite(An) at 0.1 MPa pressure based on data from Osborn and Tait (1952). Temperature is in degrees Celcius and composition is in weight pecent. Click on the diagram to see a larger, interactive version.
Di(15)-Fo(55)-An(30)
For temperature selected with a radio button, an isothermal section is shown with the equilibrium assemblages for the selected bulk compositions. The assemblages are indicated as one-phase, two-phase, or three-phase regions analogous to similar regions on the isothermal H2O-NaCl-KCl diagram in Figure 6.04.
Figure 6.07 is a fully interactive version of the Di-Fo-An liquidus diagram. Open Figure 6.07 and explore the equlibrium crystallization of other bulk compositions using the tools described in the caption. You will need to use those tools to answer the questions on the
