7.2 Ternary Reaction Curves and Points (continued)
The liquidus surfaces of Figure 7.02 (reproduced above) meet along five lines. Four of them are cotectics and one is a reaction curve. There are several ways to tell if two liquidus surfaces meet in a cotectic or in a reaction curve. An easy identification method starts with imagining a straight line passing through the compositions of the two saturating minerals. Then extend the curve in question and see where it crosses your line through the two mineral compositions. If the curve crosses the imaginary straight line somewhere between the two minerals, the curve is a cotectic. If not, the curve is a reaction curve. Look again at Figure 7.02. The line that marks the intersection of the En liquidus surface and the Crs liquidus surface passes between the compositions of En and Crs. It is a cotectic. However, the line that marks the intersection of the En liquidus surface and the Fo liquidus surface does not pass between the compositions of En and Fo. It is a reaction curve. Figure 7.03 also has five lines where two liquidus surfaces meet. Use the method just described to identify which curves are cotectics and which are reaction curves in Figure 7.03. (Note: if there is considerable curvature to the line being questioned, extend the tangent to the line to see if it passes between the two minerals.)
Equilibrium diagrams like those in Figure 7.02 and Figure 7.03 are used by experimental petrologists to summarize and interpret the data from their laboratory experiments. Petrologists studying igneous rocks can use these diagrams to help understand the history of specific rocks. As we have done, petrologists can follow the equilibrium crystallization of a particular bulk composition based on the data in the diagram and compare natural rocks to these laboratory model systems. For example, rock textures may have clues to the order of crystallization such as mineral inclusions (the included mineral must have formed first). Similarly, reaction textures (such as olivine surrounded by enstatite) provide clues to magma composition and temperature.
Figure 7.02 (again). Mg2SiO4-CaAl2Si2O8-SiO2 Diagram. Ternary phase diagram for melting the system Mg2SiO4-CaAl2Si2O8-SiO2 at 0.1 MPa pressure based on data from Irvine (1974). The pertitectic point in Figure 01 becomes a reaction curve in this diagram. Click on the diagram to see a larger version with the ternary lever rule animated by the "Show Phase %" button.
Figure 7.03. Lct-An-SiO2 Diagram. Ternary phase diagram for melting the system KAl2Si2O6-CaAl2Si2O8-SiO2 at 0.1 MPa pressure based on data summarized by Morey (1964, Fig.78). Click on the diagram to see a larger version with more information.
