We follow the evolution (in red) of a liquid produced by the fusion of a solid, a mixture of the 3 minerals A, B and C. The first liquid occurs at the ternary eutectic, then moves, as the temperature increases, along a cotectic line, then onto the Liquidus surface. When melting is completed, the liquid leaves this Liquidus surface.
To work on such a diagram, it is helpful to project onto the A-B-C triangle. As an example, we can see the fusion of the mantle at the origin of basaltic magmas .
The dotted lines delineate isothermal planes.
