Real rocks are chemically more complex than the end-member phase diagrams, but it is possible to extract information about fractional crystallization from chemical analyses of igneous rocks using logic and the chemical compositions of common minerals. Chemical analyses of rocks are most commonly given in weight percentages of the oxides for major elements. A starting point for evaluating rock chemistry for evidence of fractional crystallization in a series of related igneous rocks is to plot two oxide weight percentages against one another on a variation diagram.
On the previous page you examined the liquid compositions for a simple rock composition based on laboratory experiments (see Figure 8.04). The only major element oxides in an analysis of the Figure 8.04 rocks are SiO2, Al2O3, MgO, and CaO. The following variation diagrams show liquid compositions produced by forsterite removal from a bulk composition of Di(30)Fo(60)An(10) on Figure 8.04. On each of the Figure 8.05 diagrams, one of the oxides Al2O3, MgO, or CaO is plotted against SiO2. Click on any diagram to see a larger version.
Figure 8.05a. MgO vs. SiO2. Click on the diagram to see a large version.
Figure 8.05b. CaO vs. SiO2. Click on the diagram to see a large version.
Figure 8.05c. Al2O3 vs. SiO2. Click on the diagram to see a large version.
Several important observations of these diagrams can be summarized as follows:
- The paths defined by the liquid compositions on these diagrams are straight lines. This makes sense because the composition of the forsterite being removed does not change. All the liquid compositions must be on a line defined by the bulk composition and the forsterite composition.
- The MgO weight percentage of the liquid declines as forsterite is removed because forsterite has a higher weight percentage of MgO than the liquid. You can see this on the larger version of Figure 8.05 by clicking the "Show Forsterite Composition" radio button.
- The SiO2 weight percentage of the liquid increases as forsterite is removed because forsterite has a lower weight percentage of SiO2 than the liquid.
- Both the CaO and Al2O3 weight percentages of the liquid increase as forsterite is removed, even though no CaO or Al2O3 is added or removed from the liquid when forsterite crystallizes. When expressing a composition in percentages, all percentages change when any one of the oxides are removed or added.
