8.9 Fractional Crystallization Questions
(3) Look at the following variation diagrams. You will see a larger version if you mouse over the diagram. The black dot starting is the starting bulk composition named in the label. Check the boxes above the diagram(s) that have liquid composition paths caused by fractional crystallization of plagioclase?
Yes. Each one of these diagrams was produced by fractional crystallization of plagioclase. The different directions of chemical change on the diagrams is the result of different starting bulk compositions. The liquid compositions move directly away from the plagioclase composition. If the plagioclase in equilibrium with the melt has a lower wt.% SiO2 than the liquid, then the liquid compositions will increase in SiO2 during fractionation. If the plagioclase in equilibrium with the melt has a greater wt.% SiO2 than the liquid, then the liquid compositions will decrease in SiO2 during fractionation. The same reasoning applies to the alkalis (Na2O+K2O). Because the equilibrium plagioclase composition changes as the liquid composition changes (see equation (1)), the liquid path may be curved on a variation diagram. You should be able to reproduce any of these paths with Figure 8.10 by choosing 100% plagioclase as the fractionating mineral and the appropriate starting bulk composition.
No. In fact, each one of these diagrams was produced by fractional crystallization of plagioclase. The different directions of chemical change on the diagrams is the result of different starting bulk compositions. The liquid compositions move directly away from the plagioclase composition. If the plagioclase in equilibrium with the melt has a lower wt.% SiO2 than the liquid, then the liquid compositions will increase in SiO2 during fractionation. If the plagioclase in equilibrium with the melt has a greater wt.% SiO2 than the liquid, then the liquid compositions will decrease in SiO2 during fractionation. The same reasoning applies to the alkalis (Na2O+K2O). Because the equilibrium plagioclase composition changes as the liquid composition changes (see equation (1)), the liquid path may be curved on a variation diagram. You should be able to reproduce any of these paths with Figure 8.10 by choosing 100% plagioclase as the fractionating mineral and the appropriate starting bulk composition.
You are correct. None of the fractional crystallization options listed produce the Sierra Nevada Batholith straight line trend using the Figure 11 model. Compositional variation by the minerals causes the model fractionation paths to curve. This result suggests that there are other reasons for the straight line trend for the Sierra Nevada Batholith. One possibility is the mixing of magmas from two sources.
No. None of the fractional crystallization options listed produce the Sierra Nevada Batholith straight line trend using the Figure 8.11 model. Compositional variation by the minerals causes the model fractionation paths to curve. This result suggests that there are other reasons for the straight line trend for the Sierra Nevada Batholith. One possibility is the mixing of magmas from two sources.
