Plagioclase Phase Diagram

Use the diagram at left to answer the following.  (It is slightly different from the one in your text.)

Consider a melt that has composition Ab60An40.

1. If the melt cools, at what temperature will the first crystals form?

2. What will be the composition of those first crystals?

		

3. At what temperature will the ratio of crystals:melt be 1:1?

Suppose, at the time the ratio of crystals to melt is 1:1, the crystals settle to the bottom of the magma chamber and no longer react with the melt.  After this, the remaining melt crystallizes in the normal way, with crystals and melt staying in equilibrium.  

4. What will be the composition of the final drop of melt?  

5. What will be the composition of the last plagioclase to crystallize?