GLY306                                   Study Questions for Exam II                                     Petrology

(30 point) Questions

1. Make a diagram of a typical island arc subduction zone.  Label all major features.  Where is magma generated in such a system?  Why is it generated in those places?  What controls the general chemical character of the produced magmas?  Use a K₂O vs Si₂O diagram to illustrate your main points regarding magma generation and evolution.  What role does dehydration of the down going slab have in the generation and composition of the magmas produced?

2. Use data from Rb and Sr isotopes to clculate the radiometric ag of a comagmatic series of rocks. You will be provided the necessary formulas, constants, and graph paprer for you answers.

(20 point) questions

1. Describe the mechanism of basaltic underplating as it applies to the formation of flood basalts.  Give an actual example to prove your argument.

2. Discuss the three mechanisms by which the mantle can undergo partial melting to produce basaltic magmas.  Give examples of geological environments where each type is manifest.

3. Discuss the roles played by fractional crystallization and partial melting in producing either an alkalic series or a tholeiitic series of products.  Use the Q-Ne-Ol triangular diagram to explain your answer.

4. Explain how a specific phase diagram would govern the melting of a rock of specific composition (e.g. using the system of olivine-SiO₂ polymorph or another similar system with an eutectic or cotectic).

5 . Ansower a series of questions related to the SIAM classification of garnitoid rocks. Relate the petrogenesis to their  geochemistry.

(10 point) questions

1. Name some igneous rocks given their chemical composition and an appropriate diagram on which to plot the data.

2. What is the role of depth (pressure and source composition) and % melting on the formation of melts of tholeiitic, high-Al₂O₃, and alkalic compositions?

3. How can the Q-Ab-An triangular phase diagram be used to determine the water pressure and depth at which granites form?

4. How can a Harker-type diagram be used to show the effects of fractionation of olivine from a basaltic magma?  The data at hand would be chemical analyses of major elements in basalts erupted at the surface.  I will probably provide a diagram and ask some questions.

5. Explain the distinction between alkaline, calc-alkaline, and tholeiitic series using appropriate diagrams.  You should be able to plot these series correctly on the diagrams.

6. Explain the petrogeneitc significance of certain trace elements: compatible/incompatible, fractionation trands, substitution in minerals, source in the mantle, etc.  You would probabluy have a table to complete for a  list of elements.