Petrology: Review Questions on Chapters 4-5

C:\Courses\320\fall2002\review-questions\questions02.wpd; September 4, 2002

1. Through fractional melting, would it be possible to develop a anorthite-rich liquid by melting albite? See Figure 4.12. (For these first few questions, it may help to do the phase diagram questions 8-12, first.)

2. On page 85, top-left, the book discusses various rock types that have been brought up as xenoliths from the mantle: gabbro, eclogite, amphibolite, granulite, peridotite. In the phase diagram at the bottom, another type of rock is mentioned: "lherzolite."

a. What do these six names mean - that is, what minerals do they include and are they plutonic or volcanic? Are these rocks found exclusively in the mantle (when they are not xenoliths)?

b. What other evidence do we have regarding the petrology of source regions of magmas? Which evidence do you find most compelling?

3. Why do most petrologists believe that basaltic magmas come from the mantle? Generally they invoke two types of melting: flux melting and decompression melting. What do these two terms mean?

4. Why do magmas tend to move upwards through the Earth?

5. Define, contrast and compare the following: primitive magma, primary magma, parental magma, derivative magma.

6. Define and explain how magma differentiation occurs.

7. a. Petrologists agree that the source regions for most magmas are very mafic. So, how/why do we get silicic magmas at the Earth's surface?

b. Could petrologists be wrong? Could the source regions actually be granitic, for example? (For example, is it possible for granitic source regions to produce basaltic magmas?) Explain your answere clearly and completely.

8. Consider the phase diagram below. It is for a strange system, containing only two components: NaAlSi₃O₈ and Fe₂SiO₄. There are three possible phases: fayalite, albite and melt.

a. Is this diagram consistent with the phase rule? Where on the diagram does F=0? F=1? F=2?

b. The number of degrees of freedom has something to do with probability. How likely is it to find, for instance, a place in the Earth where albite, fayalite and a melt all coexist? Compare this to the probability of finding a place where fayalite and albite coexist without a melt. Explain.

c. Consider a piece of solid Earth composed of 70% fayalite and 30% albite. Describe what happens as temperature increases to some value well above the liquidus, assuming all crystals and melt stay in equilibrium. How will the composition of crystals and melt change as this goes on?

d. Consider a melt with composition equivalent to 70% fayalite and 30% albite. Describe what happens as temperature decreases to some value well below the solidus, assuming all crystals settle out as they form and so do not stay in equilibrium with the melt Be sure to describe (use a drawing if it will help) what the cumulate at the bottom of the magma chamber will look like, and how the melt changes composition with time.

9. Consider the phase diagram below. The labels at the bottom are confusing (see sketch), but it describes part of the system NaAlSiO₄ - SiO₂.

a. What are the possible phases according to this diagram.

b. The diagram has two eutectics. Where are they on the diagram?

c. The diagram also has a "thermal divide." (The thermal divide is the "hump" in the liquidus curve that corresponds to albite melting.) This means that small variations in composition of a melt can drastically affect how things crystallize. Consider two melts, one of composition Q and the other of composition O. Describe what happens as they cool to temperatures well below the solidus. What will be the final product(s)?

10. Consider the phase diagram below.

a. How many components are involved - what are they? How many possible phases are there - list them?

b. Point R is called a "peritectic." Another term for it is a "reaction point." At a peritectic, strange things happen. A melt and crystals may be in equilibrium, but when the temperature reaches the peritectic, some or all of the crystals disappear and are replaced by different ones. Consider two melts, one of composition O and the other of composition O'. Describe what happens as they cool to temperatures well below the solidus. What will be the final product(s)?

11. Consider the diagram below. It is a two component, one mineral, diagram.

a. There are only two possible phases. What are they?

b. Describe what happens to a melt of composition O as it cools to some temperature below the solidus assuming melt and crystals stay in equilibrium.

c. Describe what happens when olivine of composition 60% fayalite is slowly heated to some temperature above the solidus assuming that all melt squirts off (is separated from the olivine) as soon as it forms. Give me details so I know you know what goes on. (This is a tough question.)

12. In the three preceding questions, you looked at simple systems. We study simple systems - use them as models for nature - because they are easier to understand (and to draw in 2-D) than natural systems. In many cases they describe natural systems reasonably well. Some mineral assemblages (subsolidus) in the diagrams are given below. Natural analogs are in parentheses:

a. fayalite + albite (olivine + plagioclase)

b. nepheline + albite (nepheline + plagioclase)

c. albite + tridymite (plagioclase + quartz)

d. leucite + K-feldspar (leucite + K-feldspar)

e. K-feldspar + silica (K-feldspar + quartz)

f. olivine alone

What rock types correspond to each of the five assemblages? Be sure to consider both intrusive and extrusive. There may be multiple answers.