Petrology
Study Guide for Exam 2
Basics of thermodynamics & kinetics
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What is thermodynamics? What is it useful for?
What is kinetics? How is it different from thermodynamics? What is it useful for?
What does it mean for a system to be open or closed with respect to matter? What does it mean for a
system to be open or closed with respect to energy?
Which of the following are reactions:
o Crystallization of quartz from a magma
o Evaporation of water
o Replacement of feldspar by epidote during metamorphism
o Dissolution of salt in water
o Dissolution of Calcite by HCl(aq)
o Combustion of hydrogen with oxygen
o Rusting of iron metal
What is equilibrium? What happens in a system which is at equilibrium?
How does a metastable state compare to an equilibrium state? What is different, and what is the
same?
If G = 0, what does that tell us about a system/reaction? If G > 0, what should happen? If G < 0,
what should happen? Along a phase boundary line on a phase diagram, what is the value of G for
the reaction between the phases found on each side of the line?
Why are solid phases usually favored over liquids phases at high pressures?
Why are liquid phases favored over solid phases at high temperatures?
What is an equilibrium constant (K)? Write one for a simple reaction.
What is a reaction quotient (Q)? How is Q different from K? What does it mean if Q=K? What does
it mean if Q>K? What does it mean if Q<K?
Describe the importance of energy barriers, reaction mechanisms, and reaction rates to kinetics.
What factors can affect reaction rates?
Use kinetics and reaction rates to explain why metastable states occur.
Phase Relations
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What is a phase? What is a component? How are they different? Give two examples of 1-component
systems with multiple phases.
1 atm
Using the diagram to the right, list all of the
components of the system.
Using the same diagram, list all of the possible
phases shown. What is the maximum number of
phases that can coexist together and what are they?
What is a liquidus? Label it on the diagram.
What is a solidus? Label it on the diagram.
What is a eutectic? Label it on the diagram.
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What is Gibb’s phase rule? Use the equation to calculate the number of degrees of freedom at the
eutectic.
What is an intensive variable? Give two examples.
What is an extensive variable? Give two examples.
How does Goldschmidt’s Mineralogical Phase Rule explain the tendency for rocks to contain only a
few abundant phases?
Note: Phase diagram exercises similar to those on the lab homework will be part of the lab exam.
Magma generation
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List the three main ways in which rocks can be made to melt. Which of these is the most important?
Which occurs at ocean ridges? Which is important for melting the mantle at subduction zones?
Which can result from repeated intrusions of mafic magma into the crust?
What effect does the addition of water have on the temperature at which a rock may melt? What
effect can the addition of water have on the extent to which a rock melts at a given temperature?
Usually, we expect higher pressures to require higher temperatures for melting. Why does a granite
in equilibrium with water instead require higher temperatures to melt at the lower pressures found
near the Earth’s surface? (Hint, think about what happens to the volatiles in a magma at low
pressure.)
Why do nearly all rocks progressively melt over a range of temperatures? Or, in other words, why
does partial melting occur?
Summarize the plate tectonic settings in which magma generation occurs. Include the cause of
melting in each setting and the type of material being melted. Also include for each setting, the
compositions (felsic, intermediate, mafic) of the most commonly erupted magmas.
Chemical petrology
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How do major, minor, and trace elements differ in terms of abundance?
What is a Harker diagram or Harker plot? How can the trends on such diagrams be used to test
hypotheses related to fractional crystallization and magma mixing? For example, if a suite of rocks
shows increasing Na2O with increasing silica at lower silica contents and then decreasing Na2O
with silica at higher silica contents, what might explain this pattern? (see slide 7) What might be
happening if CaO first increases and then decreases with decreasing MgO? (see slide 9)
What are the three major assumptions involved in making the above interpretations using Harker
diagrams?
What are some of the chemical classification diagrams used for igneous rocks? How are they used?
What diagram is used to distinguish basalt, rhyolite, phonolite, etc.? Which is used to distinguish
calc-alkaline from tholeiitic rocks?
Why do the ternary classification diagrams for igneous rocks have a top half with quartz and a
bottom half with feldspathoids? Relate this to the Nephelene (Ne) – Quartz (Q) binary phase
diagram.
What is the most abundant igneous rock composition or type?
Relate alkaline, tholeiitic, and calc-alkaline magma series to their plate tectonic setting(s).
Why are trace-elements so important for igneous petrology?
What are the rare earth elements? What kinds of plots are commonly used to examine REE data?
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How does the trace-element content of typical MORB (mid-ocean ridge basalt) compare to typical
OIB (ocean island basalt)?
What is an incompatible element? What happens to the concentration of incompatible trace elements
during fractional crystallization?
What is a compatible element? What happens to the concentration of compatible trace elements
during fractional crystallization?
What is a distribution coefficient? What does it mean if a trace element has a KD=1? What does it
mean if a trace element has a KD>1? What does it mean if a trace element has a KD<1?
Explain how the solid phase(s) present govern whether a trace element will be compatible or
incompatible.
Do melt composition, pressure, and temperature matter when making use of distribution
coefficients?
Which trace element(s) is/are especially compatible in K-feldspar? Explain why. (Think about
charge and ionic radius or about the position in the periodic table.)
Which trace element(s) is/are especially compatible in Ca-plagioclase? Explain why.
Which trace element(s) is/are especially compatible in olivine? Explain why.
Related to the above: be prepared to use distribution coefficients from a table to interpret what will
happen to a given trace element during crystallization or melting.
Which trace elements tend to be particularly mobile when a fluid phase is present? (i.e. Which trace
elements tend to be enriched in subduction zone magmas?)
During small degrees of partial melting, which elements (in terms of distribution coefficients or
degree of compatibility/incompatibility) are enriched in the liquid relative to the bulk composition of
the source rock? Which are depleted in the liquid relative to the bulk composition of the source
rock? What happens to the relative concentrations of these two groups of elements as the degree of
melting increases?
If 50% of a melt crystallizes, by what multiple do the concentrations of highly incompatible
elements increase in the remaining liquid?
 Analytical Methods
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How are reference standards of known composition important for quantitative chemical analysis
using EPMA, SIMS, AA, etc.?
What would you need to do to test the accuracy of an analytical technique?
What would you need to do to test the precision of an analytical technique?