Name
April 27, 2017
GLY4310
88 points
16 took exam
Florida Atlantic University
PETROLOGY -- FINAL EXAMINATION KEY
Scores to the left of the answer in red are the number of incorrect responses. Instructor comments
and answers are in blue.
True-False - Print the letter T or F in the blank to indicate if each of the following statements is
true or false. Illegible answers are wrong. (1 point each)
1
T
1. According to the SCMR definition of metamorphism, metamorphism may coexist
with partial melting.
0
F
3
T
3. At low pressures, fluids associated with metamorphic reactions may be either liquid
or gas.
2
F
4. Retrograde reactions are endothermic. They often involve dehydration or
decarbonation, or both.
4
F
5. In the isograd scheme for the Buchan type metamorphism, the introduction of
andalusite and cordierite as index minerals indicate that Buchan metamorphism occurred
at higher pressures than Barrovian metamorphism.
3
F
6. The Sanbagawa belt lies nearer the former subduction zone than the Ryoke belt. The
Ryoke belt is richer in alkaline elements than the Sanbagawa belt.
3
T
7. Isotherms within a subducted plate will have lower values than isotherms at the same
depth adjacent to the subducted plate.
5
F
8. All rocks formed under high-strain conditions, such as occurs during fault
metamorphism, are non-cohesive.
3
T
9. If two alternative assemblages are compositionally-equivalent, we must be able to
relate them by a reaction
2. During deviatoric stress, pressure is equal in all directions.
Exam Total
/208
%
Grade
1
3
T
10. Pentii Eskola’s development of the concept of metamorphic facies, was based primarily
on studies of mafic rocks, and is much more dependent on pressure than the grade system
is.
2
T
11. Most metamorphic rocks now exposed at the surface of the Earth belong to the
greenschist, amphibolite, or granulite facies
5
F
12. At low grades, equilibrium is always obtained.
1
F
13. Granulite facies rocks can only be produced from water-saturated protoliths.
2
T
14. Hydration reactions release large quantities of energy and may elevate the temperatures
of well insulated rock bodies substantially.
3
T
15. When one facies is plotted on an ACF diagram, crossed tie-lines indicate nonequilibrium conditions.
7
F
16. The presence of a single pyroxene phase, either cpx or opx, in an assemblage is
diagnostic of the granulite facies. Both cpx and opx must be present
0
F
17. Garnets are always seen in granulite rocks.
3
T
18. Granulite facies rocks are anhydrous.
4
T
19. The assemblage Jadeite + Quartz is characteristic of the high pressure end of the
blueschist facies
6
F
20. None of the garnets associated with the eclogite facies contain calcium. Grossular is
present on the ACF diagram for eclogite, along the A-C line, indicating it contains Ca.
0
T
21. Eclogites span a greater temperature range than any other facies.
7
F
22. P-T-t paths associated with orogenic belt crustal thickening are counterclockwise.
5
T
23. For P-T-t paths associated with orogenic belt crustal thickening , Pmax precedes Tmax by
a significant time period.
0
F
24. Barrow found a greater change in the sandstones than the pelitic rocks he studied.
2
T
25. Ca-bearing minerals, like laumonite, prehnite, and pumpellyite are stable in water-rich,
carbonate-free fluids.
1
T
26. The high P/T baric series is characteristic of below normal geothermal gradients, such
as in subduction zones.
2
4
F
27. Fractional crystallization under low pressures (at a shallow depth) is likely to produce a
silica-undersaturated magma, leading to a rock like nephelinite.
7
F
28. Seismic discontinuities in the earth's interior at depths of 410 and 660 km below the
surface are believed to be due to changes in composition of the earth between layers.
2
T
29. Undulatory extinction results from post-solidification deformation.
Multiple-Choice - Choose the best response to each statement or question. Print the letter
corresponding to your choice in the blank. (1 point each)
1
D
1. The source of meteoric water is:
A. Water from dehydration reactions inside the earth
B. Water from degassing of the mantle
C. Water from pieces of broken asteroids that hit the earth
D. Rainwater
0
B
2. What is the most effective mineralizer?
A. Carbon dioxide
B. Fluoride ion
C. Water
D. None of the above
0
A
3. Pyrometamorphism is a sub-category of what type of metamorphism?
A. Contact
B. Hydrothermal
C. Impact
D. Regional
3
C
4. If the protolith of a metamorphic rock is ultramafic, an increase in which of the
following suites of chemical elements would be expected?
A. Al, K, Si
B. Nearly pure SiO2
C. Mg, Fe, Ni, Cr
D. Ca, Mg, CO2
4
B
5. In the standard Barrovian Zone Classification, which mineral delineates the lowest
grade zone?
A. Biotite
B. Chlorite
C. Kyanite
D. Staurolite
3
7
B
6. The pelitic rocks in the Skiddaw Aureole started as unaltered slates, which grade into
an outer zone of spotted slates, a middle zone of andalusite slates, and an inner zone of
hornfels. What was the intruding magma?
A. Diorite
B. Granite to granodiorite
C. Quartz monzonite
D. None of the above
1
C
7. Which of the following is the correct description of a microbreccia?
A. A cohesive rock containing around 50% clasts
B. A non-cohesive rock containing more than 70% clasts
C. A non-cohesive rock containing less than 70% clasts
D. A cohesive rock containing glass, produced by partial melting, in the matrix
1
D
8. Which of the following is the correct description of pseudotachylyte?
A. A cohesive rock containing around 50% clasts
B. A non-cohesive rock containing more than 70% clasts
C. A non-cohesive rock containing less than 70% clasts
D. A cohesive rock containing glass, produced by partial melting, in the matrix
10
A
9. A striped gneiss is associated with which of the following zones?
A. A wide ductile shear zone
B. Cohesive brittle fault rock
C. Narrow ductile shear zone
D. Non-cohesive brittle fault rock
3
A
10. The presence of the zeolite and prehnite-pumpellyite facies in modern facies charts
was due to the work of:
A. Coombs
B. Eskola
C. Fyfe
D. Tilley
2
B
11. Which of the following is a high-pressure facies?
A. Amphibolite
B. Eclogite
C. Hornblende hornfels
D. Prehnite-pumpellyite
4
10
C
12. Which of the following is a low-pressure facies?
A. Amphibolite
B. Eclogite
C. Hornblende hornfels
D. Prehnite-pumpellyite
2
C
13. Who proposed classifying series of facies into “baric series”?
A. Fyfe
B. Harker
C. Miyashiro
D. Tilley
9
B
14. A sequence of facies (zeolite facies) - (prehnite-pumpellyite facies) - greenschist
facies -amphibolite facies - (granulite facies) is associated with which baric series?
A. Low P/T
B. Medium P/T
C. High P/T
D. This sequence is fictional, and does not correspond to any of the above
10
D
15. In the ACF diagrams, first employed by Eskola in 1915, A stands for alumina, Al2O3.
The corrected value of alumina is computed how?
A. Addition of ferric iron
B. Subtraction of [Na2O]
C. Subtraction of {K2O]
D. All of the above
8
C
16. Which of the following minerals is seen only in the Zeolite facies?
A. Albite
B. Calcite
C. Heulandite
D. Quartz
8
D
17. Jadeite is a sodium type of which mineral?
A. Amphibole
B. Mica
C. Olivine
D. Pyroxene
5
6
C
18. Coleman’s Group A eclogites, xenoliths associated with basalt or kimberlite, are
associated with which of Carswell’s temperature classifications?
A. Low 450°- 550°C
B. Medium 550° - 900°C
C. High 900° - 1600°C
D. Either A or B
12
A
19. Coleman’s Group C eclogites, Bands or lens-shaped deposits in blueshist, are
associated with which of Carswell’s temperature classifications?
A. Low 450°- 550°C
B. Medium 550° - 900°C
C. High 900° - 1600°C
D. Either A or B
5
D
20. On the P-T diagram shown in class for various well studied metamorphic terranes,
which of the following had the highest P/T ratio/
A. Contact metamorphic rocks, around shallow plutons
B. Barrovian
C. Buchan
D. Franciscan
3
A
21. Who proposed the addition of the granulite, epidote amphibolite, and glaucophane
schist facies to Eskola’s original set of five facies?
A. Eskola
B. Coombs
C. Fyfe
D. Goldschmidt
4
A
22. In comparison with MORB, OIT magmas are lower in which substance?
A. Al2O3
B. K2O
C. P2O5
D. TiO2
5
B
23. Pyroxenes which contain relatively large cations (Ca2+, Na+) belong to which crystal
system?
A. Hexagonal
B. Monoclinic
C. Orthorhombic
D. Triclinic
6
6
C
24. Which of the following minerals contains the most calcium? All Old Andean Ladies
Buy Anchovies - now which end is the Ca-rich end?
A. Albite
B. Andesine
C. Labradorite
D. Oligoclase
6
D
25. Sanidine is a type of:
A. Pyroxene
B. Plagioclase feldspar
C. Silica
D. Potassium feldspar
Fill-Ins - Write in the word or words which best completes each statement or answers each
question. (1 point per blank)
0
1. The starting material from which a reaction or recrystallization begins from is called the
PROTOLITH
11
.
2. Suppose a fluid phase contains water, carbon dioxide, and methane. Write an expression for
the total fluid pressure, Pfluid.
8
Pfluid =
PH20 + PCO2 + PCH4
3. An increase in metamorphic grade with time as a rock is subjected to gradually more severe
conditions is known as PROGRADE
1.5
metamorphism.
4. Who made one of the first systematic studies of metamorphic rock types, their variation, and
mineral assemblages in an episode of progressive metamorphism in the Scottish Highlands?
George Barrow
6.5
5. A METAMORPHIC FACIES
is a set of repeatedly associated mineral
assemblages
8.5
6. The terms high-stress metamorphism and dynamic metamorphism are used for two distinct
types of metamorphism. What are they? FAULT-ZONE AND IMPACT METAMORPHISM
9
7. The second most abundant element in the earth is
SILICON
.
8-9. Name and describe two factors that are important in determining whether a crystal is
euhedral or anhedral?
7.5
8). CRYSTALS WHICH FORM VERY EARLY IN THE CRYSTALLIZATION PROCESS
ARE OFTEN EUHEDRAL, BECAUSE THEY ARE GROWING INTO LIQUID, WHICH
OFFERS LITTLE RESISTANCE TO GROWTH.
7
11
9). FORCE OF CRYSTALLIZATION - THE SURFACE ENERGY OF SOME MINERALS IS
MUCH LOWER THAN OTHERS. MINERALS WITH LOW SURFACE ENERGY WILL
USUALLY BE EUHEDRAL. THE METAMORPHIC MINERALS GARNET AND
STAUROLITE, AND THE IGNEOUS MINERALS ZIRCON, APATITE, AND TITANITE
ARE EXAMPLES.
Problem - SHOW ALL WORK - there will be no credit for answers not supported by sufficient
work to justify your answer. Show the formula used in your calculation, and identify all
parameters, including units, used in the formula. Clearly label your answer, including units if
any. Be sure to express your answer to the correct number of significant figures.
19.5
1. Suppose a reaction takes place with a constant energy barrier of 3420 cal/mol. Calculate K at
635C. (4 points)
where K = equilibrium constant (dimensionless)
E = energy barrier (calories/mole)
R = gas constant = 1.987 cal/ mole
T = temperature (Kelvin) - C + 273.15 = Kelvin
e = natural logarithm base
8
Discussion Questions - Answer all of the following questions completely but concisely. Points
as indicated. Diagrams (labeled) may be used to supplement your written answers, where
appropriate.
15.5
1. If the country rock is permeable and fluids are available, how will these conditions affect the
country rock near an igneous intrusion, and the intrusion itself, over a substantial period of time?
(2 points)
Fluids will cool the intrusion quicker, and will heat the country rock faster and at a
greater distance from the contact.
11
2. What agent(s) is associated with impact metamorphism? (1 point)
A transient, extremely high pressure wave, accompanied by rapid very large temperature
increase
40.5
3. The Crestmore, California area was studied by Burnham in a classic paper from 1959 which
clearly indicated pyrometamorphism had occurred. What was the country rock? What type of
rock did the intruding magma produce? Why is the texture of this rock significant? How did this
create a pyrometamorphic event at shallow depth here? (4 points)
The country rock was Mg-bearing carbonates which had been previously metamorphosed
to brucite-bearing calcitic marbles. The magma body produced a quartz monzonite
porphyry. The porphyry texture of the intrusive rock suggests that silica rich fluids were
released from the magma. When the magma is rising, crystals form and grow, becoming
phenocrysts. When the magma gets close to the surface, the volatiles may escape. This
quickly undercools the magma (raises the melting point) and the remaining magma
solidifies in tiny crystals. The escaping volatiles, especially water, are hot and loaded with
dissolved silica. The heat from the escaping water adds to convective heat loss, and
creates a pyrometamorphic situation. The silica-rich fluids reacted with the carbonates at
very high temperatures despite the shallow depth. The Mg-bearing carbonates contained
little silica, so it is clear that the fluids had to be the source of silica.
9
43
4. How did Burnham ultimately present the data from his study? How does this serve to
emphasize his explanation of what occurred at Crestmore? (3 points)
He formulated a series of ten assemblages, arranged in four zones, and numbered 1-10.
One was the lowest grade, ten the highest. He plotted these assemblages on e ternary
phase diagram using SiO2, CaO, and MgO as the end members. Alumina and the volatiles
were omitted during this plotting. A very strong pattern emerged with number 1 plotting
near the CaO apex and the successive assemblages moving upward toward the SiO2 apex,
thus making it clear that silica was being added to the country rock.
19.5
5. What is a mylonite? How does a mylonite differ from a blastomylonite? ( 2 points)
A mylonite is a foliated cohesive rock containing around 70% clasts, and produced by
ductile deformation, usually along a fault. A blastomylonite is similar, but contains clear
evidence of recrystallization of clasts.
24
6. Rocks in the Zeolite and Prehnite-Pumpellyite Facies are usually only partly altered, and the
products are extremely fine-grained, messy-looking minerals. Most workers chose to ignore
them, simply calling them altered. Why do the rocks at a location in New Zealand show clear
evidence of being composed of equilibrium assemblages? (2 points)
New Zealand protoliths includes graywackes and tuffs, which are fine-grained and very
reactive, so the reactions went further there then in most other locations. In addition,
metamorphism in New Zealand was related to the degree of circulating fluids, and is thus
partially hydrothermal.
10
27
7. The change from the Greenschist to the Amphibolite facies is marked by two important
mineral transitions. What are the two transitions? Which of the new mineral species produced
will appear first under Barrovian conditions? Which appears first under Buchan conditions? (4
points)
The transitions are:
1. Albite to oligoclase (across the peristerite gap).
2. Actinolite to hornblende
Under higher pressure Barrovian conditions, hornblende appears before plagioclase (this
led to the proposal for the transitional albite-epidote-amphibolite facies.
Under lower pressure Buchan conditions, oligoclase appears before hornblende.
0
8. In rocks of the amphibolite facies, hornblende is observed to change from green to brown in
thin section. What produces the color change? (1 point)
As grade increases, the titanium content of hornblende increases, which is responsible for
the changing color observed in thin section.
48.5
9. Pyralspite is a term used to represent certain minerals in a mineral family. Explain what the
family is, and which three minerals are involved. Pyralspite minerals are absent from rocks
associated with which pressure regime? Why? (4 points)
Pyralspite garnet is a term used to represent garnets with the composition M3Al2(SiO4)3,
where M = Mg, Fe2+, or Mn2+, which includes pyrope, almandine, and spessartine, as well
as intermediate forms. Pyralspite garnets are not associated with low pressure regimes,
because they are dense minerals, requiring higher pressures to form.
11
42.5
10. Precambrian blueschists are rare. Three possible explanations were discussed in class. What
were they? Which was said to be very unlikely, and why? (4 points)
1. There was no subduction before the Cambrian. This is very unlikely, since the early
earth was hotter than today internally - the heat engine that drives plate tectonics should
have been even more active then.
2. Geothermal gradients were higher, and the P/T ratio would have been lower, making
blueschist minerals impossible.
3. Early metamorphism has been overprinted by later events, and is no longer
recognizable.
38.5
11. There are three approaches used to try to determine P-T-t paths. Describe them. (3 points)
1. Look for partial overprints of one mineral assemblage on another - Relict minerals may
include part of the path, either prograde or retrograde. We have previously seen that
equilibrium is apt to be maintained during prograde metamorphism. Relict minerals
reflecting prograde metamorphism are therefore relatively common. The necessity for
hydration reactions to occur during retrograde metamorphism makes the maintenance of
equilibrium more difficult. Nevertheless, relict minerals reflecting retrograde paths have
been observed.
2. Apply the techniques of geothermometry and geobarometry to studies of core to rim
changes in the geochemistries of individual, chemically zoned, mineral grains - Electron
microprobes have greatly enhanced our ability to do this. Other techniques can be used as
well. These include oxygen isotope geothermometry, data from fluid inclusions, and
isotopic cooling ages, such as the 40Ar/39Ar method. All of these methods are examples of
"reverse" methods. They begin with studies of the product, and attempt to determine the
process that created the product. At best, they give us clues to part of the P-T-t path.
3. Heat-flow models can be used to theoretically recreate complete P-T-t paths - These
models are evaluated by comparison with the partial data obtained from the reverse
methods.
12
3.5
12. Why is lead thought to be a good indicator of contamination of mantle rocks? (1 point)
The mantle contains very little lead, so mantle samples with high lead concentrations are
probably contaminated by non-mantle rocks.
15.5
13. OIA rocks are depleted in both Ni and Cr relative to OIT and MORB. What does this
indicate? (Type of event, and timing of the event) (2 points)
This indicates the OIA’s have experienced fractional crystallization of olivine prior to
eruption.
HAVE A GREAT SUMMER AND ENJOY
SFC IF YOU ARE GOING ON IT.
IT'S BEEN FUN!
13
Final Exam Grades (out of 88)
83.0
82.0
73.0
67.0
65.5
62.0
60.5
59.0
58.5
55.5
55.0
54.5
52.5
52.0
50.0 - 2
44.0
A
B
C
CMEAN = 60.8 (69.0%)
MEDIAN +58.8
D+
D
D-
Previous Final Exam Results
Spring, 2017
Mean = 60.8 (69.0%)
Spring, 2016
Mean = 74.5 (84.7%)
Spring, 2015
Mean = 79.0 (89.8%)
Spring, 2014
Mean = 63.9 (72.7%)
Spring 2013
Mean = 67.1 (76.3%)
Spring 2012
Mean = 73.6 (83.7%)
Spring 2011
Mean = 76.4 (86.8%)
Spring 2010
Mean = 79.5 (90.3%)
Spring 2008
Mean = 76.2 (86.6%)
Spring 2006
Mean = 81.3 (92.4%)
Fall 2001
Mean = 76.4 (86.8%)
Eleventh out of last eleven years
14
Total Exam Results (Possible = 208)
203.5
188.0 - 2
185.0
182.5
173.0
172.5
170.0
168.5
163.0
159.5
152.0
143.5
142.5
142.0
119.0
A+
AB+
B
MEDIAN = 169.3
BC+
MEAN = 165.8 (81.8%)
C
D+
F
Exams Overall
Spring, 2017
Mean = 81.8%, B_
Spring, 2016
Mean = 79.9%, C+
Spring 2015
Mean = 83.4%, B
Spring 2014
Mean = 77.4%, C+
Spring 2013
Mean = 78.1%, C+
Spring 2012
Mean = 80.3%, B-
Spring 2011
Mean = 83.0%, B
Spring 2010
Mean = 82.9%, B-
Spring 2008
Mean = 80.7%, B-
Spring 2006
Mean = 84.8%, B
Fall 2001
Mean = 81.9%, B-
Sixth out of last eleven years.
15