Midterm 2 Key - Florida Atlantic University

GLY4310
Name
60 points
March 23, 2015
15 took exam - Numbers to the left of the question number in red are the number of incorrect
responses. Instructor comments are in blue.
Florida Atlantic University
PETROLOGY -- MIDTERM TWO KEY
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. Along MOR’s, most of the fastest spreading rates observed are along the East Pacific
Rise.
3
T
1
F
5
F
3. In the system Forsterite-Nepheline-Silica as pressure increases the eutectic shifts
toward the silica apex.
1
T
4. Fast spreading ridges have persistent magma chambers, whereas slow spreading
ridges have small, ephemeral magma bodies
0
T
5. Shield volcanoes, such as Mauna Loa, cover a much larger area, and have greater rock
volume, than stratovolcanoes, such as Mt. Rainier.
2
F
6. Alkaline basaltic magmas are likely to be richer in water than tholeiite magmas,
whereas tholeiitic magmas are likely to be richer in carbon dioxide than alkaline basalt
magmas.
0
F
7. The seismic Moho and the petrologic Moho are the same depth below the surface.
3
T
8. The slopes of the 87Sr/86Sr will change after a melting event. The slope for the rocks
formed from the melt will be steeper, while the slope for the residuum will be less. The
143
Nd/144Nd ratio will increase more in the residuum than in the melt.
4
T
3
T
2. A system in which the pressure is constant is known as an isoseismic system.
9. N–MORB shows a large LREE depletion, and a positive slope.
10. Although plumes associated with hot spots are thought to be less than 100 km in
diameter, the thermal effects of a plume beneath the ocean are much greater. The high
heat flow causes the plate to swell for about 600 km around the plume, with an average
height above sea-bed of 1200 m at the plume location.
1
1
F
11. Water solubility is less in felsic minerals like feldspar and quartz, which have
bridging oxygens, than in olivine, which doesn’t.
5
F
12. The composition of slow-spreading ridges are more varied than those of fastspreading ridges, and show either a larger proportion of evolved liquids or a greater
degree of partial melting.
Multiple-Choice - Choose the best response to each statement or question. Print the letter
corresponding to your choice in the blank. (1 point each)
5
A
1. The calc-alkaline series is found in which setting?
A. Convergent plate margin
B. Divergent plate margin
C. Within an oceanic plate
D. Within a continental plate
2
B
2. Which of the following lherzolite phases occurs at the shallowest depths?
A. Garnet
B. Plagioclase
C. Spinel
D. None of the above
4
D
3. Which isotope of lead does not have a radiogenic precursor?
A. 208Pb 238U
B. 207Pb 235U
C. 206Pb 232Th
D. 204Pb
3
D
4. Which of the following set of conditions would most like result in the petrogenesis of
tholeiite basalt?
A. Deep, carbon-dioxide rich
B. Deep, water-rich
C. Shallow, carbon-dioxide rich
D, Shallow, water-rich
1
C
5. This type of volcanic cone result from the interaction of magma with shallow surface
water. The bedding dips inward at a steep angle, and outward at shallower angles. What
type of cone is this?
A. Maar - all bedding dips outward
B. Scoria Cone - all bedding dips outward
C. Tuff Cone
D. Tuff Ring - Layers of scoria plus ash dip inward and outward at the same angle
2
0
A
6. Which of the following is a discordant intrusion?
A. Dike
B. Laccolith
C. Lopolith
D. Sill
7
A
7. Which type of pluton would have these characteristics? The intrusion lacks chilled
border zone, has no relation to volcanoes, is generally concordant with country rock, has
gradational contacts, with the internal fabric of the pluton often foliated, passing into the
country rock, which makes identification of the plutonic rock as igneous difficult.
A. Catazone
B. Epizone
C. Mesozone
D. This does not describe the characteristics of a real pluton, since characteristics from all
three zones are mixed together
0
A
8. Bowen’s Reaction Series is based on an extensive study of what type of rock by
Norman Bowen?
A. Basalt
B. Granite
C. Ijolite
D. Syenite
7
D
9. (δG/δP)T is equal to:
A. ΔVdp - ΔSdt
B. ΔS/ΔV
C. -S
D. V
2
B
10. Which petrologist first divided magmas into two series, the alkaline and the subalkaline, in the classic paper "The Origin of Igneous Rocks" published in 1892?
A. N.L. Bowen
B. J.P. Iddings
C. A. E. Ringwood
D. C. E. Tilley
1
D
11. Lherozolite is a type of:
A. Clinopyroxenite
B. Dunite
C. Orthopyroxenite
D. Peridotite
3
1
C
12. A magma that forms by melting at depth, without any later modification, is a
description of which of the following?
A. Derivative
B. Parental
C. Primary
D. Secondary
5
B
13. On the continuous side of Bowen’s Reaction Series, what solid solution series is
crystallizing?
A. Olivine
B. Plagioclase feldspar
C. Potassium feldspar
D. Amphiboles
1
C
14. The coordination number of Si4+ below 660 kilometers depth is thought to be what?
A. III
B. IV
C. VI
D. VIII
3
A
15. The dominant type of magma in the Hawaiian Islands is:
A. OIT
B. OIA, silica undersaturated
C. OIA, slightly silica saturated
D. There is no dominant type, because the composition varies substantially over time
0
D
16. LIL elements are all enriched in OIB relative to MORB. They can be used to evaluate
which of the following?
A. Source composition
B. Degree of partial melting and residual phases
C. Subsequent fractional crystallization
D. All of the above
1
D
17. A study by Rhodes showed that clinopyroxene removal is necessary for mass balance to
be maintained in models of MORB fractionation, despite the fact that clinopyroxene is not
a liquidus phase in them. This is called what?
A. Pyroxene ambiguity
B. Pyroxene conundrum
C. Paradox enigma
D. Pyroxene paradox
4
7
A
18. The elements Th, U, Ce, Zr, Hf, Nb, Ta, and Ti are known as what type of elements?
A. HFS
B. HREE
C. LIL
D. LREE
10
D
19. Which of the following minerals is an alkaline amphibole?
A. Aegirine
B. Albite
C. Nepheline
D. Riebeckite
6
C
20. Which of the following minerals is an orthopyroxene?
A. Augite
B. Diopside
C. Hypersthene
D. Hornblende
5
C
21. Which of the following minerals is hydrous?
A. Albite
B. Augite
C. Phlogopite
D. Labradorite
Fill-Ins - Write in the word or words which best completes each statement or answers each
question. (1 point per blank)
1
1. What does EPR stand for? EAST PACIFIC RISE
8
2. The type of rock associated with the magma series OIA is ALKALINE BASALT
2
3. Persistent magma chambers just below the ridge axis were postulated as narrow (5 km) and not
too deep (up to 9 km). Magma was injected and began to fractionally crystallize to produce
derivative MORB's. Dikes could emanate upward to create sheeted dike complexes, and feed the
pillow flows. Crystallization along the walls, top, and bottom of the chamber could create
gabbros. Periodic injections of fresh magma, together with divergent plate movement, would
continually expand the chamber, preventing the crystallization from completely filling the
chamber. Indeed, Cann called this model the "INFINITE ONION
", since layers would
being added from the inside. The dense ol and opx would settle toward the floor, producing the
ophiolite layers of level 3, and suspected to be present in layer 4.
5
4
4. Why are bombs rounded to sub-rounded? THEY WERE VISCOUS LIQUIDS WHEN
ERUPTED, AND WERE ROUNDED IN FLIGHT
3
5. For the equation, V  V L  V S , ΔV is positive on going from solid to liquid. Name a
substance, which is very significant in geology, for which ΔV is negative.
WATER
3
6. The addition of water or alkali elements to magmas such as rhyolite substantially reduces the
viscosity of the magma. Why?
WATER AND ALKALI ELEMENTS REDUCE THE LENGTH OF SILICA CHAINS IN THE
LIQUID MAGMA. AS CHAINS GET LONGER, THEY TWIST AROUND EACH OTHER,
INCREASING VISCOSITY, SO REDUCING CHAIN LENGTH REDUCES VISCOSITY.
Omit
7. Maars are negative topographic features, with craters excavated by hydromagmatic (phreatic)
explosions. Such explosions occur when magma interacts with meteoritic water (either surface or
groundwater). The water flashes into steam, with a huge increase in volume.
4
8. The origin of columnar jointing is somewhat controversial, but it is often observed that joints are
present at the top and bottom of a flow, while the middle of the flow, called the entablature, is
composed of irregular curved columns. What explanation is usually given for this phenomenon?
THE TOP AND BOTTOM OF THE FLOW COOL FASTER, WHEREAS THE CENTER
STAYS HOT. TENSIONAL CRACKING DEVELOPS AS THE CONTRACTING LAVA
PULLS AWAY INTO POLYGONAL COLUMNS
5.5
9. A FLUID-SATURATED
melt contains the maximum amount of dissolved
volatile species possible at a given set of P-T-X conditions.
7.5
10. What is the primary reason some gabbros may melt at considerably lower temperatures than
basalt, despite similar magmatic chemistries?
BASALT MAGMAS ARE USUALLY ANHYDROUS, WHEREAS SOME GABBROIC
MAGMAS HAVE A HYDROUS COMPONENT. WATER DEPRESSES THE MELTING POINT.
6
8
11. In 1967, the late A.E. Ringwood of Australian National University performed a series of
experiments on a synthetic material called pyrolite, which was supposed to represent the earth’s
mantle. The pressure was varied to simulate various depths below the surface, down to about 140
kilometers below the surface. What important phase was never found in any of the runs made in the
Ringwood lab? SPINEL LHERZOLITE
12-13. Why are divergent boundaries more likely to be found in oceans than on a continent?
Discuss two reasons for this.
10
12) ONE REASON IS THAT OCEANS COVER 70% OF THE EARTH'S CRUST, CONTINENTS
30%.
1.5
13) ANOTHER IS THAT THE OCEANIC LITHOSPHERE IS MUCH THINNER (10 KM) THAN
CONTINENTAL LITHOSPHERE. THIS RESULTS IN RIFTS ON LAND EITHER OPENING,
SPLITTING A CONTINENT AND CREATING A NEW OCEAN, OR FAILING, AND
BECOMING AN AULACOGEN. SINCE A RIFT ON THE OCEAN FLOOR IS NO LONGER
CONTINENTAL, CONTINENTAL RIFTS SIMPLY DO NOT LAST FOR LONG.
14-15. Basalts displaying a “pillow” structure are believed to have formed in one of two ways.
These are:
1
14)BY SUBMARINE OR SUBLACUSTRINE ERUPTION.
5
15)BY SUBAERIAL ERUPTION, FOLLOWED BY FLOW INTO THE OCEAN.
7
Discussion Question - Write a complete, concise answer to the following question. Diagrams
(labeled) may be used to supplement your written answers, where appropriate.
10
1. Peter Wyllie extended the work done by A.E. Ringwood, which considered only the effects of
partial melting. Explain how he did this. 2 points
PETER WYLLIE TOOK THE PROCESS A STEP FURTHER, CONSIDERING BOTH
PARTIAL MELTING AND THE REMOVAL OF A PHASE BY FRACTIONAL
CRYSTALLIZATION. AT A GIVEN DEPTH OF MELTING, FRACTIONAL
CRYSTALLIZATION IS SIMPLY THE REVERSE OF PARTIAL MELTING. BY ALLOWING
THE MELT TO RISE TO A SHALLOWER DEPTH, THE FRACTIONAL
CRYSTALLIZATION PROCESS REMOVES A DIFFERENT MATERIAL THAN WAS
ORIGINALLY MELTED, PRODUCING A WIDER SPECTRUM OF PRODUCTS. FOR
EXAMPLE, USING THE 30% MELT OF RINGWOOD AT 60 KM, AND REMOVING AN
ALUMINOUS ENSTATITE PHASE RESULTS IN AN ALKALINE BASALT, THE SAME
PHASE PRODUCED BY 20% PARTIAL MELTING AT 60 KM.
8
2. On an REE diagram shown in class, results for microprobe analyses of MORB samples were
shown. What pattern was observed? Discuss the implications of this result. (2 points)
13
IT SHOWS A PATTERN OF HEAVY RARE EARTH ELEMENT (HREE) ENRICHMENT.
NEITHER PARTIAL MELTING NOR FRACTIONAL CRYSTALLIZATION CAN PRODUCE
THIS EFFECT. THIS SUGGESTS THE MANTLE IS ALREADY LIGHT RARE EARTH
ELEMENT (LREE) DEPLETED! THE MANTLE IS THOUGHT TO HAVE A COMPOSITION
LIKE THE CI CHONDRITES. IF SO, THE PLOTS SHOULD ALL HAVE A VALUE OF 1 OR
EACH ELEMENT, AND NO SLOPE. ONLY IF THE MANTLE WERE PREVIOUSLY
DEPLETED IN LREE COULD WE GET A MELT LIKE MORB, WITH A POSITIVE SLOPE.
3. On the plot of REE for both MORB and OIB, the ordinate was shown as rock/chrondrite. Why is
this normalization performed? (1 POINT)
9
THE ODDO-HARKINS EFFECT WOULD PRODUCE A JAGGED SAWTOOTH PATTERN IF
NO0RMALIZATION IS NOT DONE. THIS PATTERN OBSCURES THE TREND OF THE
ANALYSES.
9
Problem - Use Figure 7-22 to answer the questions. ( 1 point each)
3
1. Start at a pressure of 0.3 GPa, with a melt containing 10% H2O. At what temperature will the
first liquid appear?
810EC
4
2. At what temperature will their be 25% melt?
925EC
5
3. At what temperature will there be 50% melt?
1060EC
±10EC for full credit; ±20EC for half credit
10
Short Answer - Write a complete, concise answer to the following two questions. Diagrams
(labeled) may be used to supplement your written answers, where appropriate. 5 points each
48
1. One of the problems in producing MORB is the position of the geotherm well below the solidus.
One possible solution to this problem is decompression melting. Discuss this process, including the
reason upwelling might occur. It is thought that 20-30% melting must occur to produce the observed
amount of MORB on earth. How deep would the rising rock body likely have to start to produce this
amount of melting. Roughly sketch a phase diagram illustrating your discussion.
Decompression melting under adiabatic conditions, without loss of heat. This curve is a loss of
about 12EC/GPa. It is impossible to follow a true adiabat, but rapidly rising masses would come
close. The melting curve is decreasing about 130EC/GPa, so it is possible for the rock to initiate
melting. Once melting begins, the heat of fusion will consume energy, and keep the adiabatic path
to a shallow slope. Melting will occur, but slowly, and with limited quantities of melt.
Why would the upwelling occur at all? At a divergent plate boundary, the plates are pulling apart,
leaving a potential void. The ductile mantle flows upward, creating a rapid upwelling. It has been
calculated that 10-20% melting/GPa, or about 0.3%/km of upwelling, will occur. If the upwelling
material began at the solidus temperature, rising 65-100 km would produce the 20-30% melting
thought necessary to produce MORB. But since rock is more likely at the geotherm, more heat
would be necessary, and the probable starting point is about 150 km deep.
11
29.5
2. Hawaii is by far the best studied of the oceanic islands. A typical sequence of four stages of
eruption has been discovered, beginning before the shield volcano has formed. Name and describe
these four stages of eruption. Which stage(s) brings a wide variety of xenoliths to the surface?
1. PRE-SHIELD STAGE - SUBMARINE ERUPTIONS OF ALKALINE BASALT AND
HIGHLY ALKALINE BASANITES, FOLLOWED BY THOLEIITIC BASALT. LOIHI
SEAMOUNT, SITUATED ABOUT 35 KM SOUTHEAST OF THE ISLAND OF
HAWAII, IS AT THIS STAGE. THE ALKALINE BASALTS REPRESENT LOW
DEGREES OF PARTIAL MELTING.
2. SHIELD-BUILDING STAGE - TREMENDOUS OUTPOURINGS OF THOLEIITE
BEGIN TO BUILD A SHIELD VOLCANO. THE ISLAND OF HAWAII IS COMPRISED
OF FIVE SHIELDS. MAUNA LOA AND KILAUEA, THE MOST ACTIVE SHIELDS,
ARE PRESENTLY IN THIS STAGE. MAUNA LOA ALREADY REPRESENTS 40,000
KM3 OF BASALT. 98-99% OF ALL LAVA IN HAWAII RESULTS FROM THIS
STAGE.
3. POST-SHIELD STAGE - MAUNA KEA, HUALALAI, AND KOHALA, THE
REMAINING THREE SHIELDS ON THE ISLAND OF HAWAII, HAVE REACHED
THIS STAGE. ACTIVITY SLOWS, BECOMES EPISODIC (AS OPPOSED TO NEARLY
CONTINUOUS), ALKALINE, AND MORE VIOLENT. LAVAS ARE MUCH MORE
VARIED AS THE RESULT OF SHALLOW FRACTIONATION. ACTIVITY FADES,
ENTERING A PERIOD OF DORMANCY FOR ABOUT 0.5 TO 2.5 M.Y.
4. POST-EROSIONAL STAGE - HIGHLY-ALKALINE, SILICA UNDERSATURATED
MAGMAS ERUPT. ROCK TYPES INCLUDE ALKALI BASALTS, BASANITES,
NEPHELINITES, AND NEPHELINE MELILITES.
THE ALKALINE STAGES BRING A WIDE VARIETY OF XENOLITHS TO THE
SURFACE.
12
Midterm 2 Results
62.0
57.5
56.5
55.5
55.0
52,5
48.5
44.5
42.5
42.0
41.5
41.0
39.0
36.5
32.5
A
AB+
BC
MEAN = 47.1 (78.6%)
MEDIAN = 44.5
CD+
D
DF
Previous Results
Spring 2015
Mean = 78.6%
Spring 2014
Mean = 87.2%
Spring 2013
Mean = 81.0%
Spring 2012
Mean = 74.9%
Spring 2011
Mean = 78.6%
Spring 2010
Mean = 78.3%
Spring 2008
Mean = 70.8%
Spring 2006
Mean = 81.7%
Fall 2001
Mean = 83.1%
Tied for fifth out of previous 9 years
13
OVERALL RESULTS (120 possible)
115.5
114.5
108.5
105.0
103.5
99.0
88.3
86.0
85.5
85.0
83.0
78.5
77.3
73.5
67.5
A
AB+
B
BC
MEAN = 91.4 (76.1%)
MEDIAN = 86.0
CD+
D
DF
Previous Overall Results
Spring 2015
Mean = 76.1%
Spring 2014
Mean = 81.5%
Spring 2013
Mean = 84.9%
Spring 2012
Mean = 77.8%
Spring 2011
Mean = 75.9%
Spring 2010
Mean = 77.4%
Spring 2008
Mean = 76.3%
Spring 2006
Mean = 79.2%
Fall 2001
Mean = 78.2%
Eight out of previous 9 years
14