Florida Atlantic University PETROLOGY -- MIDTERM TWO KEY

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1 GLY4310 Name 60 points March 26, 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) 3 T 1. The chemistry of the Atlantic, Pacific and Indian Ocean ridges is remarkably consistent. 5 T 2. Fast spreading ridges have persistent magma chambers, whereas slow spreading ridges have small, ephemeral magma bodies. 0 F 3. Heat-flow at slow-spreading ridges is much higher than at fast-spreading ridges. 0 T 4. If a melt is fluid saturated, any increase in volatile content will produce one or more additional phases. 2 F 5. Carbon dioxide solubility is very high in silicate melts. Just the opposite 0 F 6. Basaltic magma generally contains more water than granitic magmas. 0 T 7. Augite is a higher temperature mineral in Bowen s Reaction Series than hornblende. 2 F 8. Ultramafic rocks like those in the mantle ordinarily contain volatiles 1 T 9. If P f = P total, the melt is saturated with volatiles. 1 T 10. Rocks form an alkaline magma series may be found within both continental and oceanic plates, but are only associated with convergent plate margins. 2 F 11. A mush zone contains more liquid than a magma lens. 1 T 12. Under adiabatic conditions, decompressive melting can turn up to at least 30% of a rock body into magma. 1

2 Multiple-Choice - Choose the best response to each statement or question. Print the letter corresponding to your choice in the blank. (1 point each) 3 A 1. Which Canadian geoscientist is widely recognized for his phase-equilibrium studies of silicate systems as they relate to the origin of igneous rocks? A. N.L. Bowen B. J.P. Iddings C. A. E. Ringwood D. C. E. Tilley 10 D 2. In the system diopside-anorthite, a binary eutectic is present. As conditions change from a dry case with pressure equal to 1 gigapascal to one where P tot = P f = 1 GPa, what happens to the position of the eutectic? A. It increases in temperature, and shifts toward the diopside end member B. It increases in temperature, and shifts toward the anorthite end member C. It decreases in temperature, and shifts toward the diopside end member D. It decreases in temperature, and shifts toward the anorthite end member 5 D 3. Who suggested dividing the sub-alkaline magma series into the tholeiitic and calcalkaline branches in 1950? A. N.L. Bowen B. J.P. Iddings C. A. E. Ringwood D. C. E. Tilley 2 D 4. The alkaline series is found in which setting? A. Convergent plate margin B. Within an oceanic plate C. Within a continental plate D. All of the above 1 C 5. Gabbro and layered gabbro are found in which oceanic crustal layer? A. 1 B. 2 C. 3 D. 4 1 A 6. Deep sea sediment defines which oceanic crustal layer? A. 1 B. 2 C. 3 D. 4 2

3 7 A 7. The dominant rock type associated with volcanic activity within plates, rather than at interacting plate margins, is: A. Basalt B. Granite C. Granodorite D. Tonalite 8 D 8. Which of the following are formed when a basaltic magma gets close to the surface before encountering meteoritic water? The magma:water ratio is high. Layers of scoria plus ash dip inward and outward at the same angle. Diamond Head in Hawaii is an example. A. Maar B. Scoria Cone C. Tuff Cone D. Tuff Ring 1 A 9. When early-formed, dense crystals settle to the bottom of a magma chamber without further reaction, the process is called: A. Fractional crystallization B. Anatexis C. Forceful injection D. Zone melting 0 D 10. We have never successfully drilled into the upper mantle. Nevertheless, sample fo the upper mantle are thought to be available. Which of the following sources might provide upper mantle rock samples? A. Dredged samples from oceanic fracture zones B. Nodules in basalt C. Xenoliths in kimberlites D. All of the above 2 B 11. The work of the late A.E. Ringwood on the melting and crystallization of the synthetic mantle material called pyrolite failed to show the formation of which of the following minerals at any depth? A. Plagioclase B. Spinel C. Garnet D. Olivine 3

4 1 D 12. A magma formed by melting at depth, without any subsequent modification process, is a: A. Derivative magma B. Parental magma C. Primitive magma D. Primary magma 2 A 13. Normalizing the observed concentrations of elements in REE and spider diagrams relative to C1 chondrites helps to eliminate: A. The Oddo-Harkins effect B. Partial melting effects C. Fractional crystallization effects D. All of the above 4 B 14. In the Hawaiian Island sequence of basaltic eruptions, alkaline basalt is seen in all of the following stages except: A. Pre-shield B. Shield-building C. Post-shield D. Post-erosional 0 C 15. The following set of characteristics describes plutons of what depth of emplacement? Partially concordant, partially discordant, contacts sharp to gradational Country rock C Low grade regional metamorphism Planar foliation, sometimes with lineation, is generally present, especially near the contact Contact metamorphic aureole is usually present No relation to volcanoes No chilled border zone (or very minor) Most major batholiths are mesozonic Generally late stage syn-tectonic, or post-tectonic Spotted slates and phyllites are common A. Catazone B. Epizone C. Mesozone D. None of the above 3 C 16. Lherzolites are a type of: A. Clinopyroxenite B. Orthopyroxenite C. Peridotite A. All of the above 4

5 0 C 17. In the two-layer mantle model, where is the separation between layers within the mantle thought to occur? A. At the Moho B. At 100 kilometers C. At 660 kilometers D. At 2900 kilometers 4 C 18. The lack of HREE depletion excludes which of the following as a source for MORB? A. Plagioclase lherzolite B. Spinel lherzolite C. Garnet lherzolite D. None of the above 4 A 19. MORB s do not show a Eu anomaly. Which of the following layers would likely produce a Eu anomaly if it were the source of MORB. A. Plagioclase lherzolite B. Spinel lherzolite C. Garnet lherzolite D. None of the above 5 B 20. Element ratios are sometimes useful for discriminating different rock types. On the is frequently used is K/Ba. For which of the following rock types is this ration very high? A. E-MORB B. N-MORB C. OIA D. OIT 4 A 21. Pahoehoe is seen in magmas of which viscosity? A. Low B. Medium C. High D. Either B or C 0 C 22. Submarine flows or eruptions result in: A. Columnar structures B. Pahoehoe C. Pillow structures D. Tephra 5

6 Fill-Ins - Write in the word or words which best completes each statement or answers each question. (1 point per blank) 4 1. What does EPR stand for? EAST PACIFIC RISE 8,5 2. If plates move slowly, the individual eruptions over a hot spot coalesce to produce an ASEISMIC RIDGE. 4 3.Anhydrous olivine basalt magma has a LOWER (higher or lower) viscosity than anhydrous rhyolitic magma, if both are at the same temperature 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 the PYROXENE PARADOX Cann proposed a name for a model of axial model chambers. The model envisioned narrow, shallow magma chambers into which magma was injected. Fractional crystallization began to produce MORB. Dikes might emanate upward, to produce sheeted dike complexes. They would also feed pillow flows. Crystallization along the walls, top and bottom of the magmas would produce gabbro. Periodic injections of fresh magma, together with divergent plate motion, would continually expand the magma chamber, preventing crystallization from filling it up. Cann called this model THE INFINITE ONION MODEL Despite the attractiveness of the model outlined in the previous question in explaining many physical features of the ocean crust at a divergent boundary, it was discarded because of one fatal flaw. What was the flaw? THE AXIAL MAGMA CHAMBERS SHOULD BE READILY DETECTED SEISMICALLY, BUT THEIR PRESENCE IS NEVER SEEN A MAGMA SERIES is a group of rocks that share some chemical, and sometimes mineralogical, characteristics Where are the DUPAL volcanoes located? 14)DUPAL VOLCANOES ARE LOCATED IN THE SOUTHERN HEMISPHERE, CLUSTERED AROUND THE 30 S LATITUDE LINE. 6

7 Discussion Questions - Write a complete, concise answer to the following questions. Diagrams (labeled) may be used to supplement your written answers, where appropriate In Fenner diagrams for MORB, what element is chosen for the absicca? Why? Is the concentration of this element plotted increasing from left to right, or right to left? (3 points) Mg is chosen because it varies more than 25% during the evolution of a basaltic magma. The ratio of Mg/Fe 2+ decreases as magmas evolve, so the Mg concentration is plotted increasing from right to left REE diagrams are often used to differentiate E-MORB from N-MORB. What do N-MORB and E-MORB stand for? How do the LREE and HREE patterns for both rock types compare? (5 points) N MORB stands for normal Mid-Ocean Ridge Basalt. E-MORB stands for enriched Mid-Ocean Ridge Basalt. The LREE pattern for E-MORB has a negative slope, but overall the LREE are enriched in E-MORB. The LREE pattern in N-MORB has a positive slope. The HREE patterns for both types are very similar OIA rocks are depleted in both Ni and Cr relative to OIT and MORB. What does this indicate? (2 points) This indicates the OIA s have experienced fractional crystallization of olivine prior to eruption. 7

8 Figure Figure 1 was used to explain what observed trend in the evolution of a basaltic magma from the AMAR region? How does it explain this trend? (3 points) Examination of the CaO/Al 2 O 3 ratio shows a decrease as differentiation proceeds. The ratio drops from 0.90 to Removal of plagioclase cannot account for this. Plagioclase has a CaO/Al2O3 of 0.55, and removal of plagioclase should increase the melt CaO/Al2O3 ratio. Since olivine and opx contain neither Ca nor Al, they cannot be responsible. Cpx, which contains Ca, is the likely candidate for Ca removal. 8

9 Problem - Use the figure below. You should be able to get within 10 C Start at a pressure of 0.5 GPa, with a melt containing 9% H 2 O. At what temperature will the first liquid appear? 775 C At what temperature will their be 25% melt? 1010 C At what temperature will there be 50% melt? 1150 C 9

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. 4 points each The terms derivative magma, parental magma, and primary magmas were discussed in lecture and the textbook. Define each type, and discuss their interrelationships. A PRIMARY MAGMA IS ONE THAT FORMS AT DEPTH, WITHOUT ANY LATER MODIFICATION. A DERIVATIVE MAGMA IS ONE THAT HAS BEEN MODIFIED BY SOME MAGMA DIFFERENTIATION PROCESS AS IT IS RISING. SUCH PROCESSES CAN PRODUCE A MAGMA SERIES. WITHIN A GIVEN SERIES, THE MOST PRIMITIVE MAGMA MAY BE CALLED THE PARENT MAGMA. PARENT MAGMAS MAY OR MAY NOT BE PRIMARY. 10

11 22 2. Which of the following ratios is the most sensitive indicator of mantle contamination? 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb. Why is this ratio the best? How does 204 Pb differ from 208 Pn, 207 Pb, and 206 Pb? The HIMU reservoir is characterized by a high value of µ. What is µ? How is the HIMU reservoir different isotopically from other reservoirs? 206 Pb/ 204 Pb IS THE MOST SENSITIVE, BECAUSE THE RADIOACTIVE PRECURSOR OF 206 Pb IS 238 U, WHICH IS FAR MORE COMMON THE 235 U OR 232 Th. 204 Pb IS NON- RADIOGENIC, WHILE ALL THE OTHERS HAVE A RADIOACTIVE PRECURSOR ISOTOPE. : = 238 U/ 204 Pb. THE HIMU RESERVOIR IS QUITE DISTINCTIVE IN THE Pb SYSTEM, HAVING A VERY HIGH 206 Pb/ 204 Pb RATIO, SUGGESTIVE OF A SOURCE WITH HIGH U, YET NOT ENRICHED IN Rb, AND OLD ENOUGH (> 1 Ga) TO DEVELOP THE OBSERVED ISOTOPIC RATIOS BY RADIOACTIVE DECAY OVER TIME 11

12 Midterm 2 Results 54.0 A B B MEDIAN = C C- MEAN = D D F Previous Results 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% 12

13 OVERALL RESULTS A A B 97.5 B- MEDIAN = C+ MEAN = C 84.0 C D F Previous Overall Results 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% 13