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
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