GEOL 2312 Igneous and Metamorphic Petrology Name KEY Spring 2016 Score / 58 Midterm 1 Chapters 1-10 1) Name two things that petrologists want to know about magmas (1 pt) Formation, source, composition, crystallization, 2) Label the proper term pertaining to the layers of earth on the figure below (2.5 pts) Continental crust Asthenosphere Lithosphere Outer Core Oceanic crust Mesosphere Mantle Inner Core Core COMPOSITIONAL LAYERS PHYSICAL LAYERS Continental Crust Oceanic Crust Mantle Mesosphere Lithosphere Asthenosphere Core Outer Core Inner Core 3) The processes of Plate Tectonics can be thought of as a two-stage distillation process to make the Earth s continental crust (1 pt) A) What is distilled in the first stage? Mantle B) What is distilled in the second stage? Oceanic Crust 4) Name two important goals of any useful classification scheme for igneous rocks (1 pt) Practical, Descriptive, Systematic, Hierarchical, Natural 5) While the mineralogy of an igneous rock may tell us about the composition of the magma from which it crystallized, what can texture tell us? (0.5 pt) Sequence of crystallization, rate of cooling,
6) In addition to a modal rock name, name two other descriptors that are useful to completely describing a rock (1 pt) Alteration, foliation/layering, general textures, grain size (abs/rel), accessory minerals,. 7) Circle the three essential minerals by which MAFIC intrusive rocks are classified (1.5 pts) Olivine Alkali Feldspar Apatite Plagioclase Quartz Pyroxene Gabbro Calcite Biotite Peridotite 8) Circle the three essential minerals by which FELSIC intrusive rocks are classified (1.5 pts) Olivine Alkali Feldspar Apatite Plagioclase Quartz Pyroxene Granite Calcite Biotite Diorite 9) Rank the order of crystal shapes (mineral habits) from 1- closest to its ideal shape to 5 - least resembling its ideal shape (1 pt) 5 Poikilitic 1 Euhedral 4 Subpoikilitic 2 Subhedral 3 Anhedral 10) Ophitic texture in gabbro is a very significant texture because it gives an indication of the evolved (differentiated) state of the mafic magma (1.5 pts) a. What mineral forms the oikocryst in ophitic texture? Pyroxene, Cpx, or Augite b. What mineral habit listed in 9 does the oikocryst mineral have? Poikilitic c. What mineral forms the chadacryst inclusions in ophitic texture? Plagioclase 11) How might we classify volcanic rocks that are too fine-grained or altered to determine their mineralogy? (0.5 pt) By chemical composition, By CIPW norm mineral abundances 12) Fill in the table below with relative terms of low or high for the physical properties of mafic and felsic magmas (1.5 pts) General Magma Type: Mafic Felsic Temperature: Viscosity: Gas Content: High Low Low Low High High 13) What is a major difference between shield volcanoes and stratovolcanoes in terms of: (2 pts) Shield Stratovolcano Dominant volcanic rock type? Basalt Andesite&/or Rhyolite Volcano shape (draw):
14) Name two physical features that you might find in a subaerially erupted plateau basalt (1 pt) Amyg/vesicle zone, pahoehoe (ropey) top, AA top, toe lobes, tabular shape, columnar jts, 15) Name a physical feature that you might find only in a submarine-erupted basalt (0.5 pt) Pillow structure 16) Which of these are true statements about thermodynamics of igneous rocks (Circle T or F; 3 pts): T F All systems in nature strive to achieve the highest level of energy T F Entropy (S) is a measure of the randomness of a chemical system T F A stable phase will be one with the lowest volume and highest entropy at a particular pressure and temperature (see equation in question 3 below) T F A phase is a mechanically separable portion of a chemical system T F The reference state of a phase is measured at 0 K and 0.1 MPa pressure T F Most rocks we see at the earth s surface are in a state of metastability 17) An extensive variable is dependent of the amount of material present; an intensive variable is not. Give an example of each (1 pt). Extensive Mass, volume, Intensive Density, Temperature, Pressure, Element ratios 18) The diagrams to the right shows the Gibbs free energy curves against T and P. Considering the Gibbs free energy equation - dg = VdP SdT, (2 pts) a. Why does the G of the solid and liquid decrease with increasing temperature? Since dg/dt= -S, the slopes will be negative. b. Why does G liquid decrease at a faster rate than G solid? Entropy of a liquid increases faster than that of a solid with increased T c. Why does the G of the solid and liquid increase with increasing pressure? dg/dp= V, the slopes will be positive d. Why does G liquid increase at a faster rate than G solid with increasing P? A liquid is more compressible than a solid, so V changes more in a liquid with changes in P 19) Indicate if the statements about the crystallization behavior of magmas are true or (3 pts): T F Cooling melts crystallize from a liquid to a solid at one temperature when P is constant T F The number of mineral phases crystallizing from a melt commonly increases as T decreases T F The minerals that form from a cooling melt do so sequentially, generally with considerable overlap T F Minerals that involve solid solution will not change composition as cooling progresses T F The melt composition typically changes during crystallization T F The type and sequence of minerals that crystallize from a melt depend on T and X (composition) of the melt (some may add P and thus say this is false)
20) The Gibbs Phase Rule for a P-T phase diagram is F = C - φ + 2 a) How many degrees of freedom (F) exist at points A, B, and C on the phase diagram for SiO 2? (1 pt) A) 2 A C B) 0 B C) 1 b) What can be said about the molar volume of Coesite relative to β Quartz at high pressures? (0.5 pt) V of Coesite is lower the B Quartz c) What can be said about the entropy of β Quartz relative to α Quartz at high temperatures? (0.5 pt) S of B Quartz is higher that A Quartz 21) Match the statement that best applies to the letter on the solid solution phase diagram for equilibrium crystallization of plagioclase (1.5 pts) G Last melt composition A Starting bulk composition of melt D Melt composition at 1450 o C B Beginning of melt crystallization C Composition of first plag to crystallize F Composition of plag at 1450 o H or I Final composition of plagioclase 22) In the same diagram above (1.5 pts): a) What are the relative proportions of plagioclase and liquid at 1450 o C? %Pl ~40 % Liq ~60 b) What is the composition of plagioclase at 1450 o C? An = 80 c) What could be the last composition of liquid during fractional crystallization? An= 0
23) Both solid solution minerals plagioclase - (Na,Ca)(Al,Si) 4O 8 and olivine - (Fe,Mg) 2 SiO 4 commonly develop compositional zoning as they crystallize, as shown in the adjacent figure. However, only plagioclase retains that zoning in intrusive igneous rocks. Why? (1 pt) Redistribution of Ca-Na is linked to Al-Si to maintain charge balance; very hard to break Al-O and Si- O bonds 24) For the binary phase diagram for pyroxene (Di) and plagioclase (An), answer the following. Assume equilibrium crystallization throughout. (5 pts) a. What type of phase diagram is this? solid solution, eutectic, or peritectic (circle one) b. For a starting melt composition at a, at what temperature will crystallization begin? 1450 c. At 1360 C, what will be the relative proportions of liquid to crystals? 33 %xtals: 67 %liq d. What is the composition of the solid mineral assemblage at this temp? 0 %Di:100%An e. At what temperature will Di start to crystallize? 1274 f. At what temperature will Di stop crystallizing? 1274 g. What will be the proportion of Di crystals to An crystals in the final rock? 30%Di: 70%An h. If you heat up a rock containing 80% Di and 20% An, what will be the composition of the first liquid to form? 42wt% An i. What percentage of melting of this rock must take place before all of the An is gone? ~50% j. At what temperature will all of this rock be melted? ~1350
L1 R2 25) For the ternary phase diagram for pyroxene (Di), olivine (Fo), and plagioclase (An), answer the following. Assume equilibrium crystallization unless otherwise noted (5 pts) a. At what temperature will a melt of starting composition L1 begin to crystallize? ~1430 b. With a dashed line and arrows of one color, show the path that a starting liquid of composition L1 would take during equilibrium crystallization. c. Show with a dashed line and arrows the path that the bulk solid would take during crystallization of melt L1. d. When a liquid evolving from starting composition L1 just reaches point M, what are the approximate proportions of liquid and solid of the system? ~30 %Liq ~70 %Solid e. At the same point, what are the proportions of plagioclase and olivine in the bulk solid? ~ 47 %Fo 53 %An f. If a melt of composition L1 fractionally crystallizes in a mafic layered intrusion, what will be the stratigraphic sequence of minerals one would expect progressing from bottom to top?\ Ol Ol+Pl Ol+Pl+Cpx g. What will be the composition of the first melt to form from a rock of composition R2? ~50 %Di ~43 %An ~7 %Fo. h. At what is the temperature will this melting occur? 1270 i. At what % of melting will one of the three mineral phases disappear? ~40-50% j. What mineral will that be? Plagioclase
26) Refer to the periodic table above to answer the following questions about the role of elements in igneous rocks (element abbreviations OK) (3 pts) a. What is the major anion of most rock-forming minerals? O b. What is the most abundant cation found in igneous rocks? Si c. Name two other major element cations found in igneous rocks Al, Fe, Mg, Ca, Na, K, (Ti, P) d. Name a trace element that commonly substitutes for Ca Sr, Ba e. Name a high field strength (HFSE) trace element REE s, Y, Zr, Nb, Ta, Hf, f. What is the valence state of large ion lithophile elements like Na, K, and Rb? +1 27) CIPW norm calculations estimate the idealized mineralogy of a rock from it whole rock composition. Match the CIPW phase (or phases) that is(are) the major carrier(s) of these elements (2 pts) a. K - or ap apatite b. P - ap il ilmenite c. Na - ab or orthoclase d. Ti - il di diopside e. Ca an, di ab albite f. Al an, ab, or an anorthite 28) Most trace elements are incompatible; what are they incompatible with? Minerals/solid phase 0.5 pt)
29) For a magma crystallizing 30% Ol and 70% Pl and mineral-liquid partition coefficents (Kd) values of 20 and 0.01 for Ni in each mineral phase, and 0.01 and 0.1 for La in each mineral phase, write an equation that would calculate the bulk rock partition coefficient (D) for each element (2.5 pts) a. D Ni =.3 * 20 + 0.7 * 0.01 = 6.07 b. D La =.3 * 0.01 + 0.7 * 0.1 = 0.073 c. Which element will become enriched in the magma from the fractional crystallization of this mineral assemblage? La 30) The graph to the right illustrates the effect of % partial melting (F) on element abundance in the melt relative to its original abundance (Cl/Co) for different bulk rock distribution coefficients (D). (1.5 pts) Normal Range of Partial Melting in the Mantle T F Strongly incompatible elements are most enriched at higher degrees of partial melting. T F Variable degrees of partial melting between 0 and 40% don t have a strong effect on changing compatible element concentrations. T F Compatible elements will always have depleted concentrations in the magma relative to the original mantle from which it melted 31) In addition to olivine, clinopyroxene and orthopyroxene, the mantle is composed of Al-bearing phases of either plagioclase, garnet or spinel, depending on depth/pressure. Which of these is the low pressure phase? Plagioclase; which is the high pressure phase? Garnet(1 pt) 32) Which of these Al-bearing phases (or mantle depths) was involved in the generation of basalts with the REE normalization patterns shown below. (1 pt) sample/chondrite 10. 8. 6. 4. 2. Al phase: Plagioclase (Shallow mantle) sample/chondrite 10. 8 6 4. 2 Al phase: Garnet (Deeper Mantle) 0. La Ce Nd Sm Eu Tb Er Yb Lu 0. 5 La Ce 5 Nd 6 Sm 6 Eu 6Tb 6 Er 6 Yb 7 7
33) Under normal geothermal gradient conditions, the mantle will not melt. Name two processes or conditions that allows the mantle to melt (1 pt) Temperature rise (in a plume), adiabatic decompression, Addition of water or volatiles 34) Name three processes or conditions that occur in the mantle that will yield variable magma compositions. (1.5 pts) Different depths of melting Different degrees of melting Heterogeneous mantle compositions Fractionation of magma as it rises through the mantle Variable concentrations and species of volatiles 35) Name two specific conditions of mantle melting that will favor the creation of alkaline basalt magma over tholeiitic compositions (1 pt) Deeper melting Lower degrees of partial melting CO2-rich volatiles