ES 4971-4973 Petrology
Final Examination
Individual work, open notes and text
Print your name: ______
Pledge: I pledge my honor that I have neither given nor taken unauthorized assistance during the course of this examination. Signed: ______
A. Chapter 8
1. A rock sample contains 47.31 wt. % SiO2 ,
2.18 wt.% Na2O and 0.58 wt. % K2O. Name the rock.
a. Phonolite
b. Rhyolite
c. Basalt
2. What causes a single magma to produce many different rocks, if assimilation is not important?
a. alkylation
b. acidification
c. fractionation
3. Which of the following minerals is undersaturated?
a. Albite
b. Nepheline
c. Enstatite
4. Tholeiitic magmas produce pyroxenes, for example the orthopyroxene solid solution (Mg,Fe)SiO3 , from Enstatite to Ferrosilite. As the magma cools, crystals form, sink, cumulate, and are isolated from the melt.
Look at the phase diagram. What is the correct sequence of crystal composition as fractionation progresses?
a. first Mg-rich orthopyroxenes, then Fe-rich
b. first Fe-rich orthopyroxenes, then Mg-rich
B. Chapter 9
5. In Table 6 below from a research paper are partition coefficients CS/CL for some Trace Elements. Which one concentrates the most in Orthopyroxenes, column Opx?
a. La
b. Yb
c. Hf
6. For a Basalt sample from eastern Puerto Rico, the lab measures 35% plagioclase, 40% clinopyroxene, 25% orthopyroxene (all by weight). Using the data in Table 6 above, the bulk distribution coefficient DNb for Niobium Nb is:
a. DNb = (0.35 · 0.0100) + (0.40 · 0.2160) + (0.25 · 0.0150)
b. DNb = (0.35 · 0.1800) + (0.40 · 0.2880) + (0.40 · 0.0260)
7. For the following batch melting process
The weight fraction of melt produced is F =0.2
and the distribution coefficient for Rubidium is DRb =0.1
TO DO: Calculate CL/CO for Rubidium. Show all work.
Answer ______
8. A lab reports a basalt has minor element concentrations
(shown as wt.% x 10 ) listed below right. Use Mullen’s 1983 discriminant chart provided to determine the tectonic setting of the basalt.
a. an ocean island alkaline basalt
b. a mid-ocean ridge basalt MORB
C. Chapter 21
9. RINGWOOD, A.E. 1974. The petrological evolution of island arc systems. Journal of the Geological Society, London 130, 183-204.
Ringwood, (1974) suggested that magmas with a composition of Island Arc Tholeiites (IAT) could be related to dehydration of the hydrated ocean crust (amphibolite) as it transforms to dense eclogite at depths of ~150km +. The hydrous fluids rise up into the peridotite mantle wedge, promoting partial melting of the mantle, thus generating a Tholeiitic basalt.
Question: Melting occurs
a. at much lower temperatures in the presence of water.
b. at much higher temperatures in the presence of water.
D. Chapter 22 Match the terms
Abbreviations T temperature; M metamorphic, metamorphism; Ms metasomatism
10. Gneissose ___a. felsic minerals melt, refreeze.
11. Schist ____ b. Dehydration of amphibolite forms this
12. Blueschist___ c. piece of oceanic crust + upper mantle
13. Ophiolite ____ d. anhedral phaneritic equi-granular texture
14. Eclogite ___ e. visible flat grains roughly parallel
15. Granoblastic ____ f. foliations w/ alternating bands
16. Migmatite ___ g.Glaucophane schist from near subduction zone
______
17. Augen ___ a. gneissic texture in M igneous rock
18. Porphyroblast __ b. large crystal grows within matrix.
19. Porphyroclasts __ c. gneissic texture in M sedimentary rock
20. Orthogneiss __ d. eye-shaped, mineral grains sheared
21. Paragneiss__ e. large crystal older than the matrix
E. Chapters 25 and 26 Match the terms
22. George Barrow ____ a. form in/near wet subduction zones
23. V. Goldschmidt ____] b. Sediment, basalts, dikes, gabbro, peridotites
24. P. Eskola ____ c. forms in dry conditions at high temperatures
25. Granulites _____d. recognized sequence of stable M phases of basalt
26. Ophiolite ____ e. realized M minerals near Oslo same atoms as parent
27. Blueschists ____ f. System changes to restore equilibrium
28. Le Châtelier’s Principle _____ g. discovered M zones in Scotland
F. Chapter 27
29. (5 Points) You prepare a thick section of a metamorphic rock containing Plagioclase, Biotites, Garnets and aluminosilicates (Al2SiO5), then you run electron microprobe scans across interesting areas. In a scan where garnet contacts biotite, you find
Find the Pressure and Temperature of metamorphism
Procedure:
i. Calculate KD then draw in a Garnet-Biotite line
ii. Calculate Pressures in Kilobars for 400 and 700C
Notice 1000 bar = 1 kilobar
Draw in the GASP Line
iii. Crossing Point gives the
P-T conditions
Question:
The PT conditions
during metamorphism were
_____ kbar and _____oC
G. Project skills Part 1, Minerals (1 point each)
Identify the following minerals:
30. Note the “tartan” twins
This is ______
31. Note the polysynthetic “Albite” twins
This is a ______
32. Notice the undulose extinction, and no cleavages.
This is ______
33. Here is an Orthopyroxene in PPL. Note the very faint Pleochroism. Name the two end members of the “hypersthene” solid solution series. ______.
34. Here is Biotite, pleochroic from brown to tan. Name the two end members of the biotite solid solution series. ______
35. This mineral is strongly pleochroic. In some orientations, the cleavages make angles of about 60 and 120 degrees.
This is
a. a garnet
b. an amphibole
36. Take a look at this garnet below under plane-polarized light. It has very strong relief because it has a refractive index of more than 1.7, much more than the 1.54 of epoxy. It looks like a squashed tomato. There are no cleavages, just fractures. What colors will you see if you cross the polars and rotate the stage?
a. 2nd order red and blue
b. black in all positions
H. Project skills Part 2, Protolith Determinations
37. (5 Points) You examine multiple fields of view in a thin section of a leucocratic rock from the Losee Metamorphic Suite, and identify each mineral at every crossing line on a grid. In total you get 63 quartz, 57 Plagioclase or sericitized Plagioclase, 5 Potassium feldspars, 10 Almandine Garnet and 12 mafic minerals. Calculate a multiplier for, and tabulate the percents of, Quartz, the K-spars, and the Plagioclases, then plot your sample on the graph below.
38. (5 points) Plot the wt% of SiO2 versus Na2O + K2O for sample 7 on the Le Bas chart (Figure 2-4) below. Use crossing lines and a colored dot. Assuming no metasomatism, what are possible parent rocks?
Answer______
39. (10 points) For a metasediment in Table 8 of Volkert and Drake, convert the oxide wt% concentrations into atom% for sample 426.
40. (5 points) From the atom % column for the previous problem, column 7, use the values for Si, Al, Ca, Na and K and calculate
a. Si/(Si + Al) = ______
b. Ca + Na/ (Ca + Na + K) = ______
c. Now plot sample 426 on this chart. Use crossing lines and a colored dot.