GEOL 3041 Answers for Chapter 8: Chemical Petrology I: Major and Minor Elements. 100% Score.

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GEOL 3041 Answers for Chapter 8: Chemical Petrology I: Major and Minor Elements. 1. What is AA spectroscopy, and in what way(s) does it differ in principle from XRF? 2. In what principal way does... the EMP differ from AA, XRF, and INAA? 3. If the intensity of some emitted or absorbed spectrum is the type of data supplied by instrumental analysis, how do we convert the counts/second to meaningful concentrations? 4. Use Figure 2.4 to determine whether the rocks analyzed in Table 8.3 are correctly named. 5. Convert the Fe2O3 and FeO in the basalt in Table 8.3 to FeO* 6. Explain in a single sentence what a norm attempts to do. How does a norm differ from a mode? 7. How would you recognize a silica-oversaturated plutonic rock from a silica-undersaturated plutonic rock? 8. Suppose you have the chemical analysis of a volcanic glass. How would you determine whether it represents a silica-oversaturated or undersaturated composition? 9. Suppose you have chemical analyses for two volcanic rocks with identical wt. % SiO2, but one has a higher alkali/Al2O3 ratio. Which is more likely to be silica-undersaturated? Why? 10. Given the following mineral compositions, explain the trends in Fig. 8.2 assuming that fractional crystallization is responsible. Olivine: (Mg,Fe)2SiO4 Orthopyroxene: (Mg,Fe)SiO3 Clinopyroxene: Ca(Mg,Fe)Si2O6 Albite: NaAlSi3O8 Anorthite: CaAl2Si2O8 Orthoclase: KAlSi3O8 Quartz: SiO2 11. What would you call the set of rocks collected at Crater Lake and shown in Fig. 8.2? Why? 12. What would you conclude if the D-P trend in Fig. 8.6b did not extrapolate to a point between A and B on the A-B line? 13. How does the tholeiitic trend differ from the calc-alkaline trend on an AFM diagram (Fig. 8.14)? What binary phase diagram in Chapter 2 helps you understand the initial tholeiitic trend? Explain. 14. In what way is the Ab composition in Fig. 8.13 a “thermal divide”? What is the effect of the divide on cooling silica-oversaturated vs. undersaturated melts? Problems The ability to use a spreadsheet will greatly reduce your workload throughout your petrology course (and beyond). If you are inexperienced with spreadsheets, there is a document called EXCEL.DOC available on my web site (http://www.prenhall.com/winter) that will get you started. Just click on How to Use Excel on the main page. If you are doing any problem using a spreadsheet as a class exercise, I recommend you hand in your spreadsheet for grading electronically, either as an email attachment or into an class drop-box, so your instructor can check your cell formulas if a correction is necessary. 1. Calculation of C.I.P.W. Norm Use the method supplied in Appendix B to calculate a Norm for the following volcanic rock: Tholeiite, Imnaha of the Columbia River Basalts SiO2 51.1 TiO2 2.24 Al2O3 15.1 FeO* 13.0 MnO 0.22 MgO 5.07 CaO 9.31 Na2O 2.58 K2O 0.91 P2O5 0.42 Total 99.95 Be sure you show your subsidiary calculations, such as Y, D, D1, Fe/Mg, formula weights for Di and Hy, etc. 2. Next change the SiO2 value in your spreadsheet to 49%. Because you did not change any other oxides, all of your normative mineral quantities should remain the same, except that you now have a deficit of silica (not enough to make the provisional silicate minerals) in your norm. Return to your norm calculations and proceed through steps 9 and 10. Hand in that spreadsheet also and explain what you had to do to “free up” some silica to accommodate all of the other oxides into appropriate minerals. 3. Open the Excel® file NORM.XLS (available from my web site: http://www.prenhall.com/winter). This spreadsheet was created by Kurt Hollicher of Union College. Enter the wt. % oxide data for the basalt in Table 8.3 in column 2. This spreadsheet calculates a reasonable norm (if you do not try some very unusual compositions). It follows the norm procedure described in Appendix B. Are the minerals appropriate for a basalt? Determine the plagioclase composition (scroll to the right). An54 Now vary the silica content between 52% and 46% in successive 1% increments. Explain the effect this has on the normative minerals. Then return to your initial SiO2 value and vary Al2O3 between 19% and 14% in 1% increments, and explain the effect on the norm. Return again to the original values and increase Na2O in 1% increments to 5%. Then do the same for K2O to 5%. Explain the changes. 4. Use IGPET® to calculate norms for the rocks in Table 8.3. Discuss the results. Much the same. 5. In Chapter 7 we studied the system diopside-anorthite-forsterite (Fig. 7.2). The file CH8-3.XLS is a partially completed spreadsheet for the liquid line of descent for composition a to the ternary eutectic, M. I have calculated the compositions of diopside, anorthite, and forsterite in weight % oxides, and estimated the Di:An:Fo ratios of several points along the liquid line of decent. Complete the spreadsheet, calculating the composition of the liquid at each point. This is easily done for each oxide by multiplying the weight fraction of each phase times the wt.% of the oxide in that phase, and summing for the oxide. Thus the wt.% MgO in “a 1700” is 0.36 • 18.61 + 0.10 • 0 + 0.54 • 50.22 = 33.82, etc. You will next create Harker-type diagrams of the data for liquids along the liquid line of descent. Choose an oxide to act as a good differentiation index. Note that wt.% SiO2 is not a good one, why? If you choose one such as wt. % MgO (as I did), you may want to scale the x-axis from high to low, so the more evolved liquids plot on the right. Create 3 Harker-type variation diagrams (X-Y scatter charts), using your differentiation index and the other three oxides. Print the results, and then connect linear segments of the liquid lines of descent, and extrapolate back to the bulk mineral extract appropriate for each segment. Discuss your results in conjunction with the analysis that accompanied Fig. 7.2. 6. The file TUOLUMNE.ROC on my web site is an IGPET® file for a series of rocks from the Tuolumne Intrusive Complex, shown in Fig. 4.30. Using IGPET®, load the file, and create a series of Harker diagrams. Choose the Diagrams, option, and print all 8 options. You will have to adjust the x-axis to 55-75% SiO2 in order to get all of the points to plot. You can print a set of Harker diagrams by printing each in order, and only affirming the last diagram to be printed when you finish the P2O5 diagram. In Chapter 4 we proposed that the sequence of injections that comprise this intrusive complex were part of a consanguineous magma series. Discuss this proposal in light of the Harker diagrams. Which elements are depleted, and which are conserved? The symbols are: + = marginal quartz diorite, open triangle = non-porphyritic Half Dome granodiorite, filled triangle = porphyritic Half Dome granodiorite, open square = Cathedral Peak granodiorite, filled square = Johnson granite porphry. Use the Diagrams option and plot an AFM diagram (Irvine and Baragar, or “irvbar.dgm”) to determine if the Tuolumne Series is tholeiitic or calc-alkaline. The Miyashiro (1974) diagram under “petrol.dgm” will also discriminate based on SiO2 vs. FeO/MgO. As a final test, plot an X-Y diagram using X = SiO2 and Y = (Na2O+K2O)/CaO. Y will take some fiddling about with the parameters. For example, you must choose A+B and then A = Na2O and B = K2O, then A/B with A = Na2O+K2O and B = CaO before you can select this as Y. The SiO2 value at which this value exceeds 1.0 will give you Peacock’s (1931) “alkali-lime index,” which can be used to determine his estimate of alkalinity. Are your results for the various tests consistent? You may want to experiment with other diagrams. If you plot any diagrams that use normative minerals, you must first run the norm program (preferably cation norm). 7. The analyses for two volcanic rocks from the same vent are given in the accompanying table: Both rocks have less than 5% phenocrysts, and it is believed that the more silicic rock #2 was derived from the more mafic rock #1 by a process of fractional crystallization in a shallow magma chamber beneath the vent. The few phenocrysts in the more mafic rock were plagioclase and augite. Microprobe analyses of each are included in the table. Enter all 4 analyses into either an Excel® spreadsheet or the program IGPET®. Because IGPET® will not plot zero values, I substituted 0.01 for 0.0 in the data. Then create six Harker diagrams with SiO2 as the X-axis and the other six major oxides as the Y-axis. If you use IGPET®, use the XY Plot option. Print the results each time, and connect the olivine and plagioclase points on each diagram. Next draw a line from rock #2 to rock #1, and extend it back until it intersects the olivine-plagioclase line. Determine the ratio of olivine/plagioclase required to produce rock #2 from rock #1 from each plot. Is this bulk mineral extract consistent for all of the diagrams? Discuss the probability that rock #2 was derived from rock #1 by a process of fractional crystallization. Next find the silica content of the bulk mineral extract (note that the SiO2 content at which K2O = 0 on a Harker can give it directly). Use the silica content for the bulk mineral extract and equation (8.4), using rock #1 and rock #2 to calculate the CaO content of the bulk mineral extract. Does this agree with your graphical result? Plot both rocks on an alkali-silica diagram, and name them. This can be done in IGPET® using the Diagrams option and choosing the “TAS (total alkalis-silica) LeBas diagram” under “irvbar.dgm.” As what does the plagioclase + augite rock plot? # 8. The analyses for two volcanic rocks from the same vent plus their contained phenocrysts are given in the accompanying table: Enter all 5 analyses into either an Excel® spreadsheet or the program IGPET®. Because IGPET® will not plot zero values, I substituted 0.01 for 0.0 in the data. Then create six Harker diagrams using the XY Plot option with SiO2 as the X-axis and the other six major oxides as the Y-axis. Print the results each time, and connect the olivine, plagioclase, and augite points on each diagram. Carefully label the olivine, plagioclase, and augite points for each triangle. Next draw a line from rock #2 to rock #1, and extend it back until it passes completely through the olivine-plagioclase-augite triangle. Note the sides of the triangle that are pierced by the extended line. Determine the two points of interception for each triangle. Express the points as a percentage along the triangle side (for example: 27% of the distance from plagioclase toward olivine, etc.). Be sure you do this carefully, because the triangle may invert from one diagram to another. Then, on a sheet of triangular graph paper, mark the points for each of the Harker triangles and connect them with the line (equivalent to the extension from point #2 through point #1). Use the point of mutual intersection of these lines to determine the percentages of olivine, plagioclase, and augite in the bulk mineral extract. Discuss the probability that rock #2 was derived from rock #1 by a process of fractional crystallization. 9. The IGPET® file BOQUERON.ROC contains a series of analyses for the Paraguayan volcano. There is an older, calc-alkaline suite, and a younger, tholeiitic suite that issued from the vent. Fairbrothers et al. (1978), in a detailed analysis employing a great number of least-squares mixing calculations, determined that the calc-alkaline fractionation trend was caused by fractional crystallization of plagioclase + clinopyroxene in the proportion [plag/ (plag+cpx)] = 0.55, whereas the tholeiitic trend was caused by the fractionation of the same minerals in the proportions [plag/(plag+cpx)] = 0.72. Run IGPET and load the file BOQUERON.ROC. Plot Harker diagrams for MgO, FeO, K2O, CaO, and Al2O3 vs. SiO2 (at least). This can be done as an X-Y diagram or as a Special Diagram and choosing the Harker option. I prefer the former, because IGPET® for Windows makes smaller Harker diagrams on the screen via the latter option. The green triangles represent the calc-alkaline series, and the small circles the tholeiitic series. Discuss your results. Note the scatter for some diagrams. Much of the scatter may be the result of the “closure problem” that results from the requirement that any analysis must total 100%, regardless of whether it relates to another analysis by material loss. We can compensate for this by dividing the x and y parameters by a “conserved” element, one not lost by fractional crystallization. Pick a Harker diagram with large scatter, and divide both x and y by a conserved element, such as K2O. This will take some fiddling about with the parameters. For example, you must choose A/B and then A = SiO2 and B = K2O before you can select this as X. Compare your results to the original Harker diagram. Choose Pearce Parameters (leaving the ferric/ferrous ratio). Return to the X-Y plots and plot x = Si/K vs. y= [0.5(Mg+Fe) + 2Ca + 3Na]/K. The y parameter should have a slope of 1.0 if plagioclase fractionation causes the liquid trend, and 1.25 if clinopyroxene does. For example, if pure anorthite fractionates and Si changes by 2, y changes by 2 times the one Ca in anorthite, or 2. The slope is thus 1.0. If albite fractionates and Si changes by 3, y changes by 3 times the one Na in albite, or 3. The slope is thus 1.0. For diopside, if Si changes by 2, y changes by 2 times the one Ca plus 0.5 times the one Mg, or 2.5. The slope is thus 1.25. Discuss the resulting diagram, including the scatter. Compare it to your Harker diagrams. Now plot x = (2Ca + Na)/K and y = Al/K. What is the slope of this line for plagioclase? For clinopyroxene? How will this slope vary with plag/(plag+cpx)? Use the Symbols option and select each symbol group separately to plot. Then Regress the data for each set. Discuss your results for the fractionation of the Boqueron suite. 10. Use IGPET® and load in the file CERRONEGRO.ROC. Create a SiO2 vs. MgO X-Y diagram. Label the points. Note the positions of CN4 (more primitive) and CN1 (more evolved). Next use the program MIXING.EXE that comes with IGPET® to simulate the evolution of liquids from the Cerronegro volcano. Select the CN.MIN file and the CERRONEGRO.ROC file. Then choose Fractional Crystallization as the process to model. Select a mineral of each type (magnetite, plagioclase, olivine, and clinopyroxene). Then choose CN4 as the parent and CN1 as the daughter. The Coefficients in your result are the amounts of each mineral that must be removed (or added if negative) to generate the daughter from the parent. The program uses these values and the mineral analyses to generate a model parent composition that best approximates the true parent. The value of “Sum of squares of residuals” is a measure of the closeness of the fit between the real parent and the model. A value close to zero is good. A value close to 100 is awful. Interpret your results. [Show More]

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