Chapter 08 Ans

Chapter 08 Ans - Answers for Chapter 8: Chemical Petrology...

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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? AA is atomic absorption spectroscopy. In AA a solution of dissolved sample is aspirated into a flame or a graphite furnace, and a beam of light (of a predetermined wavelength characteristic of the element to be analyzed) is also passed through the flame or furnace. The absorption (the reduction in intensity of the resultant light) is measured and compared to the reduction in standards of known composition. XRF is an emission technique and uses solid samples. It measures excited characteristic x-rays and compares them to the spectra of standards. 2. In what principal way does the EMP differ from AA, XRF, and INAA? The electron microprobe is similar to XRF in that it measures emitted x-rays. The x-rays in this case are excited by bombarding the sample with a focused electron beam. Like XRF and INAA, solid samples are used and emissions measured. INAA emits radioactive particles excited by a neutron flux in a reactor. EMP analysis is capable of analyzing very small areas of a sample directly in a thin section. 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? We must analyze standards of known concentration and compare the intensity of the sample to that of a series of standards. 4. Use Figure 2.4 to determine whether the rocks analyzed in Table 8.3 are correctly named. Yes, all volcanic rocks are properly named. Peridotite, of course is plutonic, so Figure 2.4 is not appropriate for it. The volcanic equivalent, in this case, is picrobasalt. 5. Convert the Fe 2 O 3 and FeO in the basalt in Table 8.3 to FeO* Fe 2 O 3 = 3.79 and FeO = 7.13 FeO* = 7.13 + 0.8998 x 3.79 = 10.54 6. Explain in a single sentence what a norm attempts to do. How does a norm differ from a mode? A norm attempts to construe a chemical analysis as an idealized mineralogy so that volcanic rocks can be classified and compared mineralogically. A mode is the actual mineralogy observed whereas a norm consists of “designer minerals.” 7. How would you recognize a silica-oversaturated plutonic rock from a silica-undersaturated plutonic rock? Silica-oversaturated plutonic rocks contain quartz in the mode whereas silica-undersaturated rocks contain olivine of a feldspathoid. 1
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8. Suppose you have the chemical analysis of a volcanic glass. How would you determine whether it represents a silica-oversaturated or undersaturated composition? Calculate a norm and see if there is normative q (quartz). 9. Suppose you have chemical analyses for two volcanic rocks with identical wt. % SiO 2 , but one has a higher alkali/Al 2 O 3 ratio. Which is more likely to be silica-undersaturated? Why? The one with the higher alkali/alumina ratio, as alkalis tie up silica in feldspars (more so than
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Chapter 08 Ans - Answers for Chapter 8: Chemical Petrology...

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