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Question: 22a exercise 2 radiometric ages radiogenic isotope data pb206v23804755 the...

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22A Exercise 2 Radiometric Ages Radiogenic Isotope Data Pb-206/V-238-.04755 The Carboniferous-Permian boundary beds at Usolka contain nine bentonite beds (labeled a through ), two tuffaceous sandstone beds, and a graded quartz sandstone bed (figure 2.3). Each of these beds yields zircon crystals (figure 2.5) that contain small amounts of uranium-238 (parent) and lead-206 (daughter). Table 2.2 shows the ratio of P- 206/U-238 in zircon crystals from ash beds e, t, & and i and from a graded sandstone bed labeled 18/4 2. Using the isotopic data listed in table 2.2, calcu late the average Pb-206/U-238 values for each sample using data from bed 18/4 as an example. Record your answers in the row labeled sam- ple aeaeforach clumn in table 2.2. Use FIGURE 2.5 Saing electren microsecope image of a the average Pb-206/U-238 values to approximate zircon crystal the age of each ash bed using figure 2.6 4. Use the relationships shown in figure 2.3 along with the conodont and isotopic data provided to calculate the age of the Carboniferous-Perm- ian boundary. Suggest an absolute age for the boundary between the Carboniferous and Perm 3. Do the radiometric ages make sense stratigraph ically? Are superpositionally younger strata also radiometrically younger? If not, which date(s) seem(s) to be wrong and why? million years. Compare your estimate with the date shown on figure 1.1 TABLE 2.2 Lead-206/uranium-238 ratios of individual zircon crystals from Usolka River samples. Bentonite Bentonite Bentonite Bentonite Bed 18/4 0.04762 0.04753 0.04729 0.04717 0.1712 0.04758 0.047470.047260.04720 0.1731 .04767 0.04763 0.04735 0.04723 0.1698 0.04753 0.047380.047320.04712 0.047450.047450.047240.04718 0.047720.04754 0.047280.04719 0.04751 0.04737 0.04714 0.04741 0.04759 0.1714 Sample average
Exercise 2 Radiometric Ages 23 area enlarged in ureBillions of Years Before Present A. 04795 04785 04780 04775 co 04770 04765 04760 04755 04750 04745 04740 04735 04730 04725 04720 04715 04710- 04700 297 298 299 300 301 Millions of Years Before Present B. FIGURE 2.6 Graphs showing lead-206/uranium-238 ratio relationship to age of sample.
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