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U_Pb_Geochronology - Earth Sciences 4502/6510 Radiogenic...

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Earth Sciences 4502/6510 Radiogenic Isotopes II: U-Pb Geochronology Stephen J. Piercey Department of Earth Sciences Memorial University St. John’s, NL, Canada A1B 1N3 [email protected] Wednesday, 15 August, 12
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Outline Basics of the U-Pb system. Minerals we can date by U-Pb methods. Methodologies and analysis. Diffusion and closure temperatures. Concordia and interpreting concordia diagrams. Examples. Costs. Summary. Wednesday, 15 August, 12
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U-Pb Isotope System U-(Th)-Pb has three isotopes that decay to isotopes of Pb: 238 U 206 Pb ( λ 1 = 1.55125x10 -10 yr -1 , t 1/2 = 4.47 Byr) 235 U 207 Pb ( λ 2 = 9.8485x10 -10 yr -1 , t 1/2 = 0.704 Byr) 232 Th 208 Pb ( λ 3 = 0.49475x10 -10 yr -1 , t 1/2 = 14.01Byr) 238 U and 235 U are ones used primarily in U- Pb geochronology. Wednesday, 15 August, 12
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U-Pb Isotope System These isotope systems are described by the following decay equations: 206,207 Pb/ 204 Pb = Pb/Pb isotopes at present day. 238,235 U/ 204 Pb = U/Pb isotope ratios at present day. 206,207 Pb/ 204 Pb o = initial Pb/Pb isotope ratios λ 1 , λ 2 = decay constants. COMMON Pb = all 204 Pb and 206,207 Pb/ 204 Pb o Wednesday, 15 August, 12
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U-Pb Isotope System In most minerals we aim to have no common Pb as it increases error. With this we can rewrite the above to have equations with no common Pb (i.e., we can remove 204 Pb and 206,207 Pb/ 204 Pb): Since we have two decay schemes we get two independent ages for a mineral/rock. If the ages are the same they are CONCORDANT. If the ages are different they are DISCONCORDANT. 206,207 Pb* = Pb derived from radiogenic breakdown of U. 238,235 U = U present in the minerals. λ 1 , λ 2 = decay constants Wednesday, 15 August, 12
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U-Pb Isotope System By dividing the above equations we also get what is called the 207 Pb/ 206 Pb age (Pb-Pb age). This is solved iteratively and these ages are often used for discordant data arrays (we’ll see significance of this in future slides). 206,207 Pb* = Pb derived from radiogenic breakdown of U. 238,235 U = U present in the minerals. λ 1 , λ 2 = decay constants Wednesday, 15 August, 12
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Minerals for U-Pb Dating Key with U-Pb dating is finding U-rich minerals. Just as important, however, is finding U-rich minerals with very low common Pb (common Pb increases errors and gives less precise ages). Example: Zircon Takes U into its structure, but excludes Pb. Hence, any Pb present in the zircon grain has come from radiogenic breakdown of U. By comparing the amount of Pb derived from radioactive decay, relative to the amount of U present we can get ages for the rocks. More on this later ...... Wednesday, 15 August, 12
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Common Minerals Used for U-Pb Dating K = kimberlite, C = carbonatite, UM = ultramafic, M = mafic, A = alkaline, SG = S-type granitoids, IG = I-type granitoids, GP = granitic pegmatite, HV = hydrothermal veins, GN = granitic gneiss, PS = pelitic schist, SK = skarn. From Heaman and Parrish (1991) and Richards and Noble (1998) Mineral Formula Uranium Range (ppm) Common Pb (ppm) Rock Type Zircon ZrSiO 4 1-10000 <2 Most Baddeleyite ZrO 2 58-3410 <2 K,C,UM,M,A Monazite CePO 4 282-13730 <2 SG,GP,PS,HV Titanite CaTiSiO 5 4-500 5-40 Most,SK Rutile TiO 2 <1-390 <2-95 GP,GN,HV Perovskite CaTiO 3 21-348 <2-90 K,C
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