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RADIOGENIC ISOTOPES

RADIOGENIC ISOTOPES. AT THE HOME FOR OLD ATOMS…. Willy. Al. HISTORY OF RADIOMETRIC DATING. 1896 Henri Becquerel discovers that uranium is radioactive. 1898 Marie Curie discovers that thorium is radioactive.

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RADIOGENIC ISOTOPES

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  1. RADIOGENIC ISOTOPES

  2. AT THE HOME FOR OLD ATOMS… Willy Al

  3. HISTORY OF RADIOMETRIC DATING 1896 Henri Becquerel discovers that uranium is radioactive 1898 Marie Curie discovers that thorium is radioactive 1902 McGill University professor, Ernest Rutherford discovers law of radioactivity with PDF Frederick Soddy - dates uraninite from He content. 1907 Bertram Boltwood provide first U-Pb dates, 535 to 2,200 Ma

  4. HISTORY OF RADIOMETRIC DATING 1911 Arthur Holmes publishes chemical U-Pb ages of rocks within 20% of modern values. Estimates age of Earth at 3.3 Ga 1919 Francis Aston invents the mass spectrometer 1938-1941 Alfred Nier lays foundations of modern U-Pb isotopic geochronometry. 1953 Clair Patterson measures the primeval Pb in the Cañón Diablo meteorite and determines the modern age of the solar system 4.55 Ga.

  5. STABLE AND RADIOGENIC ISOTOPES

  6. Proton + β-+ v Neutron BETA DECAY Radionuclides with excess neutrons undergo beta-minus decay Radionuclides with neutron deficiency undergo beta-plus decay Neutron + β++ v; Proton Neutron + v Proton + e-

  7. A - 4 4 D + He + Q Z - 2 2 4 234 Th + He + Q 2 90 ALPHA DECAY Spontaneous emission of a nuclear particle comprising 2 neutrons + 2 protons, i.e., a helium nucleus A P Z 238 U 92

  8. THE URANIUM-238 DECAY CHAIN

  9. dN - = λN dt 10 λN0 8 6 λN λ = 0.04614 4 2 0 60 100 80 120 140 20 40 0 Time, hours LAW OF RADIOACTIVITY Ernest Rutherford, McGill Professor of Physics, Nobel, 1908 Frederick Soddy, McGill post-doctoral fellow Nobel, 1921 Decay of 24Na; λ is the decay constant

  10. N dN - = λN N0 dt Integrating we obtain Ln N = - λt + C or Ln N = - λt + Ln N0 ) Ln ( or or N = N0e-λt = - λt LAW OF RADIOACTIVITY Mass of daughter + parent equals initial parent isotope mass D + P = N0 or D = N0 - P P = N0e-λt and P/N0 = e-λt or N0/P = eλt and N0 = Peλt D= Peλt – P D= P(eλt – 1) and P = D/(e-λt – 1)

  11. N0 = N0e-λt 1/2 2 Ln1 – ln2 = -λt1/2 and ln2 – ln1 =λt1/2 and ln2 t = λ 1/2 THE CONCEPT OF HALF-LIVES N0 1 = e-λt N = N0e-λt, , , = e-λt 1/2 1/2 2 2N0

  12. DECAY CONSTANTS AND HALF-LIVES

  13. THE K-Ar METHOD 40K decays to 40Ar by electron capture: 40K + e-  40Ar 40K decays to 40Ca by β- decay: 40K  40Ca + β- + v 40Ar = (λe/λ)40K(eλt – 1) λe is the decay constant for electron capture and λ is the total decay constant K-Ar dates on biotite from gneisses in the Grenville province, Canada

  14. P = N0e-λt P/N0 = e-λt N0/P = eλt N0 = Peλt 1 (D/SD)2 – (D/SD)1 ln t = λ (P/SD)2 – (P/SD)1 AGE DATING AND ISOCHRONS The isochron method depends on the idea that both radiogenic and non-radiogenic daughter isotopes (Di) are initially present. D + P= Di + N0 D = Di+ N0 – P D = Di + P(eλt – 1) Divide through by mass of stable daughter (SD) to avoid sample dependence by converting to ratios. The method requires multiple samples and assumes that the system is closed (D/SD)t = (D/SD)0 + (P/SD)t (eλt – 1)

  15. 87Sr 86Sr t1 c1 b1 a1 ) ( 87Sr to 86Sr a b c o 87Rb 86Sr (87Rb/ 86Sr)t + (87Sr/ 86Sr)t – (87Sr/86Sr)0 1 ln t = λ 87Rb (87Rb/ 86Sr)t THE Rb-Sr ISOCHRON 87Sr/86Sr = (87Sr/86Sr)0 + 87Rb/86Sr(eλt - 1)

  16. SECULAR VARIATION OF 87Sr/86Sr IN MARINE CARBONATES Mountain Building Seafloor Spreading

  17. INITIAL 87Sr/86Sr RATIOS ACROSS THE ANDES Initial 87Sr/86Sr ratios of andesites as a function of K-Ar determined age across the Andes 87Sr/86Sr increases from west to east, i.e. from trench to sub-continent, indicating increasing contribution of continental crust to magmas

  18. URANIUM-LEAD GEOCHRONOLOGY U and Th decay to stable isotopes of Pb (238U 206Pb, 235U  207Pb, and 232Th 2O8Pb). There is also a fourth stable isotope is 204Pb (common lead) 206Pb/204Pb = (206Pb/204Pb)0 + 238U/204Pb(eλt-1) 207Pb/204Pb = (207Pb/204Pb)0 + 235U/204Pb(eλt-1) 208Pb/204Pb = (208Pb/204Pb)0 + 232Th/204Pb(eλt-1) Problem: Most ages are discordant due to loss of Pb and gain or loss of U Solution: Combine 238U and 235U decay equations and relate discordant ages to hypothetical concordant ages. 206Pb* and 207Pb* take into account the initial lead. 206Pb* = 238U(eλt -1) and 206Pb*/238U = eλt-1 207Pb* = 235U(eλt -1) and 207Pb*/235U = eλt-1

  19. U loss Crystallization age Concordia U gain Pb loss Discordia Hudsonian Orogeny CONCORDIA DIAGRAM 206Pb*/238U = eλt-1 207Pb*/235U = eλt-1 Zircon

  20. ELECTRON MICROPROBE DATING (U, Th, Pb) DATING OF MONAZITE ThO2 PbO UO2 Age Y2O3

  21. DATING USING COSMOGENIC RADIONUCLIDES Isotopes created by interaction of cosmic rays with Stable atoms of atmosphere 14N + n  12C + 3H Formation Decay 14N + n  12C + 3H 3H  3He + e- + v n + 14N  14C + p 14C  14N + e- + v 10Be - Spallation of N, O and Si 10Be  10B + e- + v 26Al - Spallation of Ar and Si 26Al  26Mg + β+ v

  22. C N + v + e 14 14 n + N C + p 7 6 1 t = ln λ 14C0 14C _ β _ _ 14 14 6 7 RADIOCARBON (14C) DATING 14C/12C = (14C/12C)0e-λt

  23. Little Ice Age Maunder Sporer Dalton 10Be AS A PROXY FOR SOLAR ACTIVITY Low solar activity correlates with higher10Be production and thermal minima. Data from arctic/antarctic ice cores

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