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Radiometric Dating

Radiometric Dating. Radiometric Dating. First Attempted in 1905 Compare U and Pb content of minerals Very crude but quickly showed ages over a billion years Skepticism about utility from geologists Arthur Holmes and NAS report, 1931

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Radiometric Dating

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  1. Radiometric Dating

  2. Radiometric Dating • First Attempted in 1905 • Compare U and Pb content of minerals • Very crude but quickly showed ages over a billion years • Skepticism about utility from geologists • Arthur Holmes and NAS report, 1931 • Almost all dating now involves use of mass spectrometer (developed 1940’s)

  3. Mass Spectroscopy

  4. Exponential Decay

  5. Exponential Decay

  6. Half-Life

  7. Determining Half Life • Decay Constant λ = Fraction of isotope that decays/unit time • N= Number of atoms • dN/dt = -λN • dN/N = -λdt • Ln N = -λt + C • N = N0 exp(-λt): N0 = original number of Atoms

  8. Determining Half Life • N = N0 exp(-λt) • Solve for N = N0/2 • N0/2 = N0 exp(-λt) • ½ = exp(-λt) • -Ln(2) = -λt • Half life t = Ln(2)/λ = 0.693/λ

  9. Decay Chains • U-238 (4.5 b.y.)  Th-234 (24.5 days)  Pa-234 (1.14 min.) • dU-238 /dt = dTh-234/dt = dPa-234/dt etc. • λ(U-238)*N(U-238) = λ(Th-234)*N(Th-234) = λ(Pa-234)*N(Pa-234) etc. Or… • N(U-238)/t(U-238) = N(Th-234)/t(Th-234) = N(Pa-234)/t(Pa-234) etc.

  10. Ideal Radiometric Dating • A (parent)  B (Daughter) • A decays only one way • No other sources of B • Both A and B stay in place • Unfortunately there are no such isotopes in rocks • Branching Decay • Inherited Daughter Product • Diffusion, alteration, metamorphism

  11. Potassium-Argon • K-40 Half Life 1.3 b.y. • K-40  Ca-40 (89%) or Ar-40 (11%) • Ca-40 is the only stable isotope of Calcium • Total decays = 9 x Argon Atoms • Argon is a Noble Gas and Doesn’t React Chemically • Only way to be in a crystal is by decay • Mechanically trapped in lattice

  12. Potassium-Argon • Ar atoms mechanically trapped in lattice • Susceptible to loss from alteration or heating • One of the first methods developed • Least stable method • Little used for high-quality dates • Minerals must have K • Feldspars, Micas, Glauconite, Clays

  13. Inherited Argon • Mostly affects volcanic rocks • Usually from trapped or dissolved air in fluid inclusions • Only a problem for very young rocks • Won’t be an issue in metamorphic rocks • Diffuses out quickly in older volcanic rocks • 1 m.y. worth of argon is a problem for 100,000 year old rocks but not 500 m.y. old rocks • Detect by plotting isochron

  14. A K-Ar Isochron

  15. Rb-Sr • Rb substitutes for K, Sr for Ca • Rb-87  Sr-87 Half Life 50 b.y. • Problem: Primordial Sr-87 • But there is also Sr-86 • If there’s no Rb-87, Sr-87/Sr-86 is constant • If there is Rb-87, Sr-87/Sr-86 increases • Also Rb-87 decreases • Plot on isochron diagram

  16. Isochron Diagram

  17. Isochron Diagram

  18. What initial Sr-87/Sr-86 means • Present ratio in mantle = .703 • Ratio 4.6 billion years ago = .699 • The more Sr-87, the more Rb-87 decayed • High initial Sr-87 means old source rocks = remelted continental crust

  19. U-Th-Pb Dating • U-238  Pb 206; Half-life 4.5 b.y. • U-235  Pb-207; Half Life 704 m.y. • Th-232  Pb-208; Half Life 13.9 b.y. • Pb-204: Non-radiogenic • Methods • Isochron • Concordia/Discordia • Short-Lived Daughter Products

  20. Concordia Plot

  21. Discordia Plot

  22. Samarium-Neodymium • Sm-147  Nd-143 (Half Life 1.06 b.y.) • Nd goes into melt more than Sm • Mantle: Low Abundance, High Sm/Nd • Granite: High Abundance, Low Sm/Nd • Nd-144 = 24% of Nd • Nd-144 has half life 2.3 x 1015 years • Can use isochron methods with Nd-144 or Nd-142 (Stable, 22% of Nd)

  23. The CHUR Model:Chondritic Uniform Reservoir (CHUR) line

  24. Neodymium Model Ages • Terrestrial igneous rocks generally fall on the CHUR line • If they don’t, it’s because the suite departed from CHUR evolution at some point • Most common separation: from mantle to crust

  25. Nd-Sm Model Ages

  26. Uranium-thorium dating method • U-234  Th-230 (80,000 years) • U-235  Pa-231, (34,300 years) • U is soluble, Th and Pa are not • Precipitate in sediments

  27. Fission Track Dating • Fission of U-238 causes damage to crystal lattices • Etching makes tracks visible • Can actually count decays • Anneals at 200 C so mostly used on young materials

  28. Optically Stimulated Luminescence Dating • Radioactive trace elements cause lattice damage • Create electron traps • Excitation by light releases electrons from traps, emitting light • Emitted light more energetic than stimulating light (Distinguished from fluorescence) • Sunlight resets electrons • Measures length of burial time

  29. Cosmogenic Isotopes • Produced by particle interactions with air or surface Materials • C-14 • Be-10 • Cl-36

  30. C-14 (Radiocarbon) Dating • N-14 + electron  C-14 • Equilibrium between formation and decay • About one C atom per trillion is C-14 • C-14 in food chain • All living things have C-14 • After death, C-14 intake stops and existing C-14 decays (5730 years)

  31. C-14 (Radiocarbon) Dating • Half Life: 5730 years • Range: Centuries to 100,000 years • C-14 can be removed by solution, oxidation or microbial action • C-14 can be added from younger sources • C-14 production rate by sun variable • Calibrate with known ages like tree rings

  32. Beryllium-10 Dating • Produced by high energy cosmic rays • Spallation of N and O in atmosphere • Half Life 1.51 m.y. • Dissolves in rain water • Accumulates on surface • Also formed by neutron bombardment of C-13 during nuclear explosions • Tracer of nuclear testing era

  33. Chlorine-36 Dating • Forms by spallation of Ar in atmosphere • Forms by particle reactions with Cl-35 and Ca-40 in surface materials • Half life 300,000 years • Ground water tracer • Also formed by oceanic nuclear tests

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