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Isochron approach to dating:

Decay equation & curve (see Chapter 16) dN/dt = -  N N = number of radioactive parent atoms t = time. Isochron approach to dating: Numbers of parents and daughter atoms are changed only by radioactive decay over the lifetime of the material

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Isochron approach to dating:

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  1. Decay equation & curve (see Chapter 16)dN/dt = -NN = number of radioactive parent atomst = time

  2. Isochron approach to dating: • Numbers of parents and daughter atoms are changed only by radioactive decay over the lifetime of the material -mobility in fluids, weathering, hydrothermal alteration -diffusion during metamorphism (resetting) • No isotopic fractionation • Decay constant well-known • No other reason for linear array, such as mixing For mobile elements, there are a variety of ingenious ways to improve the use of their isotope systems

  3. Pb – Pb isochron

  4. Concordia diagram

  5. 40Ar-39Ar Dating – incremental heating curves • 39Ar is produced by neutron bombardment of 39K in a nuclear reactor – production rate is known • 39K/40K is known • Ar is driven from mineral by step-wise heating • Age is calculated from the 40Ar-39Ar ratio

  6. Cosmogenic & U-series isotopes – used for dating geologically recent events N/No = e-t N = Activity = A A/Ao = e-t For 14C, Ao = 13.56 decays per minute/gram C for year 1950

  7. Complications: • Ao changes due to changes in cosmic ray flux and Earth’s magnetic field • Atmospheric dilution by “dead” C, local excess of reactor produced 14C • C chemical exchanges occur in the environment • Chemical and biochemical reactions cause fractionation of C isotopes

  8. Sediment accumulation rates: 10Be 230Th 210Pb ln A/Ao = -t = - (thickness/rate) and slope = -/accumulation rate Sedimentation rate = thickness/accumulation time, t = thickness/rate

  9. Mantle evolution diagrams and model ages

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