The U-series Disequilibrium Method of Dating

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The U-series Disequilibrium Method of Dating
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• Uranium series, or Ionium dating, is based on the
  radioactive decay of uranium in calcium carbonate
  and other minerals that precipitate from
  solution.
• Natural 238U decays into 234Th, while the other
  isotope of U, 234U, decays into 230Th. Because
  the U is soluble in water and the Th is not,
  minerals that precipitate from solution often
  contain U but very little Th. Through time Th is
  formed in the mineral as the U decays.
• The Th is itself a radioactive element and it
  decays into daughter products. The 234Th decays
  into 234U and the 230Th decays into 226Ra.
• Ra is also radioactive, and it decays into Rn in
  a very short time. If the mineral is of
  sufficient age the entire suit of U decay
  products, from 238U through to 206Pb (238U -
  234Th - 234U - 230Th - 226Ra - 222Rn - 218Po -
  214Pb - 214 Bi - 214Po - 210Pb - 210Po - 206 Pb
  the U series), will be present.
• The critical elements in this series are 238U,
  234U and 230Th because of their half lives. For
  any mineral that initially contained only U, the
  time since its formation can be calculated from
  the 238U/ 234U and 234U/ 230 Th ratios.

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Dating Assumptions

• The rock or mineral system has neither gained nor
  lost either parent or daughter atoms so that the
  ratio of D/N has changed only as a result of
  radioactive decay. This condition is often
  expressed by the statement that the rock or
  mineral sample must be a closed system with
  respect to the parent and daughter.
• It must be possible to assign a realistic value
  to D0. This can usually be done reliably,
  especially when D is much greater than D0.
• The value of the decay constant (l) must be known
  accurately.
• The measurements of D and N must be accurate and
  representative of the rock or mineral to be dated.

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If it is a decay series
                         
If
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A series of n
If n3
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234UAactivity of 234U per unit weight of sample
at the present time 234UASactivity of 234U in
secular equilibrium with 238U 234UAXactivity of
excess 234U per unit weight of sample
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Since secular equilibrium
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230Th/232Th Method
On the earth surface U becomes uranyl ions
UO22 such as
(UO2)(CO3)3-4 Based on the assumption 230Th and
232Th simultaneously removed from sea water.
     
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• Assumptions
• The 230Th/232Th ratio in the water mass adjacent
  to the sediment in a given ocean basin has
  remained constant during the last several hundred
  thousand years.
• 230Th and 232Th have the same chemical speciation
  in sea water and there is no isotopic
  fractionation between sea water and the mineral
  phases with which the Th is associated in the
  sediment.
• 230Th and 232Th occurring in detrital mineral
  particles are excluded from the analysis.
• Th isotopes do not migrate in the sediment.

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For Excess
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For Supported
                               
If secular equilibrium reaches
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Sum up excess and supported
Consider supported is negligible
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U/Th method
Calcium Carbonate is free of 230Th. If secular
equilibrium reaches
If excess 234U presents
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Where
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If there is no excess
Fortunately it turns out that the presence of
excess 234U makes a very small difference.
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