# The U-series Disequilibrium Method of Dating

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The U-series Disequilibrium Method of Dating

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 Bi - 214Po - 210Pb - 210Po 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.

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.

If it is a decay series: If

A series of n: If n=3

234UA=activity of 234U per unit weight of sample at the present time
234UAS=activity of 234U in secular equilibrium with 238U 234UAX=activity of excess 234U per unit weight of sample

Since secular equilibrium

*On the earth surface U becomes uranyl ions
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.

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.

For Excess:

If secular equilibrium reaches
For Supported: If secular equilibrium reaches

Sum up excess and supported
Sum up excess and supported Consider supported is negligible

U/Th method *Calcium Carbonate is free of 230Th. *If secular equilibrium reaches: If excess 234U presents

Where

If there is no excess Fortunately it turns out that the presence of excess 234U makes a very small difference.

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