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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 238 U decays into 234 Th, while the other isotope of U, 234 U, decays into 230 Th. 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 234 Th decays into 234 U and the 230 Th decays into 226 Ra. 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 238 U through to 206 Pb ( 238 U Th U Th Ra Rn Po Pb Bi Po Pb Po Pb = the U series), will be present. The critical elements in this series are 238 U, 234 U and 230 Th because of their half lives. For any mineral that initially contained only U, the time since its formation can be calculated from the 238 U/ 234 U and 234 U/ 230 Th ratios.

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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 D 0. This can usually be done reliably, especially when D* is much greater than D 0. The value of the decay constant ( ) must be known accurately. The measurements of D and N must be accurate and representative of the rock or mineral to be dated. Dating Assumptions

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If it is a decay series: If

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A series of n: If n=3

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234 U A =activity of 234 U per unit weight of sample at the present time 234 U AS =activity of 234 U in secular equilibrium with 238 U 234 U AX =activity of excess 234 U per unit weight of sample

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Since secular equilibrium

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230 Th/ 232 Th Method *On the earth surface U becomes uranyl ions UO 2 2+ such as (UO 2 )(CO 3 ) 3 -4 *Based on the assumption 230 Th and 232 Th simultaneously removed from sea water.

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Assumptions: The 230 Th/ 232 Th ratio in the water mass adjacent to the sediment in a given ocean basin has remained constant during the last several hundred thousand years. 230 Th and 232 Th 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. 230 Th and 232 Th 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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If secular equilibrium reaches For Supported:

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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 230 Th. *If secular equilibrium reaches: If excess 234 U presents

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Where

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

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