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.. •The Th is itself a radioactive e

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 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 - 234 Th - 234 U -

230 Th - 226 Ra - 222 Rn - 218 Po - 214 Pb - 214 Bi - 214 Po - 210 Pb - 210 Po - 206

Pb = the U series), will be present.

•The critical elements in this series are 238 U, 234 U and 230 Th

N dt

t N

N = 0 − λ

N N

t e N N

D * = 0 − 0 − λ

) 1

0

2

e N

2 1

) 2

1 ln( = − λ T

2 1

) 2

λ λ

693

0 2

ln

2

1 = =

T

•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 ( λ ) 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

1 1

1

1 = −λ

01

0 1 1 2

e N

2

0 1 1

2

1

λ λ

1 2

t

t e e

N

λ λ

) )(

( 2 1 3 1

0 1 2

( 1 2 3 2

0 1 2

1 2

λ λ

λ λ

( 1 3 2 3

0 1 2

1 3

λ λ

λ λ

t t

e C

3 2

1

A series of n:

If n=3

0 1 1

2

1 2

t

t e e

N

λ λ

1 1

2

1

λ λ

−

≈

1 1

2

λ < N2 0 = 0

U

238 92

2

1 λ

λ <<

2 1

λ − ≅

1

2 2

λ =

n

nN N

N

λ

λ λ

λ

U Pa

Th

238  →α  →β−  →β−

AX AS

U 234 234

t AX

AX U e

U 234 0 234

AS A

U 0 234 0 234

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

t A

A A

U 238 (234 0 238 ) 234

234 = + − − λ

t A

) (

1 )

238 0

234 238

234

λ

−

− +

=

15 1 )

1 )

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.

• 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.

As Ax

Th 230 230

230 = +

t Ax

Th

Ax

230

0 232

230 232

230

) (

)

t

e R

R = 0 −λ

) ln(

1 0

R

R t

λ

=

For Excess:

) ( 1 2

0 1 1

−

=

) ( 234 230

0 234 234 230

−

=

AU

U 0 234 238

If secular equilibrium reaches

) 1

) 1

230 238 0

230

A

t Ax

) 1

( ) (

) (

)

232

238 0

232

230 232

230

t A

t

Th

U e

Th

Th Th

R = 0 −λ

a

h R

R = − λ

0

ln ln

a

m = − λ

m

a = − λ

Sum up excess and supported

Consider supported is negligible

) 1

0 234 234 230

−

=

) ( 234 230

0 234 234 230

230 230

230

Ax x

λ λ

*Calcium Carbonate is free of 230Th.

*If secular equilibrium reaches:

If excess 234U presents

) (

) (

)

238

234 0

234

234 230

230 238

230

t t

λ λ

−

=

AS A

U0 234 0 234

) (

)

(234 0 234 234 230

234 230

230

As A

λ λ

−

=

) )(

1 (

) 1

( )

0 234

230

230 238

230

t t

U

238

0 234

Fortunately it turns out that the presence of excess

234U makes a very small difference.