U-Th-Pb Dating

Method

Dackshta Rana

Msc. 2nd.

Contents

1) Introduction

2) Geochemistry of U,Th and Pb.

3) Nuclear properties

4) Decay scheme of the parent isotopes

5) U-Pb isochrons

6) Other techniques

Introduction

This dating method involves decay of U and Th to stable isotopes of Pb

Age determinations of rocks based on this method was first attempted in early years of early twentieth centaury by Ernest Rutherford and B.B.Boltwood.

Subsequently, A. Holmes used U-Pb and U-He dates to propose the 1st geological time scale in his book on the age of earth published in 1913.

The U, Th-Pb dating method has become most precise and most accurate method for determining terrestrial and extraterrestrial rock samples.

238U ---› 206Pb235U ---› 207Pb232Th ---› 208Pb

Geochemistry of U,Th and Pb:-

Uranium (U) and Thorium(Th) -

Members of actinide series with 5f orbitals progressively filled with

electrons.

Periodic Table

Both occur as tetravalent ions thus having similar

physical and chemical properties and ionic radii.

U4+ = 1.05 Ǻ Th4+ =1.10 Ǻ

Thus, they both can substitute each other.

But, Under oxidizing conditions, U forms uranyl ion 6+ or

8+ charge and is soluble in water when forms a

compound. Whereas, Th exists in tetravalent form only

and is insoluble in water.

In primordial times, their concentrations in chondritic

meteorites were relatively low.

U = 0.01ppm Th = 0.04ppm

With progressive increase in partial melting in the

earth’s Mantle they got concentrated in the liquid phase

and thus got incorporated in Sio2 rich rocks.

Also 4+ charge is too large to enter a mineral structure

and therefore, they are incompatible in nature.

Concentration in granitic rocks

U = 4.8ppm Th = 21.5ppm

U and Th are highly refractory elements.

Th/U ratio in Chondrites = 3.8 but we take it as 4+0.2/4-

0.2 because of mobile nature of U this ratio is not exactly

similar to that of chondrites.

Their concentrations in rock forming Sio2 minerals is low

and are found in accessory minerals as a major element

or replace other elements.

For instance, U replaces Zr in zirconium.

Accessory minerals-

Allanite, Apatite, Titanite, Monazite (less U, more Th)

Certain facts

1.Uranium is named for the planet Uranus.

2.The density of uranium is about 70% higher than

lead, but less than that of gold or tungsten, even though

uranium has the second-highest atomic weight of the

naturally occurring elements (second to plutonium-

244).

Pitchblende- U ore

3.The USA alone has enough thorium

to last for many thousands of years.

The energy value of thorium in the

earth's crust is more than the energy

value of all the uranium in the earth's

crust and fossil fuels combined.

One ton of thorium can produce as

much energy as 200 tons of uranium.The primary source of the world's thorium

is

the rare-earth-and-thorium-phosphate

mineral monazite

Thorium

Lead (Pb)

Lead has Z=82 and generally has 2 valence states of

Pb2+ and Pb4+ out of which Pb2+ is more common than

Pb4+.

Pb2+ has ionic radii of 1.29 Ǻ and replaces K+ ( 1.33Ǻ) in

K-feldspars. It rarely also replaces Sr, Ba, Ca and Na.

Just like U, Pb too is mobile

in nature and so it can be

transported in hydrothermal

solutions and dissolves in

organic acids and concentrated

alkalis.

Galena is the major ore of Pb.

Nuclear properties Uranium-Has 3 naturally occurring isotopes:-

Uranium-238 is usually an α emitter

(occasionally, it undergoes spontaneous fission),

which has 18 members, all of which eventually

decay into lead-206, by a variety of different

decay paths.

The decay series of 235U, which is called the actinium

series has 15 members, all of which eventually decay into

lead-207.

The constant rates of decay in these decay series makes

the comparison of the ratios of parent to daughter

elements useful in radiometric dating.

Uranium-234 is a member of the "Uranium Series", and it

decays to lead-206 through a series of relatively short-

lived isotopes.

Uranium

Isotopes Abundance Half life(yrs)238U 99.276% 4.47*109

235U 0.7196% 0.707*109

234U 0.006% 2.45*105

238U has half-life equal to age of earth

234U occurs as intermediate daughter in U series and does not survive.

235U has less half life as compared to 238U and so 238U has greater abundance.

Decay equations:

92U238 ―> 82Pb 206 + 8 2He4 + 6ßˉ + Q

(47.4 Mev/atom)

92U235 ―> 82Pb207 + 72He4 + 4 ßˉ + Q

(45.2 Mev/atom)

Decay chain of 238U :-

Thorium Although thorium (Th) has 6 naturally occurring

isotopes, and only one, 232Th, is stable, with a half-life of 14.05 billion years, considerably longer than the age of the earth and considered as age of the universe. This isotope makes up nearly all natural thorium. Rest all are unstable.

Th Isotopes Half life232Th 14.05*109 yrs (100% abundant) 234Th 21.4 days230Th 7.5*104 yrs228Th 1.91 yrs227 Th 18.2 days

None of them occur in nature except for 232Th.

Decay equation:-

90Th232 ―> 82Pb208 + 62He4 + 4 ßˉ + Q

Q= 39.8 Mev/atom

Lead (Pb)

Lead (Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. 204Pb is entirely a primordial

nuclide and is not a radiogenic nuclide.

Lead isotopes Abundances204Pb 1% (non-radiogenic)206Pb 24%207Pb 23%208Pb 52%

206Pb, 207Pb and 208Pb are decay products of U and Th

decay chains.

238U ---› 206Pb

235U ---› 207Pb

232Th ---› 208Pb

Decay scheme of parent Highest at weight parent gives rise to lowest at weight daughter

and vice versa.

Out of U-Pb and Th-Pb, U-Pb decay method is useful since it is

the only dating scheme which graphically allows the deviation

from a close system behavior because of mobile nature of both

the isotopes.

Th-Pb method is specifically used in case of monazite only.

Parent Decay mode Decay

Constant

(Yr -1 )

Half life

(Ga)

Radiogenic

Daughter

238U α and ßˉ 1.551* 10 -10 14 206Pb

235U α and ßˉ 9.849 * 10 -

10

0.707 207Pb

232Th α and ßˉ 4.948 *10 -11 4.47 208Pb

U-Pb isochron The term U– Pb dating normally implies the coupled use

of both decay schemes in the ‘concordia diagram'. However, use of a single decay scheme (usually 238U to 206Pb) leads to the U–Pb isochron dating method, analogous to the rubidium-strontium datingmethod.

The accumulation of radiogenic isotopes of Pb by decay is governed by following equations-

D = Do + N(eλt -1) -------(1)

D = no. of daughter atoms at present time

Do = initial no. of daughter atoms

N = no. of parent atoms

t = 1/λ ln (D – Do /N + 1) -------(2)

Considering a granitic rock of any age ‘t’.

• These equations are written in terms of the atomic ratios

w.r.t. 204Pb because 204Pb is the only stable Pb isotope.

Here, λ 1, λ2 = decay constants of 238U and 235U resp.

The above mentioned ratios are substituted in the Eq(2), thus giving us the independent dates based on their respective decay series.

U-Pb isochrons plotted are almost similar to those for Rb-Sr and if these assumption are valid then the ages should be concordant. But Unfortunately, U-Pb system rarely stays closed in silicate rocks because of mobile nature of the two.

• One area where U-Pb isochron dating has been applied

with moderate success is the direct dating of

marine carbonates, which have proven very difficult to

date by other radiometric methods.

An example of U–Pb dating of typical marine carbonates

is the study of Smith and Farquhar (1989) on Devonian

corals from Ontario.

Several coral samples, together with authigenic pyrite

from one specimen, formed a reasonably good linear

array on a 238U–206Pb isochron diagram.

A valid isochron which matches the

stratigraphic age of the corals i.e. 375-385 Ma

However, this result was only achieved by omitting one Heliophyllum

coral and three out of four Cystiphylloides corals, which lie well off the

regression line

• The assumptions of dates from U-Th method are

concordant only if:-

1) The sample analysed remained close to U and Pb and

all intermediate daughters throughout its history.

2) There has been no interference by 235U during fission

chain reaction of 238U.

3) The decay constants are known accurately.

Disadvantage

Usefulness of U-Pb isochron dates is limited not only

because of mobility of the both but also because

emission of α-particles causes radiation damage in

crystals which facilitates Pb loss.

Advantage

Since both, the parent isotopes 238U and 235U have similar

physical and chemical properties and their respective

daughters 206Pb and 207Pb too are coherent, this method

is useful in obtaining ages of altered rocks.

There are three dating techniques to exploit this

situation, namely-

a) U-Pb- Zircon method – most widely used since zircons

are found in granites and provide us with accurate

values.

b) Pb-Pb method and Galena model age method

Both these methods are rarely used and the age

obtained is not precise.

Thank you