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