This document discusses radiometric dating techniques used by geologists to determine the age of rocks. It introduces radiometric dating, the concept of half-life, and commonly dated minerals like zircon and monazite. Methods like uranium-lead dating are explained in detail. Instruments used for dating like laser ablation-ICPMS and electron microprobe are also mentioned. The document concludes that radiometric dating, especially uranium-lead dating of zircon, is a reliable method for determining the age of rocks and major geological events.

2.  INTRODUCTION
 RADIOMETRIC DATING
 HALF LIFE
 MINERALS USED IN DATING
 DATING OF DIFFERENT ROCKS
 INSTRUMENTS USED FOR DATING.
 CONCLUSION
 REFERENCE

3. By examining layers of sedimentary rock, geologists developed a time
scale for dividing up earth history.
Earlier in the 20th
century, radiometric-dating techniques allowed
scientists to put absolute dates on divisions in the geologic time
scale.
• Rocks contain radioactive minerals which are constantly
disintegrating at a steady rate
• Under certain circumstances, these atomic “clocks” can be read to
give a “time”
• The meaning of the “time” depends on what has happened to the
rock since the “clock” was set

4. How do geologists determine how old rocksHow do geologists determine how old rocks
are?are?
1.Relative dating -- determine whether the rock
is older or younger than other rocks
2. Absolute dating -- use radiometric dating
techniques to determine how long ago the rock
formed in the exact number of years

5. Radiometric dating is a technique used to date
materials such as rocks, usually based on a
comparison between the observed abundance of a
naturally occurring radioactive isotope and its decay
products, using known decay rates.
Geologists use radiometric dating to estimate how
long ago rocks formed, and to infer the ages of fossils
contained within those rocks.
Among the best-known techniques are radiocarbon
dating, potassium-argon dating and uranium-lead
dating.

6. The U-Pb Decay
 Natural U consists of 2 parent isotopes namely: 238
U & 235
U, which fall
into the range of geologic time.
235
U disintegrates much faster compared to 238
U to form 207
Pb.
8 alphas
238
U 206
Pb
6 betas
7 alphas
235
U 207
Pb
4 betas
Half life
4468 m.y
704 m.y

7. The average time needed for half the nuclei in a sample
of a radioactive substance to undergo radioactive decay.
The half-life of a substance does not equal half of its full
duration of radioactivity. For example, if one starts with
100 grams of radium 229, whose half-life is 4 minutes,
then after 4 minutes only 50 grams of radium will be left in
the sample, after 8 minutes 25 grams will be left, after 12
minutes 12.5 grams will be left, and so on.
HALF LIFE

8. Decay products
• Radioisotopes may decay to form a different isotope or a stable
isotope.
• May be a series of radioactive decays before a stable isotope is
formed.
• Stable isotope is called the "daughter" formed from decay of
radioactive "parent"
Radiometric Age Dating
Radioisotopes are trapped in minerals when they
crystallize.
Radioisotopes decay through time, and stable isotopes are
formed.
Determining the ratio of parent isotope to daughter
product reveals the number of half-lifes that has
elapsed.
Common isotopes used in age dating
U-Pb -- half-life of U-238 is 4.5 b.y.
K-Ar -- half-life of K-40 is 1.3 b.y.
Rb-Sr -- half-life of Rb-87 is 47 b.y.
Carbon 14 -- half-life of C-14 is 5730 yrs

9. Diagram indicates the radioactive disintegration of an isotope.

10. In principle, any U-Th bearing minerals such as apatite, xenotime,
allanite, uraninite or thorite can be used for dating.
U-Pb whole rock analyses have also been used for age determinations.
Mineral Zircon Monazite Sphene
Chemical
Formula
ZrSiO4 CePo4 CaTiSiO5
Primary
Occurrence
Gabbros to
granites
Granitic rocks & high grade
meta-sediments
Gabbros to
granites
U-Concentrations
(ppm)
10 - 4000 2000 - 10000 20 - 200

11. • It’s the most useful accessory mineral for U-Pb
dating
• In magmatic rocks, monazite is largely tied to
the felsic rocks.
• Monazite seems to be formed first in the course
of high grade metamorphism.
• It is a powerful tool for dating metamorphic
events.
• It is of great value to determine the age of the
provenance of sediment.
Monazite
Monazite
grain cm sized

12.  Zircon is widespread in igneous rocks as a primary mineral, makes it
valuable for dating these rocks, which have no fossil to indicate age.
 It is physically tough & easily seperable from crushed rock samples.
 Zircon is not easily disturbed by geologic events- not erosion or
consolidation into sedimentary rocks, not even moderate
metamorphism.
 Zircon chemical structure likes Uranium & hates lead. This indicates
that the ‘clock’ is truly set at zero when zircon forms.
 Dating a rock involves U-Pb measurements on many zircons there by
assessing the quality of the data.
Zircon grain
Zircon

13.  Great number of zircon data can be obtained from the felsic
rocks.
 Phanerozoic granitic rocks might contain considerable amounts
of inherited radiogenic lead in zircon grains.
 The detection of inherited radiogenic lead is of petrogenetic
importance as it is in favour of crustal processes for atleast part of
analyzed rocks.
 Zircon dating of intermediate & mafic igneous rocks is only rarely
applied. The average uranium concentrations of the Zircons are
below those of zircons from granitic rocks.
Dating Of Igneous rocks

14. Metamorphic ages can be dated using U-Pb method
Single zircon or monazite crystal differing in colour &
morphology reveals two or more different age groups.
Different age groups may reflect the time of metamorphism
& also the pre-metamorphic history of the rock.

15. Dating of Sedimentary rocks
• Zircons found in clastic sediments are of detrital origin.
• Main application of dating these zircon is to shed light on to the
provenance of the detritus.
• The data pattern may vary considerably in sedimentary rocks,depending
on the number of age groups & their quantitative contribution to the
analyzed zircon population.
• Recent lead loss results in the interpretation of the data points more
difficult & impossible.

16. 1. Isotopic age measurement of U-Pb and Rb-Sr
methods on meteorites have yielded solidification
age close to 4600 m.y
2. The oldest rock and soils from the moon yield U-Pb
and Rb-Sr ages close to 4600 m.y
3. The oldest dated rocks on earth exhibit whole rock
Rb-Sr isochron ages of ~3700 to 3800 m.y it is
probable that older rock exist, but they have been
sufficiently reworked by later metamorphism

17. Instruments frequently used in U-Pb dating
Laser ablation ICPMS:
LA ICPMS dating of U- bearing accessory phases has
recently developed into a well established tool that can be used to calculate the time
scale of igneous & metamorphic processes.
This method has almost exclusively been used for U-Pb dating of zircons, while
other geochronologically important minerals (e.g. monazite) have been only been
dated using 207
Pb/206
Pb method.
Electron probe micro analyzer:
Electron Microprobe analysis makes it possible to
determine the Th, U and Pb concentrations in an area less than few µ meter
(e.g. 5 µ) across in a single grain of monazite and zircon.
The chemical Th-U-total Pb isochron method was applied to the age determination
of monazite, and zircon.
The chemical Th-U-total Pb isochron ages coincide well with mineral and whole
rock ages isotopically determined for the samples.

18. Conclusion
o Radioactive 235U, 238U disintegrates through time into
stable isotopes of 208Pb & 207Pb respectively.
o The U-Pb present in a rock can be dated to know the
evolution of the rock.
o Zircon and Monazite are widely used minerals in U-Pb
dating.
o Laser Ablation ICPMS and EPMA are most common
instruments used for U-Pb dating of rock samples.
o The limitations of U-Pb dating are the episodic lead loss
in the rock due to alteration reactions and U-Pb is
concentrated only in certain minerals such as zircon,
monazite, etc.
o Geologists generally considers this method as a reliable
and successful method of radiometric dating of rocks.

19. References
• E Jager and J C Hunziker, Lectures in Isotope Geology, Springer-Verlag
Berlin
Heidelberg Newyork, 1979, p 1-7,105-133.
• U Aswathanarayana, Principles of Nuclear Geology, Oxonian press Pvt.
Ltd
1985, p 228-240, 248-254.
• B P Radhakrishna, R Vaidhyanadhan, Geology of Karnataka, Geol. Soc.
India,
1997, p 44.
• C.S. Pichamuthu, Archean geology, OXFORD and IBH Publishing co.
• Pp-126-127
 http://en.wikipedia.org/w/index.php?title=Uranium-
lead_dating&oldid=541087726
.