Clocks in Rocks
Radiometric Dating
Radiometric Dating
A technique used to date
materials, based on a
comparison between the
abundance of naturally
occurring radioactive
substances and their decay
products, using known
decay rates.
 Can be used to date the
absolute age of natural &
man-made materials.
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Back to Grade 9
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Elements are composed
of atoms
Protons & neutrons are
found in the nucleus
while electrons orbit the
nucleus of an atom
Protons – positive
charge & mass of 1
Electrons – negative
charge & virtually no
mass
Neutrons – neutral
charge & mass of 1
Example: Carbon
Mass Number =
protons + neutrons
 Atomic Number =
protons
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Carbon Isotope
An element may exist
in a different isotope
which differs in the
number of neutrons in
the nucleus.
 Some parent isotopes
are unstable & may
spontaneously decay
to another form of
the isotope
(daughter).
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Half Life
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A half life is
the time
required for
one half of
the parent
material to
decay into its
daughter
elements.
Exponential Decay
How old are the...
oldest rocks on Earth?
The Acasta gneisses in Canada’s
Northwest Territories are 4.03 Ga.
oldest mineral grains on Earth?
Zircon grains from western
Australia are 4.3 to 4.4 Ga.
oldest lunar rocks?
Rock samples from the moon are
as old as 4.4 to 4.5 Ga.
oldest meteorites?
The meteorite Allende is 4.559 Ga.
Parent
Isotope
Daughter
Isotope
Half Life of
Parent (years)
Effective dating
range (years)
Materials that
can be dated
238U
206Pb
4.5 billion
10 million –
4.6 billion
Zircon
Apatite
235U
207Pb
704 million
10 million –
4.6 billion
Zircon
Apatite
1.3 billion
50,000 – 4.6
billion
Muscovite
Biotite
Hornblende
10 million –
4.6 billion
Muscovite
Biotite
Potassium
Feldspar
100 - 70,000
Wood, charcoal, peat,
bone and tissue, shell
and other calcium
carbonate, groundwater,
ocean water, and glacier
ice containing dissolved
CO2
40K
87Rb
14C
40Ar
87Sr
14N
4.8 billion
5730
Three Types of Radioactive Decay
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α-decay: An alpha
particle (2 protons + 2
neutrons) is ejected.
– decreases the atomic
number by 2
– decreases the atomic
mass by 4
a-decay
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β-decay: Loss of a beta
particle (e-) causes a
neutron adding a proton.
– increases the atomic
number by 1
– atomic mass does not
change
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electron capture: An
electron and a proton
combine to form a
neutron.
– decreases the atomic
number by 1
– atomic mass does not
change
b-decay
Electron Capture
Not Really that Simple…Chain Decay!
Chain Decay
How is it measured?
A mass spectrometer
is used to measure the
composition (mass
spectrum) of a sample.
 When positively
charged ions are
accelerated then
deflected by a magnet
the molecules will
differentiate based on
weight.
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