Transcript Lecture 8: Half-Life and Radiometric Dating Techniques

Lecture 8
Radiometric Dating
• Half-life
• Radiometric
Dating
– 14C Dating
– K-Ar Dating
– U-Pb Dating
Half Life
• Half-life: the time required for half of an
original quantity of an isotope to decay
• Related to the rate of decay for a
radioactive isotope.
• Greater rate of decay = greater radioactivity
= shorter half-life
Half Life
Half-life:
• Is constant and independent of any
physical or chemical changes – cannot be
altered.
• Can be calculated at any given moment by
measuring rate of decay of a known
quantity using a radiation detector.
A certain isotope has a half-life of one day. This
means the amount of that isotope remaining at the
end of three days will be
1.
2.
3.
4.
zero.
one-quarter.
half.
one-eighth.
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3
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4
Radiometric Dating
Radiometric Dating: process that
measures the ratio of radioactive isotopes
to their decay products in order to
determine the numeric age of a natural
material.
Radioactive Isotope = Parent Isotope
Decay Product = Daughter Isotope
14C-Dating
• Carbon-14 (14C):
– Decays to Nitrogen-14 (14N).
– Good to date geologically young organic materials
(useful in archeology).
– Relatively short half-life of 5,760 years.
– Useful for dating materials within the last 50,00070,000 years.
14C-Dating
• Cosmic Rays bombard Earth’s atmosphere
resulting in the formation of C-14 from N-14.
14C-Dating
• C-14 chemically reacts with Oxygen to form CO2 which
is taken in by plants.
• C-14 transmitted to animals by consumption of plants.
• Eventually, C-14 will decay back to N-14.
14C-Dating
• When alive, organisms maintain equilibrium with the
atmosphere as they replenish C-14 in their systems.
• When an organism dies, the total C-14 decreases as it
decays back to N-14.
A gram of carbon is extracted from an ancient tree stump
exposed on the beach. The carbon is one fourth as
radioactive as a gram of carbon extracted from a currently
living tree. How long ago did the tree die? Note: half life of C14 = 5,760 years.
1.
2.
3.
4.
5,760 years
11,520 years
17,280 years
2,880 years
0%
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0%
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K-Ar Dating:
• Potassium-40 (40K):
– Decays to Argon-40 (40Ar).
– Useful for determining crystallization age of igneous
and metamorphic rocks.
– Half-life of 1.25 billion years.
K-Ar Dating:
• K-Ar Dating:
– Occasionally can be used to date rocks younger than
100,000 years.
– More commonly used for rocks with ages in the
millions of years.
Igneous rock from
the Oregon Coast
Range is
determined to be
32.5 ± 0.5 million
years old.
K-rich minerals contain
abundant 40K which decays to
40Ar that remains trapped within
crystal structure.
Ratio of 40Ar (Daughter) to 40K
(Parent) used to determine age
which is based on known decay
rate.
The 40Ar-39Ar age dating technique in which samples are subjected to
incremental heating is a more accurate version of K-Ar dating widely
utilized by geologists.
Samples to be
analyzed
Furnace for heating
and melting
samples in order to
release gases
trapped within
minerals.
Gas Extraction Line –
removes chemically reactive
gases leaving only noble
gases such as Argon to be
analyzed.
Laser used for
heating and melting
very small samples
such as individual
crystal grains.
Vacuum chamber
containing samples
Mass Spectrometer for measuring
ratios of Argon isotopes.
Utilizes Newton’s 2nd Law which can
be applied to individual atoms
Ions of different Argon isotopes have a force acting on
them when they pass through a strong magnetic field.
Which would experience the greatest accelerations?
1. Those with larger
mass numbers
(40Ar,39Ar)
2. Those with smaller
mass numbers (37Ar,
36Ar).
0%
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0%
2
• While passing
through a magnetic
field as charged
particles (ions):
– Isotopes that have
less inertia (mass)
are deflected more
(undergo greater
acceleration).
– Lower atomic mass
numbers = lower
masses.
Diagram of how a Mass Spectrometer
separates different isotopes
Uranium-Lead Dating
• One of the oldest, most refined
radiometric dating techniques.
• Measure ages of rocks of ages
ranging from 1 million to 4.5 billion
years.
• Precisions of 0.1 to 1 percent range.
• Acquired from zircon crystals which
are very resistant to chemical and
physical alteration.
Uranium-Lead Dating
• Uranium-238 and -235:
– Decays to Lead-206 and Lead-207 respectively.
– No other natural sources exist for the daughter
isotopes.
– Half-lives of 4.47 Billion Years and 704 million years
respectively.