Transcript CHMR_PNE_U17RadioaDating_V02

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Borrowed from
Evolution of the
Earth
Seventh Edition
Prothero • Dott
Chapter 5
NUMERICAL DATING OF THE EARTH
ASSUMPTIONS
• 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.
Radioactive elements
• Not all elements are radioactive. Those that are
and are the most useful for geologic dating are:
• U-238
• K-40
• C-14
Half-life = 4.5 Billion years
Half-life = 1.25 Billion years
Half-life = 5.73 Thousand years
• Also, Sm-147, Rb-87, Th-232, U-235
U-238 DECAY
• Often elements decay according to a complex decay
scheme in which a host of intermediate products,
many themselves radioactive, are produced.
• U-238 is such and element, and given its importance
to geologic dating, it is worthwhile to examine it
decay scheme.
• Keep in mind that U-238 has a half-life
approximately equal to the age of the earth, 4.5
Billion years.
Half-life for decay from U-238 all the
way to Pb-206 is 4.5 b.y. (billion years).
U-238 Decay Series
Decay rates for intermediate daughter
products range from <1 sec (polonium)
to 1,622 years (radium 226).
Fig. 5.4
Schematic diagram showing decay of radioactive parent isotope (e.g. U-238)
to a daughter (e.g. Pb-206). The original isotope was sealed in a mineral
grain at time of crystallization. Note changing ratio of parent/daughter after
2 half-lives. Note that to get an estimate of the geologicc age, you need the
ratio of the parent isotope to the daughter isotope, e.g. two measurements.
Fig. 5.5
Simple arithmetic plot of a universal isotopic decay curve. After 1 halflife 50% of parent isotope remains; after 2 half-lives, 25% remains.
What happens if the vertical axis is changed from linear to logarithmic?
BLOCKING TEMPERATURES
• The “Blocking Temperature” is an important concept; it refers
to processes that result in a “resetting” of the atomic clocks in
a rock.
• Essentially, it is possible to heat igneous and metamorphic
rocks to high enough temperatures that they no longer behave
as “closed systems”. That is some of the daughter products can
“leak” out of the primary mineral, giving an erroneous
parent/daughter ratio and hence a wrong age.
(Age for what? How could the age be interpreted in a rock in
which the blocking temperature has been reached?)
Blocking temperatures for some common minerals and decay series.
The blocking temperature is
the temperature above which
a mineral or rock no longer
behaves as a closed system
and the parent/daughter ratios
may be altered from that due
to pure radioactive
disintegration.
This can result in resetting the
isotopic clock and/or give
what are called discordant
dates.
These types of problems have
given opponents of the
radiometric dating of the
Earth ammunition to attack
the 4.5 Billion year age
geologists cite.
Inconsistent rate of
change
Use of daughter lead
isotopes for dating. The
ratios of 3 radiogenic lead
isotopes to non-radiogenic
lead-204 all change but at
different rates.
These ratios can also be
used to date a rock or
mineral.
What might be another
term for radiogenic?
Fission tracks in
an apatite crystal.
Fission tracks are
produced when an atom
of U-238 disintegrates
emitting an alpha particle,
a Helium nucleus (He-4).
This massive atomic
particle causes massive
structural damage in the
crystal that can be
revealed by etching.
The number of tracks in a
given area is proportional
to the age of the mineral.
(Why not just use the U238 to Pb-206 method
directly in such cases?)
Dating Using a System That Is NOT Closed
Metamorphic rocks go though heating and cooling cycles which
allow for redistribution of daughter isotopes.
1. Mineral crystallizes 1000 mya (1000 million = 1 billion yrs ago)
2. After 500 my (million yrs) some parent isotopes have decayed.
3. 480 mya (million yrs ago) metamorphic event redistributes
daughter atoms out of crystal into adjacent rock.
4. Dating of the mineral will now yield the age of the metamorphic
event instead of the age of the rock.
Carbon Dating
1: Formation of Carbon-14
2: Decay of Carbon-14
3: The equation is for living organisms, and the inequality is for
dead organisms, in which the C-14 then decays
• Carbon has unique properties that
are essential for life on Earth.
Familiar to us as the black
substance in charred wood, as
diamonds, and the graphite in
“lead” pencils, carbon comes in
several forms, or isotopes. One
rare form has atoms that are 14
times as heavy as hydrogen
atoms: carbon-14, or 14C, or
radiocarbon.
• Carbon-14 is made when cosmic
rays knock neutrons out of atomic
nuclei in the upper atmosphere.
These displaced neutrons, now
moving fast, hit ordinary nitrogen
(14N) at lower altitudes,
converting it into 14C. Unlike
common carbon (12C), 14C is
unstable and slowly decays,
changing it back to nitrogen and
releasing energy. This instability
makes it radioactive.
• Ordinary carbon (12C)is found in the
carbon dioxide (CO2) in the air, which
is taken up by plants, which in turn are
eaten by animals. So a bone, or a leaf
or a tree, or even a piece of wooden
furniture, contains carbon. When the
14C has been formed, like ordinary
carbon (12C), it combines with oxygen
to give carbon dioxide (14CO2), and so
it also gets cycled through the cells of
plants and animals.
• We can take a sample of air, count how
many 12C atoms there are for every 14C
atom, and calculate the 14C/12C ratio.
Because 14C is so well mixed up with
12C, we expect to find that this ratio is
the same if we sample a leaf from a
tree, or a part of your body.
In living things, although 14C atoms are constantly changing back to 14N,
they are still exchanging carbon with their surroundings, so the mixture
remains about the same as in the atmosphere. However, as soon as a
plant or animal dies, the 14C atoms which decay are no longer replaced,
so the amount of 14C in that once-living thing decreases as time goes on.
In other words, the 14C/12C ratio gets smaller. So, we have a “clock”
which starts ticking the moment something dies.
Obviously, this works only for things which were once living. It cannot
be used to date volcanic rocks, for example.
The rate of decay of 14C is such that half of an amount will convert back
to 14N in 5,730 years (plus or minus 40 years). This is the “half-life.” So,
in two half-lives, or 11,460 years, only one-quarter of that in living
organisms at present, then it has a theoretical age of 11,460 years.
Anything over about 50,000 years old, should theoretically have no
detectable 14C left. That is why radiocarbon dating cannot give millions
of years. In fact, if a sample contains 14C, it is good evidence that it is not
millions of years old unless the 14C became present by some other
What is CARBON DATING of
fossils?
The stable isotope of carbon is C-12.
There is a constant generation of C14 in the upper atmosphere by
cosmic particle bombardment of N
(nitrogen).
Nitrogen (N-15) emits a proton and
becomes C-14. This is radioactive
with a half-life of about 5,730 years.
Plants and animals ingest this
radioactive C-14 while they are
alive. When they die, the ingestion
stops, and the radioactive C-14 clock
begins to count down.
Timed Pair Share
Could there be variations in the
amount of C-14 in the atmosphere at
any given time, or is this a constant?
Non-carbon Dating Using Radioactive Isotopes Review
There are various other radiometric dating methods used today to
give ages of millions or billions of years for rocks. These
techniques, unlike carbon dating, mostly use the relative
concentrations of parent and daughter products in radioactive
decay chains. For example, potassium-40 decays to argon-40;
uranium-238 decays to lead-206 via other elements like radium;
uranium-235 decays to lead-207; rubidium-87 decays to
strontium-87; etc. These techniques are applied to igneous rocks,
and are normally seen as giving the time since solidification.
The isotope concentrations can be measured very accurately, but
isotope concentrations are not dates. To derive ages from such
measurements, some assumptions have to be made such as:
• The starting conditions are known (for example, that there was no
daughter isotope present at the start, or that we know how much
was there).
• Decay rates have always been constant which in some cases is not
true.
• Systems were closed or isolated so that no parent or daughter
isotopes were lost or added which in some cases is not true.
Resolve the Inconsistencies!
• You could be the next
Nobel Prize winner!