radiometric methods in the study of rocks` age

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Marta Miedźwiedziew
RADIOMETRIC METHODS IN THE STUDY OF ROCKS’ AGE
Introduction
The article is intended for high school students pursuing chemistry classes at the beginner level and
advanced level. Issues presented in this paper can be regarded as an extension of knowledge about
natural radioactivity and its practical use.
In the natural state many elements are presented in both the stable state and the unstable
(radioactive). Radioactive isotopes decompose radioactively over time, transforming into a stable
and lasting form. In this process, nuclear radiation is emitted, called rays (particles) of alpha (α),
beta (β) and gamma (γ).
To determine the absolute age of rocks, minerals and fossils and archaeological sites two groups of
phenomena associated with radioactive transformations are used: radioactive decay and
accumulation of radiation damage of certain minerals.
1 Fragment of a vacuum line at the Gliwice Radiocarbon Laboratory used for the
processing of the samples dated to carbon dioxide in order to determine the
concentration of 14C gas meter technique [A. Pazdur].
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Radiometric data is based on the phenomenon of radioactive decay, which rate - known
sometimes as half- decay time which means the time at which half of the total amount of the
element is broken down- it is always constant, specific for each radionuclide. Knowing the current
ratio of the radionuclide to the product of its degradation in the rock and knowing the half-decay
time, the start of this process can be determined , which usually coincides with the age of the rock
or mineral, in which these elements are present, or with the process of geological age.
The second group of phenomena is the basis of so-called dosimetry methods, which means
that they are based on the measurement of nuclear radiation dose. There are fewer number of these
methods, they are much more difficult to use and generally give poorer accuracy.
Dating on the basis of radioactive decay
The process of radioactive decay is described by a relatively simple equation
N t  N 0 e  t
where:
N0 - is the initial number of atoms of a radioactive element,
Nt - the number of atoms of this element, that remained after the time t,
λ - the value, called the constant of radioactive decay.
The values of λ are known exactly for all radionuclides. In order to characterize more palpable the
decay process it is introduced the quantity called half-life T1/2 in place of λ , and the value of which
is expressed by the formula
ln 2 0,692
T1 / 2 



The value T1/2 means the time after which the number of atoms of a radioactive element
decreases by half. Using the size of T1/2 we may an equation describing the process of radioactive
decay rewrite in the following form
T
N
T  1 / 2 ln 0
ln 2 N t
The above formula shows that knowing the initial number of atoms N0 and measuring the current
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number Nt it is possible to calculate the value of T , so the time that has elapsed since the creation of
the dated object, which tested in the laboratory serves as a radioactive product - provided that the
tested object (mineral buildup or organic, remains of dead plants or pollen, buried bones of humans
or animals) was a closed system. This means that, at the time of the formation of a precipitate or
burring organic debris, to the time of the measurement the supply of atoms of the chosen element
from the environment did not occur, nor the outlet to the outside. In other words – the change of the
number of atoms of a chosen radioactive element shall be solely the result of its collapse.
Because radioactive decay occurs in a manner independent of any external factors such as
temperature, humidity, chemical composition of the preparation, the texture, physical state, as
crystallization, etc., these models are the foundation for the almost ideal isotope chronometer, of
course, under the condition of fulfilling the assumption of a closed system.
The half-lives of radioactive isotopes vary in a very wide range (eg, 5730 years in the case of
C, about 750 000 years in the case of the isotope 230Th, and only 22 years in the case of the
isotope 210Pb), which has an impact on their application. Isotopes with short half-lives sometimes
disintegrate so quickly that their number in the older rocks are too small to be able to be measured,
and therefore are used for dating very young rocks, while playing these long times, for dating rocks
older ( in the young rocks there is not enough child isotopes).
The most important radiometric methods for dating include, among others radiocarbon
method, rubidium-strontium, potassium-argon and uranium-lane.
14
1. The radiocarbon method
Radiocarbon Chronometry - the best known method of all radiometric methods - was
developed by Willard Libby in 1949 (for which he received in 1960 Nobel Prize in Chemistry). It
uses the fact that the prevalence of the carbon isotope 14C in the form of small admixtures
(approximately 1 atom in 1012 carbon atoms - a concentration of 1 ppt) in all the substances
containing carbon, both inorganic and organic.
The isotope 14C is formed in the upper atmosphere due to cosmic ray neutrons
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n  147 N 146 C  11H
and together with the other carbon isotopes involved in the geochemical cycle of this element. The
collected data show that the concentration of 14C is approximately constant during the life of the
organism. At the time of the withering away of the body's the exchange of carbon with the
environment is broken, and since then there is only the process of decay to the stable isotope 14N:
14
14

6 C  7 N  e  ve
On the basis of the ratio of 14 C to the total amount of carbon in the material, it can be calculated
how much time has passed since the so-called radiometric clock reset. The calculated value is called
2. The accelerator ‘s view in Kilonia used for AMS radiocarbon dating
technique [A.Pazdur]
the conventional radiocarbon age and is expressed in years BP ( in English called before present time counted backwards from the present, conventionally taken as the calendar year 1950).
Because of the very short half-life of 14C (5730 years) radiocarbon method can only be used for the
dating of materials whose age does not exceed 70 000 years. He has a great importance in the dating
of the remains of living organisms or archaeological findings [photo 3] - materials so young that
most of the other radioactive elements present in them does not decays sufficiently to allow a
reliable measurement. Radiocarbon age may also be used to track carbonate age, such as cave
formations, calcareous tufa, shells of snails.
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3. The paintings on the wall of the cave Chauvet, France. Age of about 31
thousand. years, determined by AMS radiocarbon dating of organic dyes in
the Radiocarbon Laboratory in Gif-sur-Yvette (France) [A.Pazdur]
2. Rubidium-strontium method
Rubidium occurs as a trace element in many igneous and metamorphic rocks, and even in
the few sedimentary rocks. Due to the long half-life of rubidium 87Rb (48.6 billion years),
rubidium-strontium method can be used in dating the very old rocks, of the age of over 100 million
years.
3. Potassium-argon and argon-argon method
Potassium-argon method was developed in the 50s of the twentieth century. It is based on
the phenomenon of radioactive decay 14K (half-life decay of 14K is 1.3 billion years old) to a stable
argon isotope gas of 40Ar and is used to test the age of the volcanic rocks and archaeological sites
covered by volcanic rock (eg position of hominids in Africa).
The so-called clock reset occurs during the formation of the rocks as a result of volcanic
activity, when it occurs before the release of the accumulated argon, it is believed that from this
moment it arises only as a breakdown product of the isotope 40K. Unfortunately, the actual process
does not take place in accordance with this assumption. Before the complete solidification of rock
when it is in a semi-fluid state which results in the appearance of argon which can still escape into
the atmosphere and it lowers the calculation results of the initial amount of potassium. There is also
the possibility of gas absorbed from the atmosphere, or its absorption or leaching by water.
More accurate results are obtained using the argon-argon method. It involves placing a sample in
the reactor and the neutron bombardment in order to transform 40K in argon. Number of argon
obtained in this manner can be measured much more accurately than the number in a sample of
rock.
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4. The method of uranium- thorium
Uranium-track method is based on the disintegration of radioactive uranium isotopes 235U and
U (half-life decay respectively 0.7 billion years and 4.47 billion years) and thorium 232Th (14
billion years) for different isotopes of lead. Uranium and thorium are naturally very clearly
separated (as a result of the high potential of the ion path it is rapidly adsorbed by the clay minerals,
and the uranium is usually in the form of easily soluble and it is transported by the water surface),
and it can be assumed that it is due to the transformation one element to another, and not the fact of
being both appeared there during the formation of a rock.
The most useful mineral for dating uranium and thorium is zirconium, found in small
concentrations in igneous and metamorphic rocks, as well as resulting from the destruction of
sedimentary rocks. During the rock crystallization into crystal lattice zirconium uranium atoms are
incorporated, which then gradually decompose lead. Using grain zircon, geologists are able to date
with precision (measurement error not greater than a few million years for the age of the rocks of
about 2-3 billion years), even very old Precambrian rocks. Uranium- thorium method can also be
used to determine the age of coral reef building to embed small amounts of uranium in their
skeletons. Uranium-lead and lead - thorium methods were also used for the dating of lunar rock
samples. The resulting age of moon rocks, about 4.6 billion years ago, agrees with the estimated age
of the Earth and the solar system, calculated on the basis of other evidence.
238
Many techniques have been developed using radiometric uranium and its child elements to
determine the age of rocks or fossils, formed a few million years ago, or even younger. One of the
technique is a method of spontaneous decay traces nuclei used when the rock is too young to be
an accumulation of the proper number of child lead. The 238U emits subatomic particles during the
decay, which move with so much energy that when they pass through the surrounding lattice they
leave traces of network disorders [Photo 4]. These marks can be enlarged in the laboratory, by
feeding them by properly selected acid and then counted directly under the microscope. After
counting all traces the sample may be subjected to neutron radiation field, leading to the
disintegration of the remaining uranium. Ratio traces formed naturally and obtained artificially
allows the calculation of the age of the rocks.
4. Traces of spontaneous disintegration of the
mineral grains, plus a chemical etching process.
The complete track in the middle of a 13.5  m
[Stanley]]
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Summation
To determine the absolute age of rocks, minerals and fossils and archaeological sites two
groups of phenomena associated with radioactive transformations are used: radioactive decay and
accumulation of radiation damage of certain minerals. Radiometric dating is to determine the age of
rocks and minerals on the basis of the ratio of the radioactive isotope to its decay products.
The choice of method depends largely on the type of test material (and thus the chemical
composition and the presence of radioactive isotopes), and the age of rocks and organic debris
(sample must contain enough of the isotope in both the original and the child element to be able to
make reliable measurements).
The most important radiometric dating methods include, among others radiocarbon method,
rubidium-strontium, potassium-argon and uranium-lane.
Bibliography
1. S. Stanley, History of the Earth, PWN, Warsaw 2002.
2. W. Mizersk Historical geology for geographers, PWN, Warsaw, 2005.
3. A. Pazdur, Absolute time scales for events in the history of the earth and man, Inaugural Lecture
of the academic year 2004/2005 at the Technical University in Gliwice [on-line].
4. www.pl.wikipedia.org
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