A8 Radioisotops and Dating
323
On the whole, igneous rocks and the sediments derived from them (mainly shales) have very similar
composition (columns 1 and 2). Note the depletion, in the sediments, of sodium (Na20) much of
which ends up in seawater. Note also the high C02 content in sediments (carbonates), the high
carbon content (coaly matter), and the sulfur (evaporites). The sea floor basalts differ from continental igneous rocks in the abundance and oxidation state of iron, in the calcium content, in magnesium,
and especially in potassium. Red clay has a composition much like sedimentary rock in general,
except for depletion in calcium carbonate (dissolution!), and enrichment in manganese. Also note
the difference·in sulfur (no evaporites) and in carbon (high degree of oxidation). (Sources for A7:
Fairbridge R. W. ed. 1972 The encyclopedia of geochemistry and environmental sciences, Van
Nostrand Reinhold Co. New York; A. E. J. Engel and C. G. Engel, 1971 in A. Maxwell [ed] The Sea
vol. 4 pt. 1,465-519, John Wiley and Sons, New York
A8 Radioisotopes and Dating
General. Certain atoms have a nucleus which emits radiation, whereupon the nature of the atom
changes. Atoms of the same kind are called isotopes and those which emit radiation from the
nucleus are radioactive isotopes. Many elements have both stable and unstable (= radioactive)
nuclides (= isotopes), but elements heavier than lead (see A4) generally tend to be unstable. The
radiation emitted from a nucleus may be 0:, ~, or y-radiation. An alpha particle consists of two
neutrons and two protons (= helium nucleus); a beta particle is a high-speed electron, and gamma
rays are like X-rays except more energetic.
The probability that a given atomic nucleus emits radiation (or "decays") is independent of its
age and of environment (pressure, temperature, chemical conditions). From this it follows that the
number of atoms in a given set of radioactive isotopes decreases according to a simple exponential
law, which may be written:
NINo = e- Al ,
where NINo is the proportion of remaining atoms after time t, and ' A is the decay constant. Solving
the equation for NINo = 1/2 we obtain a value for t which is called the half-life
tl/2 = ln2/A. .
The decaying isotope is called parent and the resulting new isotope is called daughter. Geochronological dating is done by assessing the abundance of parent and/or daughter elements. When NINo
is known, and A., then t can be calculated.
Types of Radioactivity. Three kinds of radioactivity are useful in measuring time in the geologic
record.
The first is primary radioactivity from radioisotopes with very long half-lives, which have been
around since the Earth was formed. These isotopes are ;B0tassium-40, rubidium-87, thorium-232,
uranium-235, and uranium-238 (4
magnetic lineations on the sea floor (Chap. 1). The decay of 87Rb to 87Sr has been used to date
igneous rocks and meteorites, and to establish the timing of metamorphism (rubidium-strontium
method). The method is also used to study inhomogeneities in the upper mantleA~s reflected in
different 87Srltisr rations in oceanic basalts on the seafloor. 232rh decays to 2~Pb in a series
involving six alpha steps (each decreasing the atomic weight of the preceding parent by 4). 2~i also
is produced in this decay. The 232rh decay has been used to date the time of crystallization of certain
minerals. The two uranium decay series are especially important in dating Pleistocene deposits.
323
On the whole, igneous rocks and the sediments derived from them (mainly shales) have very similar
composition (columns 1 and 2). Note the depletion, in the sediments, of sodium (Na20) much of
which ends up in seawater. Note also the high C02 content in sediments (carbonates), the high
carbon content (coaly matter), and the sulfur (evaporites). The sea floor basalts differ from continental igneous rocks in the abundance and oxidation state of iron, in the calcium content, in magnesium,
and especially in potassium. Red clay has a composition much like sedimentary rock in general,
except for depletion in calcium carbonate (dissolution!), and enrichment in manganese. Also note
the difference·in sulfur (no evaporites) and in carbon (high degree of oxidation). (Sources for A7:
Fairbridge R. W. ed. 1972 The encyclopedia of geochemistry and environmental sciences, Van
Nostrand Reinhold Co. New York; A. E. J. Engel and C. G. Engel, 1971 in A. Maxwell [ed] The Sea
vol. 4 pt. 1,465-519, John Wiley and Sons, New York
A8 Radioisotopes and Dating
General. Certain atoms have a nucleus which emits radiation, whereupon the nature of the atom
changes. Atoms of the same kind are called isotopes and those which emit radiation from the
nucleus are radioactive isotopes. Many elements have both stable and unstable (= radioactive)
nuclides (= isotopes), but elements heavier than lead (see A4) generally tend to be unstable. The
radiation emitted from a nucleus may be 0:, ~, or y-radiation. An alpha particle consists of two
neutrons and two protons (= helium nucleus); a beta particle is a high-speed electron, and gamma
rays are like X-rays except more energetic.
The probability that a given atomic nucleus emits radiation (or "decays") is independent of its
age and of environment (pressure, temperature, chemical conditions). From this it follows that the
number of atoms in a given set of radioactive isotopes decreases according to a simple exponential
law, which may be written:
NINo = e- Al ,
where NINo is the proportion of remaining atoms after time t, and ' A is the decay constant. Solving
the equation for NINo = 1/2 we obtain a value for t which is called the half-life
tl/2 = ln2/A. .
The decaying isotope is called parent and the resulting new isotope is called daughter. Geochronological dating is done by assessing the abundance of parent and/or daughter elements. When NINo
is known, and A., then t can be calculated.
Types of Radioactivity. Three kinds of radioactivity are useful in measuring time in the geologic
record.
The first is primary radioactivity from radioisotopes with very long half-lives, which have been
around since the Earth was formed. These isotopes are ;B0tassium-40, rubidium-87, thorium-232,
uranium-235, and uranium-238 (4
magnetic lineations on the sea floor (Chap. 1). The decay of 87Rb to 87Sr has been used to date
igneous rocks and meteorites, and to establish the timing of metamorphism (rubidium-strontium
method). The method is also used to study inhomogeneities in the upper mantleA~s reflected in
different 87Srltisr rations in oceanic basalts on the seafloor. 232rh decays to 2~Pb in a series
involving six alpha steps (each decreasing the atomic weight of the preceding parent by 4). 2~i also
is produced in this decay. The 232rh decay has been used to date the time of crystallization of certain
minerals. The two uranium decay series are especially important in dating Pleistocene deposits.