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Isotope Geology (Geo 448)

Department of Geology

The University of Qatar

Prof. S. Nasir

DOWNLOAD PROGRAM Isotope for age determination

This program is for calculating Temperatures using the Calcite-Dolomite thermometers

Coarse Contents:

  1. Introduction: The discovery of radioactivity, how old is the earth, impact on geology
  2. Structure of atmoms
  3. Deacy mechanisms of atoms
  4. Radioactive Decay and growth
  5. Mass spectrometry
  6. K-Ar dating
  7. Rb-Sr dating
  8. Sm-Nd dating
  9. U, Th-Pb dating
  10. Pb-isotope dating
  11. C-14 dating
  12. Stable isotope
  13. Oxygen and hydrogen
  14. Carbon
  15. Nitrogen
  16. Sulfur
  17. Application of isotope geology in igenous, sedimentary, metamoprhic rocks, water, sulfides, petroleum geology

Text Books:

  1. Principles of Isotope Geology. Gunter Faure 1986. 2nd edition. John Wiley & Sons.
  2. Applied geochronology. Hamilton, E.I. (1965). Academic press. London.
  3. Lectures in isotope geology. Jaeger, E. and Hunziker, C. (1979) Springer Verlag.
  4. Nuclear and chemical dating techniques; interpreting the environmental record. Currie, L.A. (1982) American chemical Society.


Age of the earth:

Bishop Ussher (1650): creation of the world took place in the year 4004 B.C.

Lord Kelvin (William Thomson): (1897): the possible age of the earth between 20 and 40 million years.

1896: Henri Bequerel had announced the discovery of radioactivity.

1898: Marie and Pierre Curie discovered two new active elements: Polonium and radium

1917: Discovery of mass spectrograph by Thomson and F. Aston.

1905: Rutherford: ages of uranium minerals of about 500 million years

1913: ages of archean rocks of about 1600 million years.

Structure of atoms

Proton (p): + charged particles

Neutron (n): neutral charge, larger mass than proton

The atomic number (Z): number of protons

Neutron Number (N): number of neutrons

Mass number (A): number of protons and neutron: A=Z+N

Isotopes: Atoms which have the same atomic number (Z) and different values of neutron (N). They are atoms of the same element. They differ only in their mases. (found in a horizontal row).

Isotones: have the same valus of neutron (N) but different atomic number (Z). They are atoms of different elements (Vertical rows)

Isobars:Have the same value of mass number (A) and different (Z) and (N). They are different elements (Diagonal row)

The atomic weight:

The totla number of different nuclides is about 1700. About 260 of these are stable. The atomic weight of an element is the sum of of the masses of its nuturally occuring isotopes in accordance with the abundance of each isotope. The atomic weight is measured by atomic mass unit (amu).

The gram atomic weight or mole :atomic weight of element in grams.

1 amu=1/N gram where N is the Avogadro's number (6.02252 *10^23)

Most stable nuclides have even numbers of protons and neutrons. They have nearly equal Z and N.

Most known nuclides are not stable (radioactive nuclides) but decompose spontaneously until they achieve a stable configuration. Isotopes become increasingly unstable as the number of neutrons increases .

Abundances of the elements in the solar system:

The abundances of elements in the solar system are primarly the results of the nuclear reactions :

Radioactive Mechanisms

Radioactive decay results in changes of Z and N of the parent elements and thus leads to the transformation of an atom of one element into that of another element. The process of decay continues until at last a stable atom is produced. Radioactivity invloved the emission of three different types of rays: alpha, beta and gamma.

  1. Beta (Negatron )Decay:

    Beta decay can be regarded as a transformation of a neutron into a proton and an electron. . The electron is then expelled from the nucleus as a negative beta particle. By this process, the atomic number (Z) of the daughter increase by one , its neutron number is reduced by one. Consequently, the daughter atom is an isobar to the parent atom (the same atomic mass (A), different Z and N).


    40 ------40

    K------- Ca + B + v + Q

    19 ------20


    ----------------Atomic Number Neutron Number Mass Number


    Parent --------Z --------------------N---------------------- A=Z+N

    Daughter---- Z+1 ----------------N-1-------------------- A=(Z+1)+(N-1)


    2. Positton Decay (positive beta)

It is a decay by emission of a positvely charged electron (positron). It occurs by a transformation of a proton in the nucleus into a neutron, a positron and a neutrino. The atomic number of the daughter atom will decrease by one, its neutron number will increase by one. The daughter element is an isobar and has one less proton than its parent


    ----------------Atomic Number Neutron Number Mass Number


    Parent --------Z --------------------N---------------------- A=Z+N

    Daughter---- Z+1 ----------------N-1-------------------- A=(Z+1)+(N-1)



18------- 18

F -------- O + B + v + Q


3. Electron Capture

This process can be visualized as a reaction between extranuclear electron and a proton in the nucleus to form a neutron and a neutrino. The daughter is isobaric with the parent .


    ----------------Atomic Number Neutron Number Mass Number


    Parent --------Z --------------------N---------------------- A=Z+N

    Daughter---- Z-1 ----------------N+1-------------------- A=(Z-1)+(N+1)


4. Branched Deacy

The isobar rule implies that two stable isobare e.g Ca and Ar, must be separated by a radioactive isobar e.g. K, that can undergo branched decay and thus forms two stable isobaric daughters:


K -----89%---Ca + B

19-------------- 20

40-------------- 40

K ----11%--- Ar + B


5. Alpha decay

Alpha particles are composed of two protons and two neutrons. The emission of alpha particles reduces both the atomic number (Z) and the neutron number by two and the atomic mass by four. The duaghter is an isotope of a different element:


    ----------------Atomic Number Neutron Number Mass Number


    Parent --------Z --------------------N---------------------- A=Z+N

    Daughter---- Z-2 ---------------- N-2-------------------- A=A-4


6. Nuclear Fission

This process invloves the bombarding of neuclei with neutrons, protons, alpha particles, gamma rays or X-rays.

Radioactive Deacy

The rate of decay of an unstable parent nuclide is proportional to the number of atoms (N) remaining at any time (t):

-dN/dt= lamdaN

and the mean half time t is equal to the reciprocal of the decaay constnt:


which is longer than t by a factor of 1/0.693

Units of Radioactivity:

Curi (Ci) is the basic unit for the measurement of radioactivity: it is the quantitiy of any radioactive nuclide in which the number of disintigration is 3.700*10^10 per second. Millicurie (1 mCi)= 10^-3 Ci. Microcurie (1 uCi)=10^-6 Ci.

Roentgen ( R ) : The quantity of x or gamma radiation for which the associated corpuscular emission per 0.001293 grams of air produces ions carrying 1 electrostatic unit of electricity.

Rad: the dose corresponding to the absorption of 100 ergs per gram of tissue

Rem: A measure of the dose of any ionizing radiation to body tissues in terms of its estimated biologicaeffect relative to a dose of one roentgen of x-rays

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