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Radiometric Dating of Rocks

Radiometric Dating of Rocks. 1896-1898 Radioactivity discovered by: Henri Becquerel and Marie and Pierre Curie. Phenomenon of Radioactivity. Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously. The Atom. Uranium Nucleus.

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Radiometric Dating of Rocks

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  1. Radiometric Dating of Rocks 1896-1898 Radioactivity discovered by: Henri Becquerel and Marie and Pierre Curie.

  2. Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

  3. The Atom

  4. Uranium Nucleus spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

  5. Alpha Particle emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

  6. Uranium - Thorium Decay U He + Th 4 234 238 92 90 2 spontaneous decay “parent” “daughter product” alpha particle = 2 protons + 2 neutrons = positively charged ion of Helium Thorium: 90 protons + 144 neutrons

  7. Beta Particle Emission But, Th is also unstable, and it emits a beta particle… 234 90

  8. Thorium - Protactinium Decay beta particle Th + Pa 234 234 90 91 beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron Protactinium: 91 protons + 143 neutrons

  9. U PbSeries This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

  10. U PbSeries The rate at which this process occurs is measured in terms of the “half life”.

  11. Half Life Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

  12. The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:   (1) As radioactive elements decay in the Earth, they heat up the surrounding rocks. (2) Radiometric Dating

  13. Radiometric Dating Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed. This is an example of uniformitarianism. The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

  14. To estimate the age of a rock: D = amount of daughter product. P = amount of parent.   For a particular radioactive element in a rock, determine the present ratio = D/P. D P

  15. Rate of decay (from theory and measurement) • Make assumptions about original ratios (from theory of geochemistry). • As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified). D D P P

  16. Half-Lives of Radioactive Isotpes

  17. Examples of Very Old Rocks Found on Earth: • 3.4 billion year old granites - South Africa. • 3.7 billion year old granites - Greenland. • Oldest terrestrial rocks > 4.6 billion years. Central Canada and Central Australia • Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

  18. Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

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