Absolute Dating - Radioisotopes

Using the sequencing of rock units we can assign relative dates to geologic formations. To assign an absolute date (a specific number of years from the present), we use radioactive decay.

  1. Some isotopes of elements are unstable and spontaneously break apart. This process is called radioactive decay and the isotopes are termed radioactive.
  2. As the unstable parent isotope decays, daughter products form. These daughter elements result from the parent isotope emitting an alpha particle, emitting a beta particle, or capturing an electron.
  3. Any of these transactions cause a new element to be formed. There may be several steps in the decay process until a stable element is created. The sequence of parent-daughter products is well defined, as are the decay rates.
  4. Parent elements are incorporated into minerals crystallizing in cooling magma. Daughter products produced by radioactive decay of the parent isotopes are trapped in the mineral.
  5. By measuring the ratio of parent to daughter elements in that mineral we can determine how a long ago the mineral solidified.
  6. Number of Half-lives Parent Isotope Remaining (%) Daughter Isotope (%)
    0 100 0
    1 50 50
    2 25 75
    3 12.5 87.5
    4 6.25 93.75
    The usual way to describe the radioactive decay rate is half-life. This is the time it takes for one half (½) of the parent isotope to decay into the daughter product. After one half-life, 1.0 g of a parent has decayed into 0.5 g of parent and 0.5 g of daughter product. After two half-lives, there is 0.25 g of parent and 0.75 g of daughter product.
  7. Knowing the half-life of a parent isotope and determining the ratio of parent isotope to daughter product in a mineral sample allows us calculate the time since the mineral crystallized.
  8. The table below lists five parent / daughter product pairs that are useful for dating minerals. Potassium-Argon is often used due to the sensitivity of laboratory techniques for these elements and the fact that potassium is found in many common minerals such as feldspars and micas.
    Radioactive Parent Half-life (in billion years) Stable Daughter Product
    Uranium 238 4.5 Lead 206
    Uranium 235 0.71 Lead 207
    Thorium 232 14.1 Lead 208
    Rubidium 87 47.0 Strontium 87
    Potassium 40 1.3 Argon 40