Chronostratigraphy I:

Numerical or absolute dating There are many methods, each with its own strengths and limitations:

Radiometric dating

  • Antoine Becquerel (1852-1908): Discovered natural radioactivity (1896). In the following years, a large number of radioactive isotopes and their daughter products became known.

  • Pierre (1859-1906) and Marie (1867-1934) Curie: Discovered that the radioactive element radium continuously releases newly generated heat - radiogenic heat. With this discovery, it became clear that the decay of radioactive substances provided a continuous source of new heat that Sir William Thomson (aka Lord Kelvin) hadn't accounted for in his calculations that Earth was 20 - 40 million years old. The Earth might, indeed, be much older than his calculations indicated. But how old?

    History:


    The current understanding:

    Radioactive decay - unstable parent atoms change into more stable daughter atoms. This involves one of the following transformations:

    Decay constant (λ) - The probability that a given nucleus will decay at a given time. This is unique to each element. If one assumes that the parent:daughter ratio present in a crystal is determined only by the elapsed time since the parent and daughter were locked into the crystal and neither have escaped

    N = N0e-λt

    Where:

    Half-life (t1/2) - increment of time needed for half the parent atoms to decay to daughters

    t1/2 = 0.693/λ

    t = (1/λ) ln(d/p +1)

    where:


    Caveats:

  • Radiometric dating records the closure time when a crystal cooled to solid state and locked radiogenic elements into its structure.
  • Most dating is done on igneous and metamorphic rocks. Sediments are remnants of other rocks - radiometric ages obtained from sedimentary rocks are often the age of an assortment of source rocks, not the current sedimentary rock. Thus, for sediments, we typically rely on igneous marker beds that constrain the ages of adjacent sediments. (right)
  • Some parent or daughter atoms can escape if the system is not fully closed. (This is why we don't continue to use Rutherford's system of measuring the He atoms formed from alpha decay of radium.)

    Potassium (40K) → Argon (40Ar) by electron capture and γ decay.

    Uranium (238U) → Lead (206Pb) by series of α and β decays.

    Uranium (234U) → Thorium (230Th) by α decay.

    Several other systems are useful for dating igneous and metamorphic rocks, including:

    14C Dating

    This method is not used on minerals. Rather, it exploits the fractionation of radioactive 14C and stable 12C by plants during photosynthesis. 14C is produced in the upper atmosphere by bombardment of 14N by cosmogenic neutrons and is fractionated by photosynthesis such that it is incorporated into plant tissue in a fixed ratio to 12C. This fractionation is conserved across green plants and tells us the initial ratio of these isotopes when the plant was growing. Because of its short half-life (5,730 years), 14C dating is useful only as far back as 40,000 yrs.

    Note: