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1.41: Decay of Radioactive Isotopes Used For Absolute Dating

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    Decay of Radioactive Isotopes Used For Absolute Dating

    Unstable isotopes emit particles and energy in a process known as radioactive decay. A parent isotope is an unstable radioactive isotope. A daughter product isotope results from the decay of a parent.

    Radioactive decay occurs at known rates and using this you can determine the age of certain types of rocks.

    Dating of materials that contain naturally-occurring radioactive isotopes is possible because the rates of decay are known. The radiation decay clock starts the moment a mineral in a rock forms (or for 14C when an organism dies).

    A half-life is the time required for one-half (50%) of the parent to change to daughter product. The next half-life is when only a quarter of the original parent radionuclide remains, and so on. Age determinations can be determined by comparing the ratio of the parent and daughter isotome in a new (fresh) sample with the percentage in the old sample material being tested (Figure 1.100).

    Commonly referenced studies of absolute dating utilize the radioactive decay of:

    Parent Isotope

    Daughter Isotope

    Half Life

    238U (unstable uranium isotope)

    206PB (stable lead isotope)

    ~ 4.5 billion years

    40K (unstable potassium isotope)

    40Ar (stable argon isotope)

    ~ 1.25 billion years

    14C (unstable carbon isotope)

    14N (stable nitrogen isotope)

    5,730 years

    Note there are many other radionuclides used for absolute dating methods.

    Absolute datingFigure 1.100. Absolute dating methods. Different isotopes are used to study different materials and geologic time ranges.

    Sources of error in Absolute dating. Error can be caused by a variety of misinterpretation. Do we have a general good idea of the geologic history of the sample? (See Relative Dating below). Factors include:

    The sample has been within a closed system, meaning no parent or daughter atoms have entered or the sample. This is best assured by using fresh, unweathered rock samples.
    • Decay rate is constant over geologic time.

    Not all rocks can be dated by radiometric methods
    Detrital sedimentary particles are not the same age as the rock in which they formed
    Metamorphic rock age may not necessarily represent the time when the rock formed
    Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket ages
    • Bracketing sedimentary ages using igneous rocks

    Hydrogen bomb blast in the 1950sFigure 1.101. Nuclear bomb testing release large quantities of radionuclides into the global environment. Atmospheric and oceanic nuclear testing began with the first test on July 16, 1945 (Trinity Site in New Mexico). Most atmospheric testing ended in 1980, but (sadly) still continues underground.