Radiometric Dating

      Gives an absolute age of geologic material in (usually millions of) years

      Use radioactive isotopes of elements


Protons and Neutrons in Nucleus of Atoms



Radioactive Decay

      Parent is the original unstable radioactive isotope

      Daughter is the new isotope formed

   Unstable daughters along decay path

   At the end of decay series, a stable daughter



Types of Radioactive Decay

      Alpha emission

      Beta emission

      Electron capture




Half Life

Time required for half the number of radioactive nuclei in a sample to decay to a new element




Radiometric Dating

      If the following are known:

   Half life of the isotope being measured

   Parent/daughter ratio in the sample

      Then the age of the sample can be calculated



Radioactivity: Dating the Time of Crystallization


Resetting Isotopic Ages

The isotopic clock can be reset by metamorphism when temperatures become high enough for daughter isotopes to escape the minerals where they accumulated



Carbon-14 Dating

        Cosmic rays in atmosphere shatter gas atoms, releasing neutrons

        Some neutrons are absorbed by nitrogen atoms, releasing protons

        14N in atmosphere converted to 14C

        Living matter absorbs carbon dioxide containing 14C

        During life, the 12C / 14C ratio in living matter is constant

        After death, 14C in organism decays to 14N (half-life 5,730 years)

        Good for dating carbon-bearing material younger than 80,000 years



Isotopic Dating of Igneous Rocks

Igneous rocks can provide an approximate crystallization age using whole rock analysis. Minerals also provide a crystallization age provided the rock has not experienced subsequent metamorphism.



Isotopic Dating of Metamorphic Rocks

A metamorphic rock can provide an age for the last metamorphic event. If the rock experienced more than one episode of metamorphism, then usually only the most recent event can be dated.



Isotopic Dating of Sedimentary Rocks?

In general, clastic sedimentary rocks do not give meaningful ages because the minerals contained in these rocks were derived from other sources. At best, clastic minerals can only provide the age(s) of their parent source rocks.



Isotopic Dating of Sedimentary Rocks?

Isotopic dating of a sedimentary rock is only possible if it contains an authigenic mineral or material (e.g. glauconite, K-feldspar, organic matter) that crystallized in the sedimentary environment of deposition.



Bracketing the Ages of Sedimentary Rocks

The ages of sedimentary rock packages can be bracketed by dating underlying and overlying igneous and metamorphic rocks, interlayered volcanic ash deposits and cross-cutting igneous dikes.