Radiometric Dating

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

ï Use radioactive isotopes of elements

Protons and Neutrons in Nucleus of Atoms

Radioactive Decay

ï Parent is the original unstable radioactive isotope

ï Daughter is the new isotope formed

ñ Unstable daughters along decay path

ñ At the end of decay series, a stable daughter

Types of Radioactive Decay

ï Alpha emission

ï Beta emission

ï Electron capture

Half Life

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

Radiometric Dating

ï If the following are known:

ñ Half life of the isotope being measured

ñ Parent/daughter ratio in the sample

ï 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.Ý 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.Ý 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.Ý 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.Ý

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.