Radiometric Dating

Radiometric dating employs known rates of radioactive decay from a radioactive parent to its stable daughter element to estimate the age of a sample.

Mass number is the total number of protons plus neutrons in an isotope of an element. α-decay reduces the mass number by 4, β-decay does not alter mass number (K-40 → Ar-40), γ-decay reduces mass number by 1.

Elements may decay from radioactive parent to stable daughter through intermediate isotopes in a decay chain called a radioactive series. For the purposes of geological radiometric dating, ratios of radioactive-parent : stable-daughter provide sufficient information to estimate the length of elapsed time since an igneous rock containing radioactive elements cooled and solidified from magma or lava.

Each radioactive isotope (atoms of the same element differing in neutron number) has its own unique half-life, which is the time required for half of the parent radioactive element to decay to a daughter product. Radioactive decay occurs at a constant exponential or geometric rate, so half of parents will decay to daughters (1:1) in one half life and one half of the remaining parents will decay in the next half life (1:3).

Geologists estimate the length of time over which decay has occurred by measuring the ratio of radioactive parent element and stable daughter elements. Radioactive elements tend to become concentrated in the residual melt during the crystallization of igneous rocks, so traces of a radioactive mineral in an igneous rock will 'set the clock' at the time of cooling/solidification.

Radioactive elements and half-lives

radioactive parent

stable daughter

half-life

Sm-147 Nd-143 106 billion years
Rb-87 Sr-87 48.8 billion yrs
Re-187Os-18742 billion yrs
Lu-176Hf-17638 billion yrs
Th-232 Pb-208 14 billion years
U-238 Pb-206 4.47 billion years
K-40 Ar-40 1.26 billion yrs
U-235 Pb-207 704 million years
Be-10B-101.52 million yrs
Cl-36Ar-36300,000 years
U-234Th-230248,000 years
Th-230Ra-22675,400 years
C-14 N-14

5715 years = useful on materials less than 70,000 years old.

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