Radiometric Dating

Radiometric dating measures the steady decay of radioactive isotopes to assign absolute ages to rocks and minerals, providing the numerical backbone of the geologic time scale.

Definition

Radiometric dating is the determination of the age of a material by measuring the relative abundances of a radioactive parent isotope and its decay products, using the known, constant half-life of the parent.

Scope

This topic covers the principles and main methods of radiometric dating: radioactive decay and half-life, the requirement of a closed system, and the major decay schemes such as uranium–lead, potassium–argon, and rubidium–strontium, along with radiocarbon for young materials. It is the primary source of absolute ages in geology.

Core questions

• How does radioactive decay provide a clock for dating rocks?
• What conditions must a sample meet for a date to be reliable?
• Which decay schemes are suited to which materials and time spans?

Key theories

Radioactive decay clocks

Radioactive isotopes decay to stable daughters at constant, characteristic rates, so the measured parent-to-daughter ratio in a closed system gives the time since the system formed or last reset.

Age of the Earth from lead isotopes

Patterson used lead-isotope ratios in meteorites and terrestrial samples to determine an age of about 4.55 billion years for the Earth, establishing the planet's antiquity on a firm isotopic basis.

Mechanisms

An unstable parent isotope decays to a stable daughter at a rate described by its half-life. If a mineral incorporates parent atoms when it forms and remains a closed system thereafter, the accumulated daughter atoms record elapsed time. Different parent–daughter pairs have different half-lives, making each suited to a particular age range and material; isochron methods correct for any daughter present at formation.

Clinical relevance

Radiometric ages calibrate the geologic time scale, date geological events and resource-forming processes, establish rates of tectonic and climatic change, and underpin archaeology and forensic and environmental tracing through methods such as radiocarbon dating.

History

Following the discovery of radioactivity, Rutherford and Boltwood proposed using it to date rocks in the early 1900s. Arthur Holmes developed the method and championed the radiometric time scale, and Clair Patterson's 1956 lead-isotope study fixed the age of the Earth, cementing radiometric dating as the basis of geochronology.

Key figures

• Arthur Holmes
• Clair Patterson
• Bertram Boltwood
• Ernest Rutherford

Seminal works

• patterson1956
• holmes1913

Frequently asked questions

Can radiocarbon dating be used on rocks millions of years old?

No. Radiocarbon dating is limited to organic materials younger than about 50,000 years because of carbon-14's short half-life; dating rocks millions of years old requires long-lived isotope systems such as uranium–lead or potassium–argon.