Crustal Imaging and Characterization Team

USGS ⁴⁰Ar/³⁹Ar Geochronology Laboratory

Introduction

⁴⁰Ar/³⁹Ar geochronology is an experimentally robust and versatile method for constraining time and temperature in geologic processes. The ⁴⁰Ar/³⁹Ar isotopic dating technique has evolved over the past ten years into the most commonly used geochronologic method applied to all varieties of geologic problems that require precise and accurate time and temperature control.

It is used to date terrestrial rocks and minerals as well as meteorites and lunar samples ranging in age from approximately 30,000 years to the age of the Solar System (4.56 billion years). The method is derived from the natural occurrence of the radioactive isotope of potassium, ⁴⁰K, which has a dual decay to ⁴⁰Ca and ⁴⁰Ar and a half-life of 1250 million years. Radiogenic ⁴⁰Ar ideally accumulates in a mineral over geologic time. By irradiating a sample of unknown age with a standard of known age and then measuring the abundances of argon isotopes, we can determine an ⁴⁰Ar/³⁹Ar date, that is, the sample's geologic age. As a result of irradiation, ³⁹Ar serves as a proxy for potassium since it is produced from ³⁹K by fast neutron bombardment. After irradiation, argon isotopes are extracted from samples and standards and separately measured on a gas-source mass spectrometer. Apparent ages for the samples are then calculated by comparing the sample's ⁴⁰Ar/³⁹Ar ratio with that of the standard. Isotopic measurements on modern mass spectrometers are highly sensitive and precise. Thus, very small amounts of material, ranging in size from a single mineral grain to a few milligrams are analyzed commonly with small associated analytical errors (less than 0.2% absolute).

All samples and standards are irradiated in the USGS TRIGA reactor located at the Denver Federal Center in Lakewood, Colorado. For information and its capabilities, go to » Central Region Geology GSTR Services.

For a detailed description of the ⁴⁹Ar/³⁹Ar geochronologic methods and some examples of its application, go to http://pubs.usgs.gov/bul/b2194/.

Accessibility FOIA Privacy Policies and Notices

U.S. Department of the Interior | U.S. Geological Survey
URL: http://crustal.usgs.gov/laboratories/geochronology/introduction.htm
Page Contact Information: CICT Webmaster
Page Last Modified: 08-May-2008@12:04