40Ar/39Ar

40Ar/39Ar
Name	40Ar/39Ar dating
Type	Radiometric
Parent isotope	Potassium-40
Daughter isotope	Argon-40
Typical age range	Millions to billions of years
Main applications	Geochronology, volcanology, tectonics, planetary science

40Ar/39Ar The 40Ar/39Ar technique is a radiometric geochronology method used to date geological and extraterrestrial materials. It refines and extends concepts pioneered in Willard Libby's radiocarbon work, linking developments by Clinton Davisson, Harold Urey, Kenneth T. Kunkel, and institutions such as the United States Geological Survey, Smithsonian Institution, and California Institute of Technology. Practitioners often collaborate across facilities like the Argonne National Laboratory, Oak Ridge National Laboratory, Woods Hole Oceanographic Institution, and university laboratories during sample preparation, irradiation, and mass spectrometry.

Introduction

The method measures isotopic ratios produced after neutron irradiation in research reactors such as the High Flux Isotope Reactor and the European Commission's Joint Research Centre reactors. It builds on the potassium–argon framework advanced by scientists at the Geological Society of America and the Royal Society, adapting protocols used in studies by teams associated with the Smithsonian Astrophysical Observatory and the Lunar and Planetary Institute. Applications span investigations involving contexts from the Deccan Traps and Yellowstone Caldera to lunar samples retrieved during the Apollo program.

Principles and Methodology

40Ar/39Ar relies on conversion of a portion of stable and radioactive isotopes through neutron irradiation following principles first quantified in experiments at facilities like the Los Alamos National Laboratory and the National Institute of Standards and Technology. The method uses production of 39Ar from 39K to create an internal proxy for potassium abundance, enabling age calculations rooted in decay constants determined through collaborations among researchers affiliated with Lawrence Livermore National Laboratory, Max Planck Society, and the British Geological Survey. Analytical workflows reference standards established by organizations such as the International Union of Geological Sciences and protocols influenced by studies at the University of California, Berkeley and University of Cambridge.

Sample Preparation and Irradiation

Sample selection and mineral separation commonly follow procedures developed in laboratories at institutions like the University of Oxford, Australian National University, and the University of Tokyo. Techniques include crushing, sieving, magnetic separation, and heavy liquid isolation using equipment from manufacturers associated with the Czech Academy of Sciences and the Fraunhofer Society. Irradiation typically occurs in research reactors including the Oak Ridge Research Reactor or European counterparts under flux monitors traceable to the International Atomic Energy Agency and standards held by the United States National Institute of Standards and Technology.

Analytical Techniques and Instrumentation

Isotopic measurements are made on noble gas mass spectrometers developed by companies and research groups linked to the Johnson Matthey, Thermo Fisher Scientific, and instrument labs at the Institut de Physique du Globe de Paris. Mass spectrometers such as sector field and multi-collector instruments deployed at the Smithsonian Institution and Yale University provide high-precision determinations of 40Ar and 39Ar. Analytical protocols incorporate blank corrections, intercalibration, and data reduction approaches refined by researchers associated with Massachusetts Institute of Technology, Pennsylvania State University, and the University of Chicago.

Calibration and Standards

Calibration of the neutron flux and J-factors requires standards like biotite, sanidine, and hornblende whose ages are constrained by interlaboratory efforts involving the Geological Survey of Canada, the United States Geological Survey, and the British Geological Survey. Cross-calibration with other geochronometers—such as uranium–lead work from teams at the Australian National University and argon calibration intercomparisons organized by the International Commission on Stratigraphy—ensures consistency across datasets used in studies of regions like the Himalayas, Andes, and East African Rift.

Applications

40Ar/39Ar has been applied to problems in volcanic stratigraphy and tectonic reconstructions investigated by research groups at the Smithsonian Institution, University of California, Los Angeles, University of Arizona, and the University of Leeds. It underpins timing constraints for mass extinction studies linked to the Cretaceous–Paleogene extinction event and the Permian–Triassic extinction event, and it dates tephra layers used in archaeological contexts studied by teams from the British Museum and the Max Planck Institute for Evolutionary Anthropology. Planetary science applications include age determinations of lunar samples from the Apollo program and meteorites curated by the Natural History Museum, London and the Field Museum.

Limitations and Sources of Error

Limitations arise from neutron flux heterogeneity in reactors such as the High Flux Isotope Reactor and from argon loss or excess argon in minerals studied in settings like the San Andreas Fault and the East African Rift System. Common sources of error include recoil effects, mass discrimination in instruments produced by manufacturers linked to Thermo Fisher Scientific and Pfeiffer Vacuum, and uncertainties in decay constants debated among groups at the Max Planck Institute for Chemistry and the University of California, Berkeley. Addressing these issues involves interlaboratory comparisons coordinated by entities such as the International Union of Geological Sciences and quality assurance practices developed at the United States Geological Survey.

Category:Geochronology