Ar-Ar dating

Ar-Ar dating
Name	Argon–argon dating
Type	Radiometric
Primary isotope	40K → 40Ar
Parent isotope	39K (proxy)
Age range	up to billions of years
Applications	Geochronology, volcanology, tectonics, planetary science

Ar-Ar dating is a radiometric geochronological technique used to determine the crystallization or cooling ages of rocks, minerals, and planetary materials. Developed as a refinement of potassium–argon approaches, the method allows high-precision age determinations for samples from Mount St. Helens, Hawaii, Yellowstone National Park, and extraterrestrial bodies returned by missions such as Apollo 11. Ar-Ar dating underpins chronology in studies of Plate tectonics, Mass extinction, Pleistocene volcanism, and the timing of Cenozoic tectonism.

Introduction

Ar-Ar dating converts a portion of potassium to argon in a measured step to obtain isotopic ratios that are interpreted as ages. Practitioners commonly study minerals like biotite, hornblende, sanidine, plagioclase, and muscovite from sequences exposed in regions such as the San Andreas Fault, Iceland, Andes, and Cascade Range. Results contribute to calibrating the Geologic time scale and correlating events with stratigraphic markers used in studies of the Permian–Triassic extinction event, Cretaceous–Paleogene boundary, and Quaternary glaciation.

Principles and Methodology

The technique relies on the radioactive decay of potassium-40 to argon-40; a separate neutron irradiation converts a stable isotope, potassium-39, to argon-39 to act as a proxy for potassium content. Measured isotopic ratios such as 40Ar/39Ar and 36Ar/39Ar are used to correct for atmospheric argon and interference reactions introduced during irradiation. Precision hinges on understanding decay constants established through community efforts involving organizations like the International Union of Geological Sciences and laboratories affiliated with institutions such as United States Geological Survey, Lamont–Doherty Earth Observatory, Scripps Institution of Oceanography, and university facilities at Cambridge, Oxford, and Stanford University.

Sample Preparation and Analytical Procedures

Field collection often targets volcanic ash layers, lava flows, and metamorphic minerals from localities including Yellowstone Caldera, Mount Etna, and Mount Pinatubo. In the laboratory, samples are crushed, sieved, and mineral separates produced using techniques associated with equipment from Fritsch, Frantz, and density separation similar to methods used in studies at Smithsonian Institution and Natural History Museum, London. Single-grain analyses employ laser step-heating or furnace heating on mass spectrometers such as those developed by VG Instruments and Thermo Fisher Scientific. Analysts compare total fusion, incremental heating, and argon isotope spectra, referencing protocols used in investigations by researchers connected to Massachusetts Institute of Technology and California Institute of Technology.

Calibration, Standards, and Data Reduction

Calibration depends on standards and fluence monitors; commonly used standards include mineral and glass standards established from sites like Fish Canyon Tuff, Taylor Creek Rhyolite, and synthetic standards developed with collaborations from Geological Survey of Japan and Geoscience Australia. Data reduction software used in laboratories at ETH Zurich, Max Planck Society, and University of Tokyo applies corrections for neutron-produced isotopes, reactor spectra, and mass discrimination. Age calculations adopt decay constants constrained through intercomparisons involving Argonne National Laboratory, Oak Ridge National Laboratory, and international interlaboratory calibration exercises linked to projects by International Atomic Energy Agency.

Applications and Case Studies

Ar-Ar dating has dated key volcanic and tectonic events: establishing eruption chronologies for Mount Vesuvius, Krakatoa, and Eyjafjallajökull; constraining uplift histories in the Himalaya and Rocky Mountains; and refining ages of hominin-bearing deposits in regions like Olduvai Gorge and Zhoukoudian. Planetary applications include dating lunar samples from Sea of Tranquility and Martian meteorites associated with campaigns by NASA and European Space Agency. Geochemical studies combining Ar-Ar ages with isotopic systems from U–Pb dating, Rb–Sr dating, and Sm–Nd dating enable integrated reconstructions used in publications from institutions such as American Geophysical Union and Geological Society of America.

Limitations, Sources of Error, and Assumptions

Results can be biased by excess argon, argon loss, recoil during irradiation, and alteration documented in deposits like hydrothermally altered tuffs near Yellowstone. Assumptions include closed-system behavior since cooling and well-constrained neutron flux during irradiation at reactors such as those at McClellan Nuclear Research Center and Brookhaven National Laboratory. Minor isotope interferences require empirical corrections informed by studies at Los Alamos National Laboratory and analytical advances from groups at University of California, Berkeley.

History and Development of the Technique

The method evolved from foundational work on potassium–argon dating by investigators at Carnegie Institution for Science and early 20th-century geochronologists associated with Harvard University and Princeton University. The Ar-Ar variant was developed and refined through contributions from laboratories at California Institute of Technology and University of Cambridge, with significant methodological advances reported in collaborations involving Smithsonian Institution and USGS researchers. Its adoption expanded following interlaboratory standardization workshops supported by organizations like the Royal Society and the National Academy of Sciences.

Category:Geochronology Category:Radiometric dating Category:Geology