40Ar/39Ar dating

40Ar/39Ar dating
Name	40Ar/39Ar dating
Type	Radiometric dating
Parent method	K–Ar dating
Developed	1960s–1970s
Key figures	Hugo Benioff; George Tilton; Ralph Stacey

40Ar/39Ar dating is a radiometric geochronology technique used to determine absolute ages of minerals and rocks by measuring isotopic ratios of argon produced from potassium-bearing phases. It is widely employed in geology, volcanology, tectonics, and planetary science for dating volcanic eruptions, metamorphic events, and impact structures. Major scientific milestones in the method involve laboratories and researchers associated with institutions such as the United States Geological Survey, Carnegie Institution for Science, and the United Kingdom Atomic Energy Authority.

Introduction

40Ar/39Ar dating evolved as an improvement on K–Ar dating by permitting incremental heating, internal standardization, and smaller sample sizes. Prominent laboratories at the Lamont–Doherty Earth Observatory, Scripps Institution of Oceanography, and the Smithsonian Institution advanced practical protocols, while collaborations with accelerator facilities like Oak Ridge National Laboratory and the Australian Nuclear Science and Technology Organisation enabled neutron-irradiation production of 39Ar. The technique links to major geological problems investigated by researchers at the Geological Society of America and the American Geophysical Union.

Principles and Methodology

The method relies on the radioactive decay of Potassium-40 to Argon-40; 39Ar is produced from 39K by neutron irradiation, allowing measurement of the 40Ar/39Ar ratio rather than absolute K and Ar concentrations. Key conceptual frameworks were formalized by scientists working at University of California, Berkeley, Massachusetts Institute of Technology, and California Institute of Technology. Experimental designs often reference calibration against interlaboratory standards developed at institutions such as the International Atomic Energy Agency and the British Geological Survey. Instrumentation and vacuum systems are derived from advances at facilities including Lawrence Livermore National Laboratory.

Sample Preparation and Measurement Procedures

Sample preparation typically occurs in laboratories at universities like University of Oxford, University of Cambridge, and University of Arizona, where mineral separation, crushing, and hand-picking isolate phases such as biotite, hornblende, sanidine, and plagioclase. Neutron irradiation in research reactors—examples include the High Flux Isotope Reactor and the OPAL reactor—converts 39K to 39Ar; flux monitors or standards such as the Fish Canyon sanidine (prepared initially at institutions like the Colorado School of Mines) are used to account for neutron fluence. Measurement of Ar isotopes uses noble-gas mass spectrometers produced by companies and groups associated with Thermo Fisher Scientific and research groups at Max Planck Society facilities. Laboratories employ laser step-heating, resistance furnaces, and incremental heating protocols developed at places like the University of Michigan and the Swiss Federal Institute of Technology Zurich.

Data Reduction and Age Calculation

Data reduction follows conventions set out by working groups convened at meetings of the Geological Society of London and the European Geosciences Union, implementing decay constants refined by teams at Harvard University and the National Institute of Standards and Technology. Calculation of apparent ages, isochron analyses, and plateau criteria use software and algorithms originating from collaborations among researchers at University of California, Los Angeles, University of Edinburgh, and Johns Hopkins University. Corrections include recoil effects characterized in studies at Los Alamos National Laboratory and neutron-induced interferences quantified by groups at Argonne National Laboratory.

Applications and Case Studies

40Ar/39Ar dating has been applied to volcanic stratigraphy in regions such as the Cascade Range, the Icelandic Rift, and the Cascade Range’s Mount St. Helens studies coordinated with agencies like the United States Forest Service. It underpins tectonic reconstructions in the Himalayas, Andes, and East African Rift and is central to chronology of hominin sites investigated by teams at the National Museums of Kenya and the Max Planck Institute for Evolutionary Anthropology. Impact-event studies use the method at sites like the Chicxulub crater and the Vredefort Dome, with interdisciplinary projects involving institutions such as the Natural History Museum, London and the Smithsonian National Museum of Natural History.

Advantages, Limitations, and Sources of Error

Advantages include the capacity for single-grain dating used by researchers at California State University, Long Beach and the resolution of complex thermal histories exploited by teams at Columbia University. Limitations and potential errors arise from excess argon issues documented in studies at University of Edinburgh, alteration and diffusion effects investigated by groups at University of California, Santa Cruz, and uncertainties in neutron flux monitors addressed by the International Union of Geological Sciences. Systematic errors include uncertainties in decay constants refined through collaborative projects involving Oak Ridge National Laboratory and statistical treatments developed at Princeton University.

History and Development of the Technique

Foundational work in radiogenic argon and potassium decay originated with investigators linked to Carnegie Institution for Science and Massachusetts Institute of Technology in the mid-20th century, later extended by innovators at Scripps Institution of Oceanography and the United States Geological Survey. The 1960s–1970s saw practical development of the 40Ar/39Ar step-heating and irradiation protocols at laboratories including University of California, Berkeley and Caltech, with subsequent standardization through international bodies such as the International Atomic Energy Agency and the British Geological Survey. Modern refinements continue in academic and national laboratories like Max Planck Society, Lawrence Berkeley National Laboratory, and the Australian National University.

Category:Geochronology