K-Ar dating
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| K-Ar dating | |
|---|---|
| Name | Potassium–Argon dating |
| Invented | 1950s |
| Inventor | Clair Patterson; G. W. Wetherill |
| Discipline | Geochronology |
| Method type | Radiometric dating |
| Isotope parent | Potassium-40 (40K) |
| Isotope daughter | Argon-40 (40Ar) |
| Applications | Volcanology, Plate tectonics, Paleontology, Archaeology |
K-Ar dating is a radiometric technique used to determine the geologic age of minerals and rocks by measuring the decay of radioactive potassium to argon gas. Developed in the mid-20th century, it became a cornerstone for establishing absolute ages for volcanic provinces, calibrating the Geologic time scale, and constraining events linked to Plate tectonics, Mass extinction, and human evolution. Laboratories worldwide apply the method to samples from sites such as Hawaii (island), the Deccan Traps, and the Grand Canyon to tie stratigraphy to absolute time.
Introduction
Potassium–argon dating relies on the radioactive decay of Potassium-40 to Argon-40 and calcium, providing ages for igneous and metamorphic materials. Key historical milestones include work by Clair Patterson on terrestrial lead ages and chronologies involving G. W. Wetherill and researchers at institutions like the United States Geological Survey and the Geological Survey of India. The technique complemented earlier methods such as U-Pb dating and later evolved alongside technologies used at facilities like the Scripps Institution of Oceanography and the Smithsonian Institution.
Principles and Theory
The theoretical basis rests on the known half-life of Potassium-40 and the accumulation of radiogenic Argon-40 in a closed mineral system. Decay constants established through intercomparisons with Samarium-neodymium and Rubidium-strontium systems underpin age calculations. The closure temperature concept, applied in contexts like Metamorphism of the Himalaya or cooling histories of Oceanic crust, determines when daughter isotopes are retained. Isotopic equilibrium, diffusion, and recoil effects are considered in light of experimental findings from laboratories such as Lawrence Berkeley National Laboratory and observational constraints from localities including Mount St. Helens and the Krakatoa eruption.
Methodology and Laboratory Techniques
Sample selection often targets potassium-bearing minerals like feldspar, mica, and whole-rock basalts from volcanic provinces such as Iceland or the Aleutian Islands. Preparation includes crushing, mineral separation using heavy liquids and magnetic separators developed at research centers like Caltech and the University of Cambridge. Argon extraction uses vacuum crushing or step-heating with resistance furnaces or laser systems refined at institutions such as Massachusetts Institute of Technology and ETH Zurich. Measurement employs noble gas mass spectrometers produced by manufacturers linked to analytics in facilities like Lamont–Doherty Earth Observatory and GNS Science. Standards and interlaboratory calibrations draw on reference materials archived by bodies including the International Association of Geochemistry and national metrology institutes.
Applications and Case Studies
K-Ar results have dated volcanic flows in the Deccan Traps and the Columbia River Basalt Group to link volcanism to Mass extinction hypotheses and paleoclimatic shifts. Ages from hominin-bearing sediments near Olduvai Gorge and volcanic ash layers at Laetoli inform timelines for Human evolution. Geochronology of metamorphic belts such as the Canadian Shield and igneous provinces like the Siberian Traps uses K-Ar ages to constrain tectonic events and mineralization tied to economic deposits in regions managed by the British Geological Survey and Geological Survey of Canada. In archaeology, potassium-bearing tephra layers dated in contexts including Pompeii and sites near Jericho provide chronological anchors.
Limitations, Errors, and Calibration
Potential errors arise from argon loss, excess argon, and alteration; classic controversies involved debated ages from Mount Etna and the Himalayan orogeny where metamorphic resetting occurs. Calibration issues tie to decay constant uncertainties and inter-method discrepancies with techniques like U-Pb dating and 40Ar/39Ar dating. Laboratories apply isochron approaches and mineral-specific analyses to mitigate inherited argon or daughter redistribution, guided by protocols from organizations such as the International Union of Geological Sciences. Cross-checks using independent chronometers from sites like Grand Prismatic Spring or chronostratigraphic markers in the Cenozoic help validate results.
Variants and Related Isotopic Methods
A major derivative is the 40Ar/39Ar dating technique, developed to improve precision and assess argon loss through incremental heating; it is widely used at facilities such as the Geological Survey of Japan and the Australian National University. Complementary methods include K-Ca dating (less commonly applied), Rubidium-strontium dating for whole-rock isochrons, and U-Pb dating on zircon for high-precision crystallization ages used in conjunction with K-Ar to resolve complex thermal histories. Integrative approaches link K-Ar or 40Ar/39Ar ages with magnetostratigraphy from studies at the Royal Society-affiliated programs and biostratigraphic correlations involving sites curated by the Natural History Museum, London.