Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
The decay occurs on a logarithmic scale. For example, the half-life of C is 5, years. In the first 5, years, the organism will lose half of its C isotopes. In another 5, years, the organism will lose another half of the remaining C isotopes. This process continues over time, with the organism losing half of the remaining C isotopes each 5, years.
Fossils are collected along with rocks that occur from the same strata. These samples are carefully cataloged and analyzed with a mass spectrometer.
Radiometric Dating and the Geological Time Scale
The mass spectrometer is able to give information about the type and amount of isotopes found in the rock. Scientists find the ratio of parent isotope to daughter isotope. By comparing this ratio to the half-life logarithmic scale of the parent isotope, they are able to find the age of the rock or fossil in question.Radiometric or Absolute Rock Dating
There are several common radioactive isotopes that are used for dating rocks, artifacts and fossils. The most common is U U is found in many igneous rocks, soil and sediment. U decays to Pb with a half-life of million years.
Dating rock samples
Due to its long half-life, U is the best isotope for radioactive dating, particularly of older fossils and rocks. C is another radioactive isotope that decays to C This isotope is found in all living organisms. Once an organism dies, the C begins to decay. The half-life of C, however, is only 5, years. The zircons from the Western Australian Jack Hills returned an age of 4.
These zircons also show another interesting feature; their oxygen isotopic composition has been interpreted to indicate that more than 4.
The importance and accuracy of these interpretations is currently the subject of scientific debate. It may be that the oxygen isotopes and other compositional features the rare-earth elements record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization. Using an ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in Western Australia, and comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to the Earth's mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.
One of the oldest Martian meteorites found on Earth, ALH , discovered in the Allan Hills of Antarctica , has been reported to have crystallized from molten rock 4. The Genesis Rock Lunar sample , obtained from the Moon by astronauts during Apollo 15 mission, has been dated at 4.
Meteorites can be even older with NWA being dated to 4. From Wikipedia, the free encyclopedia. Includes rocks over 4 billion years old from the Hadean Eon.
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Human timeline and Nature timeline. Contributions to Mineralogy and Petrology.
Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4. Graham Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4. Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography, Nature Geoscience, http: Retrieved on June 15, Retrieved July 24, Clues to the age, origin, structure, and impact history of the lunar crust, Meteoritics and Planetary Science, vol 38, pp. Retrieved 1 November