Cosmogenic isotope
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Cosmogenic isotopes (or cosmogenic nuclides) are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ atom. These isotopes are produced within earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic isotopes, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic isotopes.
| element | mass | half-life (years) | typical application |
|---|---|---|---|
| helium | 3 | - stable - | exposure dating of olivine-bearing rocks |
| beryllium | 10 | 1.51 million | exposure dating of quartz-bearing rocks, sediment, dating of ice cores, measurement of erosion rates |
| carbon | 14 | 5,730 | dating of organic matter, water |
| neon | 21 | - stable - | dating of very stable, long-exposed surfaces, including meteorites |
| aluminum | 26 | 720,000 | exposure dating of rocks, sediment |
| chlorine | 36 | 308,000 | exposure dating of rocks, groundwater tracer |
| calcium | 41 | 103,000 | exposure dating of carbonate rocks |
| iodine | 129 | 15.7 million | groundwater tracer |
Contents |
Applications
Because cosmogenic isotopes have long half-lives (anywhere from thousands to millions of years), scientists find them useful for geologic dating. Cosmogenic isotopes are produced at or near the surface of the Earth, and thus are commonly applied to problems of measuring ages and rates of geomorphic and sedimentary events and processes.
Specific applications of cosmogenic isotopes include:
- exposure dating of earth surfaces, including glacially-scoured bedrock, fault scarps, landslide debris
- burial dating of sediment, bedrock, ice
- measurement of steady-state erosion rates
- absolute dating of organic matter (radiocarbon dating)
- absolute dating of water masses, measurement of groundwater transport rates
- absolute dating of meteorites, lunar surfaces
Methods of measurement
To measure cosmogenic isotopes produced within solid earth materials, such as rock, samples are generally first put through a process of mechanical separation. The sample is crushed and desirable material, such as a particular mineral (quartz in the case of Be-10), is separated from non-desirable material by using a density separation in a heavy liquid media such as LST (lithium sodium tungstate).
The sample is then dissolved, a common isotope carrier added (Be-9 carrier in the case of Be-10), and the aqueous solution is purified down to an oxide or other pure solid.
Finally, the ratio of the rare cosmogenic isotope to the common isotope is measured using accelerator mass spectrometry. The original concentration of cosmogenic isotope in the sample is then calculated using the measured isotopic ratio, the mass of the sample, and the mass of carrier added to the sample.
References
- Gosse, John C., and Phillips, Fred M. (2001). "Terrestrial in situ cosmogenic nuclides: Theory and application". Quaternary Science Reviews 20, 1475-1560.
- Granger, Darryl E., Fabel, Derek, and Palmer, Arthur N. (2001). "Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments". Geological Society of America Bulletin 113 (7), 825-836.