Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. 40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (39K) into the radioactive 39Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the 40K/40Ar* ratio, and thus to calculate the age of the unknown sample. 40Ar* refers to the radiogenic40Ar, i.e. the 40Ar produced from radioactive decay of 40K. 40Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the 36Ar (which is assumed to be of atmospheric origin) and assuming that 40Ar is found in a constant ratio to 36Ar in atmospheric gases.
The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce 39Ar from 39K via the (n-p) reaction39K(n,p)39Ar. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time. The abundance of 40Ar* increases with the age of the sample, though the rate of increase decays exponentially with the half-life of 40K, which is 1.248 billion years.
Age equationEdit
The age of a sample is given by the age equation:
where λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 39K to 39Ar than a less dense one.
Relative dating onlyEdit
The 40Ar/39Ar method only measures relative dates. In order for an age to be calculated by the 40Ar/39Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by 40Ar/39Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is the conventional K/Ar technique.[1] An alternative method of calibrating the used standard is astronomical tuning (also known as orbital tuning), which arrives at a slightly different age.[2]
ApplicationsEdit
The primary use for 40Ar/39Ar geochronology is dating metamorphic and igneous minerals. 40Ar/39Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault systems is also possible with the 40Ar/39Ar method. Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.
Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. 40Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis.
This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.
RecalibrationEdit
One problem with argon-argon dating has been a slight discrepancy with other methods of dating.[3] Work by Kuiper et al. reports that a correction of 0.65% is needed.[4] Thus the Cretaceous–Paleogene extinction (when the dinosaurs died out)—previously dated at 65.0 or 65.5 million years ago—is more accurately dated to 66.0-66.1 Ma.
^. New Mexico Bureau of Geology and Mineral Resources. Archived from the original on 2017-08-03. Retrieved 2008-09-16.
^Kuiper, K. F.; Hilgen, F. J.; Steenbrink, J.; Wijbrans, J. R. (2004). "40Ar/39Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in the eastern Mediterranean" (PDF). Earth and Planetary Science Letters. 222 (2): 583–597. Bibcode:2004E&PSL.222..583K. doi:10.1016/j.epsl.2004.03.005.
^Renne, P. R. (1998). "Absolute Ages Aren't Exactly". Science. 282 (5395): 1840–1841. doi:10.1126/science.282.5395.1840. S2CID 129857264.
^Kuiper, K. F.; Deino, A.; Hilgen, F. J.; Krijgsman, W.; Renne, P. R.; Wijbrans, J. R. (2008). "Synchronizing Rock Clocks of Earth History". Science. 320 (5875): 500–504. Bibcode:2008Sci...320..500K. doi:10.1126/science.1154339. PMID 18436783. S2CID 11959349.
External linksEdit
The Wikibook Historical Geology has a page on the topic of: Ar-Ar dating
WiscAr Geochronology Laboratory, University of Wisconsin-Madison
UC Berkeley press release: "Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years"
New Mexico Geochronology Research Laboratory
Argon Isotope Facility 2010-05-10 at the Wayback Machine of the Scottish Universities Environmental Research Council
Open University Ar/Ar and Noble Gas Laboratory
Argon Laboratory / Australian National University
October 15, 2023
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Argon argon or 40Ar 39Ar dating is a radiometric dating method invented to supersede potassium argon K Ar dating in accuracy The older method required splitting samples into two for separate potassium and argon measurements while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes 40Ar 39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium 39K into the radioactive 39Ar As long as a standard of known age is co irradiated with unknown samples it is possible to use a single measurement of argon isotopes to calculate the 40K 40Ar ratio and thus to calculate the age of the unknown sample 40Ar refers to the radiogenic 40Ar i e the 40Ar produced from radioactive decay of 40K 40Ar does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the 36Ar which is assumed to be of atmospheric origin and assuming that 40Ar is found in a constant ratio to 36Ar in atmospheric gases Contents 1 Method 2 Age equation 3 Relative dating only 4 Applications 5 Recalibration 6 See also 7 References 8 External linksMethod EditThe sample is generally crushed and single crystals of a mineral or fragments of rock are hand selected for analysis These are then irradiated to produce 39Ar from 39K via the n p reaction 39K n p 39Ar The sample is then degassed in a high vacuum mass spectrometer via a laser or resistance furnace Heating causes the crystal structure of the mineral or minerals to degrade and as the sample melts trapped gases are released The gas may include atmospheric gases such as carbon dioxide water nitrogen and argon and radiogenic gases like argon and helium generated from regular radioactive decay over geologic time The abundance of 40Ar increases with the age of the sample though the rate of increase decays exponentially with the half life of 40K which is 1 248 billion years Age equation EditThe age of a sample is given by the age equation t 1 l ln J R 1 displaystyle t frac 1 lambda ln J times R 1 nbsp where l is the radioactive decay constant of 40K approximately 5 5 x 10 10 year 1 corresponding to a half life of approximately 1 25 billion years J is the J factor parameter associated with the irradiation process and R is the 40Ar 39Ar ratio The J factor relates to the fluence of the neutron bombardment during the irradiation process a denser flow of neutron particles will convert more atoms of 39K to 39Ar than a less dense one Relative dating only EditThe 40Ar 39Ar method only measures relative dates In order for an age to be calculated by the 40Ar 39Ar technique the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard Because this primary standard ultimately cannot be determined by 40Ar 39Ar it must be first determined by another dating method The method most commonly used to date the primary standard is the conventional K Ar technique 1 An alternative method of calibrating the used standard is astronomical tuning also known as orbital tuning which arrives at a slightly different age 2 Applications EditThe primary use for 40Ar 39Ar geochronology is dating metamorphic and igneous minerals 40Ar 39Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature However in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral Dating of movement on fault systems is also possible with the 40Ar 39Ar method Different minerals have different closure temperatures biotite is 300 C muscovite is about 400 C and hornblende has a closure temperature of 550 C Thus a granite containing all three minerals will record three different ages of emplacement as it cools down through these closure temperatures Thus although a crystallization age is not recorded the information is still useful in constructing the thermal history of the rock Dating minerals may provide age information on a rock but assumptions must be made Minerals usually only record the last time they cooled down below the closure temperature and this may not represent all of the events which the rock has undergone and may not match the age of intrusion Thus discretion and interpretation of age dating is essential 40Ar 39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis This technique allows the errors involved in K Ar dating to be checked Argon argon dating has the advantage of not requiring determinations of potassium Modern methods of analysis allow individual regions of crystals to be investigated This method is important as it allows crystals forming and cooling during different events to be identified Recalibration EditOne problem with argon argon dating has been a slight discrepancy with other methods of dating 3 Work by Kuiper et al reports that a correction of 0 65 is needed 4 Thus the Cretaceous Paleogene extinction when the dinosaurs died out previously dated at 65 0 or 65 5 million years ago is more accurately dated to 66 0 66 1 Ma See also EditGrenville Turner inventor of the technique Berkeley Geochronology CenterReferences Edit New Mexico Geochronology Research Laboratory K Ar and 40Ar 39Ar Methods New Mexico Bureau of Geology and Mineral Resources Archived from the original on 2017 08 03 Retrieved 2008 09 16 Kuiper K F Hilgen F J Steenbrink J Wijbrans J R 2004 40Ar 39Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in the eastern Mediterranean PDF Earth and Planetary Science Letters 222 2 583 597 Bibcode 2004E amp PSL 222 583K doi 10 1016 j epsl 2004 03 005 Renne P R 1998 Absolute Ages Aren t Exactly Science 282 5395 1840 1841 doi 10 1126 science 282 5395 1840 S2CID 129857264 Kuiper K F Deino A Hilgen F J Krijgsman W Renne P R Wijbrans J R 2008 Synchronizing Rock Clocks of Earth History Science 320 5875 500 504 Bibcode 2008Sci 320 500K doi 10 1126 science 1154339 PMID 18436783 S2CID 11959349 External links Edit nbsp The Wikibook Historical Geology has a page on the topic of Ar Ar dating WiscAr Geochronology Laboratory University of Wisconsin Madison UC Berkeley press release Precise dating of the destruction of Pompeii proves argon argon method can reliably date rocks as young as 2 000 years New Mexico Geochronology Research Laboratory Argon Isotope Facility Archived 2010 05 10 at the Wayback Machine of the Scottish Universities Environmental Research Council Open University Ar Ar and Noble Gas Laboratory Argon Laboratory Australian National University Retrieved from https en wikipedia org w index php title Argon argon dating amp oldid 1174281803, wikipedia, wiki, book, books, library,
