Argon–argon dating

Argon–argon dating

Argon-argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassium-argon (K-Ar) dating in accuracy. In this technique, the decay of 40K to 40Ar* (* indicates radiogenic) is used to date geological events, particularly the eruption and cooling of igneous rocks and minerals. This technique differs from the K-Ar technique in that prior to measurement in a mass spectrometer, the sample is irradiated with neutrons in a nuclear reactor and some of the 39K (present as a known fraction of the total K in the rock) is converted to 39Ar. The ratio of the radiogenic daughter product, 40Ar*, to 39Ar (as a proxy for 40K) can be measured in the same sample, giving a practical advantage over K-Ar dating, where the 40K and 40Ar must be measured separately, and improving accuracy of the measurement.

Method

The sample is generally crushed and single crystals of a mineral hand-selected for analysis. These are then irradiated to produce 39Ar from 39K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Dating relies on the conversion of K to Ar, and accurate measurement of this conversion.The sample is heated in increments (step heating) which releases argon from different reservoirs within the crystal grain. Each step produces argon with a certain 40Ar:39Ar ratio, and only when 80% or more of these steps are within acceptable error is the crystal's age known. Dating via 40Ar/39Ar geochronology is generally accurate to within 1-2% for properly collected and irradiated and treated samples.

Age equation

The age of a sample is given by the age equation::t=frac{1}{lambda} ln (J imes R+1)where λ is the radioactive decay constant of 40K, J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio.

Relative dating only

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 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 isotopic dating method. The method most commonly used to date the primary standard is the conventional K/Ar technique. [ [http://geoinfo.nmt.edu/labs/argon/methods/home.html New Mexico Bureau of Geology] ]

Applications

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.

References

External links

* UC Berkeley press release: " [http://www.berkeley.edu/news/media/releases/97legacy/pompeii.html Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years] "
* The New Mexico Geochronology Research Laboratory [http://www.ees.nmt.edu/Geol/labs/Argon_Lab/NMGRL.html]
* Berkeley Geochronology Center [http://bgc.org/facilities/argon_lab.html]


Wikimedia Foundation. 2010.

Игры ⚽ Нужно сделать НИР?

Look at other dictionaries:

  • Potassium-argon dating — or K Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measuring the products of the radioactive decay of potassium (K), which is a common element found in materials such as micas, clay minerals, tephra …   Wikipedia

  • potassium-argon dating — /peuh tas ee euhm ahr gon/, Geol. a method for estimating the age of a mineral or rock, based on measurement of the rate of decay of radioactive potassium into argon. [1965 70] * * * Method for determining the age of igneous rocks based on the… …   Universalium

  • potassium-argon dating — noun geological dating that relies on the proportions of radioactive potassium in a rock sample and its decay product, argon • Hypernyms: ↑dating, ↑geological dating * * * /peuh tas ee euhm ahr gon/, Geol. a method for estimating the age of a… …   Useful english dictionary

  • potassium-argon dating — [pō tas΄ē əmär′gän΄, pə tas΄ē əm är′gän΄] n. an indirect method of dating fossils, esp. those in very ancient volcanic rock, by using potassic minerals in the same strata as a reference: a natural radioactive isotope of potassium, K 40, decays… …   English World dictionary

  • potassium-argon dating — /pəˌtæsiəm ˈagɒn deɪtɪŋ/ (say puh.taseeuhm ahgon dayting) noun a method of dating rocks which depends on the ratio of potassium 40 to radiogenic argon 40 which they contain …  

  • potassium-argon dating — noun A method of estimating the age of igneous rocks or of archaeological objects by measuring the amount of argon that has built up through the radioactive decay of potassium …   Wiktionary

  • potassium-argon dating — potas′sium ar′gon dat ing n. gel a method for estimating the age of a mineral or rock, based on measurement of the rate of decay of radioactive potassium into argon • Etymology: 1965–70 …   From formal English to slang

  • Dating methodologies in archaeology — Dating material drawn from the archaeological record can be made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence… …   Wikipedia

  • Dating methodology (archaeology) — Dating material drawn from the archaeological record can made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence… …   Wikipedia

  • potassium - argon dating — …   Useful english dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”