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. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K.
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 k/ar and 40ar/39ar dating methods 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 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 40 K to 40 Ar than a less dense one.
However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. 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 k/ar and 40ar/39ar dating methods age dating is essential. This technique allows the errors involved in K-Ar dating to be checked. Argon—argon dating has the of not requiring determinations of potassium.
Modern methods of analysis allow individual regions of crystals to be investigated. This seneca falls hook up is important as it allows crystals forming and cooling during different events to be identified. One problem with argon-argon dating has been a slight discrepancy with other methods of dating. Thus the Cretaceous—Paleogene extinction when the dinosaurs died out - previously dated at Similarly, the Permian-Triassic extinction is now dated at From Wikipedia, the free encyclopedia.
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