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So this guy just got fired from his job. Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again. After the recrystallization of magma, more 40 K will decay and 40 Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40 Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified. Despite 40 Ca being the favored daughter nuclide, it is rarely useful in dating because calcium is so common in the crust, with 40 Ca being the most abundant isotope.
Thus, the amount of calcium originally present is not known and can vary enough to confound measurements of the small increases produced by radioactive decay. The ratio of the amount of 40 Ar to that of 40 K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the equation. The scale factor 0. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required.
To obtain the content ratio of isotopes 40 Ar to 40 K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum. The potassium is quantified by flame photometry or atomic absorption spectroscopy. The amount of 40 K is rarely measured directly. The amount of 40 Ar is also measured to assess how much of the total argon is atmospheric in origin. Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample.
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Ar—Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. Due to the long half-life , the technique is most applicable for dating minerals and rocks more than , years old.
For shorter timescales, it is unlikely that enough 40 Ar will have had time to accumulate in order to be accurately measurable. K—Ar dating was instrumental in the development of the geomagnetic polarity time scale. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits. In the K—Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has been dated from its mineral ingredients while situated on another planet.
From Wikipedia, the free encyclopedia. National Nuclear Data Center.