Potassium argon dating definition dictionary, information block about the term
The scale factor 0. Despite 40Ca being the favored daughter nuclide, its usefulness in dating is limited since a great many decay events are required for a small change in relative abundance, and also the amount of calcium originally present may not be known.
Obtaining the data To obtain the content ratio of isotopes 40Ar to 39K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is melted in flame photometry or atomic absorption spectroscopy. The radioactive isotope 40K decays with a half-life of 1.
Definitions of potassium-argon dating words
Conversion to stable 40Ca occurs via electron emission beta decay in The amount of the measured 40Ar that resulted from 40K decay is then: Preconditions All the following preconditions must be true for computed dates to be accepted as representing the true age of the rock  Great care is needed in collecting a sample for dating to avoid samples which have been contaminated by absorption of argon from the atmosphere.
Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories. Calcium is common in the crust, with 40Ca being the most abundant isotope.
Specifically, its presence in solid rock cannot be explained by other mechanisms. The argon-argon dating method was developed to measure the presence of extraneous argon.
Because of the energy scales involved, this is a very good assumption, though the 40K electron capture partial decay constant may be enhanced at ultrahigh pressure.
Definition & Meaning potassium-argon dating
Extraneous argon may be incorporated into a rock depending on conditions during cooling. Examples of incorporation of extraneous 40Ar include chilled basalts and inclusions of older xenolithic material - such samples should be avoided.
Contamination is suspected when the final results are untenable. Formula The ratio of the amount of 40Ar to that of 40K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the following equation: Commonly, gases are not fully removed from magma at the time of crystallization, and so not all of the measured argon will have resulted from in situ decay of 40K in the interval since the rock crystallized or was recrystallized.
Unless some other process is active at the time of cooling, this is a very good assumption for terrestrial samples. The sample must have remained a closed system since it cooled enough to retain argon, neither admitting nor emitting either of the isotopes of interest, for example during hydrothermal alteration.
Isotopes of potassium Potassium naturally occurs in 3 isotopes - 39K When 40K decays to 40Ar, the gas may be unable to diffuse out of the host rock.
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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.
Conversion to stable 40Ar occurs via positron emission beta decay or electron capture in the remaining Argon-argon dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. For shorter timescales, it is likely that not enough Argon 40 will have had time to accumulate in order to be accurately measurable.
Ordinarily, in air samples 40Ar is 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.
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Because argon was able to escape from the rock while it was in a liquid state moltenthis accumulation provides a record of how much of the original 40K has decayed, and hence the amount of time that has passed, since the sample solidified.
One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits. K-Ar dating was instrumental in the development of the geomagnetic polarity time scale.
The amount of 36Ar may also be required to be measured - see Assumptions below. The amount of 40K is rarely measured directly.
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