http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== K–Ar dating From Wikipedia, the free encyclopedia Jump to: navigation , search *Potassium–argon dating* or *K–Ar dating* is a radiometric dating method used in geochronology and archeology . It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas , clay minerals , tephra , and evaporites . In these materials, the decay product ^40 Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallises ). Time since recrystallization is calculated by measuring the ratio of the amount of ^40 Ar accumulated to the amount of ^40 K remaining. The long half-life of ^40 K allows the method to be used to calculate the absolute age of samples older than a few thousand years.^[1] The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.^[2] Contents [hide ] * 1 Decay series * 2 Formula * 3 Obtaining the data * 4 Preconditions * 5 Applications * 6 Notes * 7 References * 8 Further reading [edit ] Decay series Further information: Isotopes of potassium Potassium naturally occurs in 3 isotopes – ^39 K (93.2581%), ^40 K (0.0117%), ^41 K (6.7302%). The radioactive isotope ^40 K decays with a half-life of 1.248×10^9 yr to ^40 Ca and ^40 Ar . Conversion to stable ^40 Ca occurs via electron emission (beta decay ) in 89.1% of decay events. Conversion to stable ^40 Ar occurs via positron emission (inverse beta decay , electron capture ) in the remaining 10.9% of decay events.^[3] Argon, being a noble gas , is not a major component of most samples of geochronological or archeological interest: it does not bind with other constituents of the material, but normally escapes into the surrounding region. Specifically, its presence in solid rock cannot be explained by other mechanisms. When ^40 K decays to ^40 Ar, the gas may be unable to diffuse out of the host rock. Because argon was able to escape from the rock while it was in a liquid state (molten), this accumulation provides a record of how much of the original ^40 K has decayed, and hence the amount of time that has passed, since the sample solidified. Calcium is common in the crust, with ^40 Ca being the most abundant isotope. Despite ^40 Ca 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. [edit ] Formula 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 following equation: t = \frac{t_\frac{1}{2}}{\ln(2)} \ln(\frac{K_f + \frac{Ar_f}{0.109}}{K_f}) * /t/ is time elapsed * /t_1/2 / is the half life of ^40 K * K_f is the amount of ^40 K remaining in the sample * Ar_f is the amount of ^40 Ar found in the sample. The scale factor 0.109 corrects for the unmeasured fraction of ^40 K which decayed into ^40 Ca; the sum of the measured ^40 K and the scaled amount of ^40 Ar gives the amount of ^40 K which was present at the beginning of the elapsed time period. 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. [edit ] Obtaining the data To obtain the content ratio of isotopes ^40 Ar to ^39 K 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 amount of ^40 K is rarely measured directly. Rather, the more common ^39 K is measured and that quantity is then multiplied by the accepted ratio of ^40 K/^39 K (i.e., 0.0117%/93.2581%, see above). The amount of ^36 Ar may also be required to be measured, see /assumptions / below. [edit ] Preconditions All the following preconditions must be true for computed dates to be accepted as representing the true age of the rock ^[4] Great care is needed in collecting a sample for dating to avoid samples which have been contaminated by absorption of argon from the atmosphere. The above equation may be corrected for the presence of such contaminating non-radiogenic ^40 Ar by subtracting from the measured ^40 Ar value the amount originally present in the air as determined by the ^40 Ar/^36 Ar ratio. Ordinarily, in air samples ^40 Ar is 295.5 times more plentiful than ^36 Ar. The amount of the measured ^40 Ar that resulted from ^40 K decay is then: ^40 Ar_decayed = ^40 Ar_measured − 295.5 × ^36 Ar_measured . Contamination is suspected when the final results are untenable. 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. Ar–Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. Extraneous argon may be incorporated into a rock depending on conditions during cooling. 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 ^40 K in the interval since the rock crystallized or was recrystallized. Examples of incorporation of extraneous ^40 Ar include chilled basalts and inclusions of older xenolithic material – such samples should be avoided. The Ar–Ar dating method was developed to measure the presence of extraneous argon. 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.^[5] A deficiency of ^40 Ar in a sample of a known age can indicate a full or partial melt in the thermal history of the area. Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories.^[6] Accuracy depends on the isotopic ratios included in the sample being normal, since ^40 K is usually not measured directly, but is assumed to be 0.0117% of the total potassium. Unless some other process is active at the time of cooling, this is a very good assumption for terrestrial samples.^[7] Accuracy also requires that the nuclear decay rate be unaffected by external conditions such as temperature and pressure. Because of the energy scales involved, this is a very good assumption, though the ^40 K electron capture partial decay constant may be enhanced at ultrahigh pressure.^[1] [edit ] Applications Due to the long half-life, the technique is most applicable for dating minerals and rocks more than 100,000 years old. For shorter timescales, it is likely that not enough Argon 40 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 .^[2] Although it finds the most utility in geological applications, it plays an important role in archaeology . One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits.^[8] It has also been indispensable in other early east African sites with a history of volcanic activity such as Hadar, Ethiopia .^[8] The K–Ar method continues to have utility in dating clay mineral diagenesis .^[9] Clay minerals are less than 2 microns thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice. [edit ] Notes 1. ^ ^/*a*/ ^/*b*/ McDougal and Harrison 1999 , p. 10 2. ^ ^/*a*/ ^/*b*/ McDougal and Harrison 1999 , p. 9 3. *^ * "ENSDF Decay Data in the MIRD Format for . National Nuclear Data Center . http://www.nndc.bnl.gov/useroutput/40k_mird.html. Retrieved 2009-09-22. 4. *^ * McDougal and Harrison & 1999 "As with all isotopic dating methods, there are a number of assumptions that must be fulfilled for a K–Ar age to relate to events in the geological history of the region being studied. , p. 11 5. *^ * McDougal and Harrison 1999 , p. 9–12 6. *^ * McDougal and Harrison 1999 , p. 11–12 7. *^ * McDougal and Harrison 1999 , p. 14 8. ^ ^/*a*/ ^/*b*/ Tattersall, 1995 9. *^ * Aronson and Lee, (1986)