K-Ar method

This problem, in which a radiogenic element is allowed to leak out of its host mineral as it forms, has found important applications in geochronology, particularly for the K-Ar method (Wasserburg, 1954) and the U-Pb method (Tilton, 1960 Wasserburg, 1963) with the so-called continuous loss model. The equation for radial diffusion of a radiogenic element in a sphere with radius a and uniform parent isotope concentration P = P0 at t = 0 can be written... 

In geochronology, the present concentrations of the parent and daughter nuclides are measured. The age will be determined by either the isochron method (Equation 1-49), or by knowing the initial concentration of the daughter (Equation l-48c) or the initial concentration of the parent. Use the °K- °Ar method as an example. " K undergoes branch decay as follows ... 

Equation 5-61 involves only Pb isotopic ratios and is called the Pb-Pb isochron. There are a few advantages of this Pb-Pb isochron compared to the individual U-Pb isochrons. One is that Pb isotopic ratios can be measured more accurately than U/Pb ratios. Hence, the above equation has the advantage of Ar-Ar method compared to the K-Ar method. Secondly, if there was recent (such as yesterday or in the last million years) U loss or gain or Pb loss, the loss or gain would not affect the Pb isotopic composition and hence would not affect Pb-Pb dating (but it would affect U-Pb isochrons). Pb gain would affect the Pb-Pb isochron. 

A popular method used to date rocks is the potassium-argon method. Potassium is abundant in rocks such as feldspars, hornblendes, and micas. The K-Ar method has been used to date the Earth and its geologic formations. It has also been applied to determine magnetic reversals that have taken place throughout the Earth s history. Another method used in geologic dating is the rubidium-strontium, Rb-Sr, method. Some of the oldest rocks on Earth have been dated with this method, providing evidence that the Earth is approximately 5 billion years old. The method has also been used to date moon rocks and meteorites. 

Bordet, P., Krummenacher, D., R. Mouterde u. M. Remy Altersbestim-mung nach der K/Ar-Methode in den zu Tage liegenden Gesteinen des Tales des Kali Gandaki (Zentral-Nepal). C. R. hebd. Stances Acad. Sci. 260, 6409 (1965). 

H. Baadsgard, and G. L. Cumming Dating of volcanic ash beds (bentonites) by the K—Ar-Method. Nuclear Geophysics, Nuclear Science Series Rept. 38, Nat. Acad. Sci. Nat. Res. Council, Publ. 1075, 70 (1962). 

Pollack, B. Altersbestimmungen einiger Lepidolithe nach der K/Ar-Methode. Diplomarbeit, Bamberg, 1957. 

Die mit der K-Ar-Methode erkannten vier groBen Tektitgruppen jeweils gleichen Alters sprechen fur die Bildung der Tektite durch wenige groBe Naturereignisse. 

In practice, two approaches are used - independent determination of N2 and N or simultaneous determination of N2 and N by mass spectrometry (MS). The second approach is not applicable if the properties of the mother nuclide and the daughter nuclide are very different, e.g. in the case of dating by the " °K/" °Ar method or by measuring " He formed by radioactive decay. Both methods require additional determination of the unknown number but in special cases A can be neglected. 

Ar and a measured quantity of Ar (>99% enriched) is added (method of isotope dilution). After purification, the isotope ratios " Ar Ar and Ar Ar are measured by means of MS. From the isotope ratios " Ar Ar and Ari Ar in the added sample and in the air, the number of radiogenic Ar atoms is calculated. The concentration of K in the sample is determined independently by the usual analytical methods. The main problem with the " K/" Ar method is the possibility that some " °Ar may have escaped or that additional amounts of Ar may have entered the system in the course of time. 

The Ar/" °Ar method is a variant on the " °K/" °Ar method. For deteimination of the amount of K present in the sample, neutron activation is applied. The sample and a standard of known age (i.e. containing a known ratio °Ar °K) are irradiated under the same conditions at a neutron flux density of about 10 " cm s for about 1 day. Ar is produced by the nuclear reaction K(n,p) Ar and the ratio 39Ar 40Ar is measured by MS. Because the half-life of Ar is rather long (h /2 = 269 y), its decay after the end of irradiation can be neglected. From the relation... 

An alkaline dolerite has intruded the lower portion of the Rundle Formation at different stratigraphic levels in the Stuart Deposit, thermally metamorphosing the invaded sequence for an average of 46m over an area of 2.25 sq. km. The age of the dolerite is set at 26.8m.y. using the K/Ar method (4). 

Scientists have been able to trace the origin of the human species back nearly 2 million years by another method—K/Ar dating. Unlike carbon dating, which relies on measuring how much carbon 14 has decayed, the K/Ar method counts the number of argon 40 atoms produced in the following reaction, which occurs in 11 percent of 40K decays ... 

Site 41-368 is located on the Cape Verde Rise (17°30.4 N, 21°21.2 W water depth 3367 m) (Lancelot et al, 1975). The Turonian-Albian black shales were horizontally penetrated by two thin ( 20 cm) and one thick (15 m) diabase sills (Natland, 1977). The age of the major intrusive unit was determined by the K/Ar method to be approximately 19 Ma (early Miocene) (Duncan and Jackson, 1977). The Cretaceous section from Site 41-368 consists of black shale with variable organic carbon and carbonate contents (0.7-12% and 1-38%, respectively) due to lithologic differences e.g. Dennis etal, 1982 Simoneit etal, 1981). These values decrease to minima near the intrusions. The thermal stress from the intrusions affected the sediment to about 12 m above and 8 m below the major sill. 

The first age determination of an Antarctic rock was reported by Goldich et al. (1958) who used the K-Ar method (Appendix 3.6.2) to date biotite from a sample of granite gneiss collected by P.N. Webb at Gneiss Point (77°24 S, 163°44 E,) on the coast of southern... 

The K-Ar method of dating arises from Eq. 3.3 which can be used to calculate the length of time in the past during which Ar has accumulated ... 

The first age determinations by the whole-rock K-Ar method of gneisses and granites in the Wilson Hills and on several small off-shore islands by Ravich and Krylov (1964) ranged from 564 to 434 Ma. A sample of biotite from Aviation Island (also called Ostrova Polyamoy) at 69°16 S, 158°47 E, yielded an even... 

The layers of the tuff near the top of the FaUa Formation and a trachyte pebble collected on Mt. FaUa were dated by isotopic methods. The pebble (F218A-19) was analyzed by the whole-rock K-Ar method which yielded a date of 197.7 2.7 Ma (Barrett and Elliot 1972). A ch ical analysis of this pebble is contained in Table 10.10 and Fig. 10.15. Barrett et al. (1986) recalculated the K-Ar date of this pebble to 203 3 Ma by using the constants of Steiger and lager (1977). Barrett and Elliot (1972) also reported a whole-rock K-Ar date of 183.2 10 Ma (recalculated to the constants of Steiger and lager 1977) for a dolerite boulder in a lenticular lahar deposit 120 m below the lowest flow of... 

Adams (1986) dated a large suite of samples of fine-grained pelitic rocks (well cleaved slates) from the Swanson and Mackay mountains and from Mt. Passel in the Ford Ranges of MBL by the K-Ar method. These dates in Fig. 15.13a have a distinctly bimodal distribution. The older suite reflects the low-grade metamorphism and deformation of the Swanson Formation during the Late Ordovician. The younger suite records the time when the rocks were heated by the intrusion of Late Devonian granites. Part B of Fig. 15.13 contains the whole-rock K-Ar dates of slates... 

The ages of the Cenozoic volcanic rocks of the McMurdo Volcanic Group were determined by Armstrong (1978), Fleck et al. (1972), Forbes et al. (1974), Wright-Grassham (1987), and others by the whole-rock K-Ar method. Under ideal circumstances, these dates indicate the crystallization ages of late Cenozoic volcanic rocks that cooled rapidly after they were erupted on land, provided they were not hydro-thermally altered and provided that all " Ar was out-gassed from the lava at the time eruption. 

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