40Ar-39Ar method

Each temperature step of the stepped heating procedure of the 40Ar-39Ar method yields a date. To calculate this date, one must determine how many 39K atoms in the sample are represented by the measured number of 39Ar atoms. To determine this number, a sample of known age is included as a monitor of the neutron flux. From measurements of this monitor, a parameter J can be calculated (see Appendix). The date for each step of the 40Ar-39Ar measurement can then be calculated from... 

Duffell, H. (1999) Contribution gdochronologique h la stratigraphic volcanique du Massif des Monts Dore par la methode 40Ar/39Ar. D.E.A. Univ. Clermont-Ferrand. 

A laser probe study of Apollo 17 basalts [4] illustrates the use of the laser dating to interpret temperature release patterns. The laser 39Ar-40Ar method has been used to study three Apollo 17 basalts 70215, a basalt with a normal well-behaved 39Ar-40Ar release pattern 70017, a basalt with a disturbed 39Ar-40Ar release... 

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... 

Turner G (1969) Thermal histories of meteorites by the 39Ar-40Ar method. In Millman PM (ed) Meteorite Research. D. Reidel Publishing, Dordrecht, The Netherlands, p 407-417 Turner G (1970a) Thermal histories of meteorites. In Runcorn SK (ed) Paleogeophysics. Academic Press, London, p 491-502... 

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