Current Version:  1.0 
Last updated:     26 Dec 1994 

A Tutorial on the ⁴⁰Ar/³⁹Ar Step-Heating
Dating Technique

Contents

• Prolog
• Introduction
• Historical Overview of the K-Ar and ⁴⁰Ar/³⁹Ar Methods
• References

Prolog

Several of my colleagues have asked me numerous questions about the actual ⁴⁰Ar/³⁹Ar step-heating dating technique and about the interpretation of the data generated from a step-heating experiment. So with their encouragement I will attempt to explain the method and the criteria used to interpret the data generated from a ⁴⁰Ar/³⁹Ar step-heating experiment.

I have just completed the data reduction on a low potassium basalt from the Medicine Lake, California, the basalt of Tionesta. The results from this sample are an excellent example of the advantages of ⁴⁰Ar/³⁹Ar over convential K-Ar dating.

Introduction

Historical Overview of the K-Ar and ⁴⁰Ar/³⁹Ar Methods

The study of geology grew out of field studies associated with mining and engineering during the sixteenth to nineteenth centuries. In these early studies the vertical succession of sedimentary rocks and structures were used to date geologic units and events relatively. In addition, faunal succession and the use of "key" diagnostic fossils were used to correlate lithologic units over wide geographic areas. Although lithologic units could be placed within a known sequence of geologic periods of roughly similar age, absolute ages, expressed in units of years, could not be assigned. Until the twentieth century geologists were limited to these relative dating methods. For a complete discussion on the development of the Geologic time scale see Berry, (1968).

Following the discovery of radioactivity by Becquerel (1896a,b,c) near the end of the nineteenth century, the possibility of using this phenomenon as a means for determining the age of uranium-bearing minerals was demonstrated by Rutherford (1906). In his study Rutherford (1906) measured the U and He (He is an intermediate decay product of U) contents of uranium-bearing minerals to calculate an age. One year later Boltwood (1907) developed the chemical U-Pb method. These first "geochronology studies" yielded the first absolute ages from geologic material and indicated that parts of the Earth's crust were hundreds of millions of years old.

During this same period of time Thomson (1905) and Campbell and Wood (1906) demonstrated that potassium was radioactive and emitted beta-particles. The first isotopes of potassium (³⁹K and ⁴¹K) were reported by Aston (1921). Kohlhorster (1930) reported that potassium also emitted gamma radiation. Following theoretical arguments by Klemperer (1935) and Newman and Walke (1935) on the existence of ⁴⁰K which radioactively decayed to ⁴⁰Ca by beta-emission, Nier (1935) discovered ⁴⁰K and reported a value of 8600 for the ³⁹K/⁴⁰K ratio. Newman and Walke (1935) also suggested the possibility that ⁴⁰K could decay to ⁴⁰Ar, however, it was Von Weizsacker's (1937) arguments, based on the abundance of argon in the Earth's atmosphere relative to the other noble gases (He, Ne, Kr, and Xe), that ⁴⁰K also decayed to ⁴⁰Ar by electron capture. As a test, Von Weizsacker (1937) suggested looking for excess ⁴⁰Ar in older K-bearing rocks. By combining Von Weizsacker (1937) argon abundance arguments, with Kohlhorster (1930) observation that potassium emitted gama-radiation, Bramley (1937) presented strong evidence that potassium underwent dual decay. Thompson and Rowlands (1943), using a cloud chamber, confirmed that ⁴⁰Ar was the decay product of ⁴⁰K undergoing electron capture. The absolute confirmation that ⁴⁰Ar was the decay product of ⁴⁰K came when Aldrich and Nier (1948) measured significantly increased ⁴⁰Ar/³⁶Ar ratios on argon extracted from potassium-rich minerals relative to the atmospheric ⁴⁰Ar/³⁶Ar ratio. This set the stage for the rapid development of the K-Ar dating method. For a more extensive discussion on the history and development of K-Ar dating see Dalrymple and Lanphere (1969) and Glen (1980).

The ⁴⁰Ar/³⁹Ar variation of K-Ar dating grew out of iodine-xenon dating studies of meteorites by Jeffery and Reynolds (1961); also see Reynolds, 1963). In these studies the isotopic ratios of all the noble gases (He, Ne, Ar, Kr, and Xe) of neutron-irradiated meteorites were measured. This led to the discovery of ³⁹Ar, which is derived from ³⁹K (Merrihue, 1965). The first ⁴⁰Ar/³⁹Ar dating results were presented in a landmark paper by Merrihue and Turner (1966). Further development of the ⁴⁰Ar/³⁹Ar method by Mitchell, (1968), Brereton, (1970), and Turner, (1971) evaluated the interfering argon isotopes derived from potassium and calcium (ie ³⁶ArCa, and ³⁹ArCa, and ⁴⁰ArK) and determination of the respective correction factors [ie (³⁶Ar/³⁷Ar)Ca, (³⁹Ar/³⁷Ar)Ca, and (⁴⁰Ar/³⁹Ar)K]. The first applications of the ⁴⁰Ar/³⁹Ar dating method of terrestrial rocks compared total fusion ⁴⁰Ar/³⁹Ar ages with conventional K-Ar ages (Mitchell, 1968; Dunham et al., 1968; York and Berger, 1970; Dalrymple and Lanphere, 1971).

The ⁴⁰Ar/³⁹Ar dating method offers a significant advantage over the conventional K-Ar dating technique, because potassium and argon are measured on the same sample by using isotopic ratios of argon, thus eliminating problems associated with sample inhomogeneity. This makes the method ideal for dating small samples such as single mineral grains. In addition, the method does not require the determination of absolute concentrations of potassium and argon to calculate an apparent age. The most significant advantage of the ⁴⁰Ar/³⁹Ar dating method over the conventional K-Ar method, however, is the ability to step-heat samples to higher and higher temperatures until the sample is fused, and calculate and ages for each step. The ⁴⁰Ar/³⁹Ar step-heating method provides information on the internal systematics of potassium and argon (ie the internal distribution of potassium relative to argon). The first ⁴⁰Ar/³⁹Ar step-heating studies of terrestrial samples were by Fitch et al., (1969), Miller et al., (1970), York et al., (1971), Lanphere and Dalrymple (1971), and Brereton (1972). See McDougall and Harrison, (1988) for a more extensive discussion on the history and development of ⁴⁰Ar/³⁹Ar dating.

References

Aldrich and Neir

Aston, F.W., (1921). The mass spectra of the alkali metals. Phil. Mag. Ser. 6 42, 436-441.

Becquerel, H. (1896a). Sur les radiations Žmises par phosphorescence. Compt. Rend. Acad. Sci. Paris 122, 420-421.

_______. (1986b). Sur les radiations invisibles Žmises par les corps phosphorescents. Compt. Rend. Acad. Sci. Paris 122, 501-503.

_______. (1986c). Sur les radiations invisibles Žmises par les sels d'uranium. Compt. Rend. Acad. Sci. Paris 122, 689-694.

Berry, William, B.N., 1968, Growth of a Prehistoric Time Scale: W.H. Freeman and Company, San Francisco and London, 136 p.

Boltwood, B.B. (1907). On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium. Am. J. Sci. 173, 77-88.

Bramley, A. (1937). The potassium-argon transformation. Science 86, 424-425. Brereton, N.R. (1970). Corrections for interfering isotopes in the ⁴⁰Ar/³⁹Ar dating method. Earth Planet. Sci. Lett. 8, 427-433.

_______. (1972). A reappraisal of the ⁴⁰Ar/³⁹Ar stepwise degassing technique. Geophys. J.R. Astron. Soc. 27, 449-478.

Campbell, N.R., and Wood, A. (1906). The radioactivity of the alkali metals. Proc. Camb. Phil. Soc. 14, 15-21.

Dalrymple G.B., Lanphere, M.A., 1969, Potassium-Argon Dating: W.H. Freeman San Francisco, 258 p.

Dalrymple, G.B., and, Lanphere, M.A., (1971). ⁴⁰Ar/³⁹Ar technique of K-Ar dating a comparison with the conventional technique. Earth Planet, Sci. Lett. 12, 300-308.

Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M.A., and Kraker, G.P. (1981). Irradiation of samples for ⁴⁰Ar/³⁹Ar dating using the Geological Survey TRIGA reactor. U.S. Geol. Surv., Prof. Paper 1176.

Dunham, K.C., Fitch, F.J., Ineson, P.R., Miller, J.A., and Mitchell, J.G. (1968). The geochronological significance of argon-40/argon-39 age determinations on White Whin from the northern Pennine orefield. R. Soc. Lond. Proc. A307, 251-266.

Fitch, F.J., Miller, J.A., and Mitchell, J.G. (1969). A new approach to radio-isotopic dating in orogenic belts. In Time and place in orogeny (eds. P.E. Kent, G.E. Satterthwaite, and A.M. Spencer), pp. 157-195. Geol. Soc. London Spec. Publ. 3.

Glen, W., 1982, The Road to Jaramillo, Stanford University Press, Stanford, 459 p.

Jeffery, P.M., and Reynolds, J.H. (1961). Origin of excess Xe129 in stone meteorites. J. Geophys. Res. 66, 3582-3583.

Klemperer, O. (1935). On the radioactivity of potassium and rubidium. R. Soc. London Proc. A148, 638-648.

Kolhorster, W. von (1930). Gammastrahlen an Kaliumsalzen. Zeitschr. Geophy. 6, 341-357. Lanphere, M.A., and Dalrymple, G.B. (1971). A test of the Dalrymple, G.B. (1971). A test of the ⁴⁰Ar/³⁹Ar age spectrum technique on some terrestrial materials. Earth Planet. Sci. Lett 12, 359-372.

Merrihue, C. (1965). Trace-element determinations and potassium-argon dating by mass spectroscopy of neutron-irradiated samples. Trans. Am. Geophys. Un. 46, 125 (Abstract).

Merrihue, C. and Turner, G. (1966). Potassium-argon dating by activation with fast neutrons. J. Geophys. Res. 71, 2852-2857.

Miller, J.A., Mitchell, J.G., and Evans, A.L. (1970). The argon-40/argon-39 dating method applied to basic rocks. In Paleogeophysics (ed. S.K. Runcorn), pp. 481-489. Academic Press, London.

Mitchell, J.G. (1968a). The argon-40/argon-39 method for potassium-argon age determination. Geochim. Cosmochim. Acta 32, 781-790.

Newman, F.H., and Walke, H.J. (1935). The radioactivity of potassium and rubidium. Phil. Mag. Ser. 7 19, 767-773.

Nier, A.O. (1935). Evidence for the existence of an isotope of potassium of mass 40. Phys. rev. 48, 283-284.

________. (1947). A mass spectrometer for isotope and gas analysis. Rev, Sci. Inst. 18, 398-411.

________. (1963). Xenology. J. Geophys. Res. 68, 2939-2956.

Reynolds, 1963

Rutherford, E. (1906). Radioactive transformations. Scribners, New York.

Taylor, J.R., 1982, An introduction of error analysis, the study of uncertainties in physical measurements: New York, Oxford University Press, 270 p.

Thompson, F.C., and Rowlands, S. (1943). Dual Decay of potassium. Nature (London) 152, 103.

Thomson, J.J. (1905). On the emission of negative corpuscles by the alkali metals. Phil. Mag. Ser. 6 10, 584-590.

Turner, G. (1971a). Argon 40-argon 39 dating: The optimization of irradiation parameters. Earth Planet. Sci. Lett. 10, 227-234.

Turrin, B.D., Champion, D.E., and Fleck, R.J., 1991, ⁴⁰Ar/³⁹Ar Laser-Fusion Ages from the Lathrop Wells Volcanic Center: Implications for Volcanic Hazards in the Yucca Mountain Radioactive Repository Site, Southwestern Nevada: Science, v.253, p. 654-657

Turrin, B.D., Champion, D.E., and Fleck, R.J., 1992, Measuring the Age of the Lathrop Wells Volcanic Center at Yucca Mountain": Science, v. 257, p. 555-558

Von Weizs...cker, C.F. (1937). †ber die Mšglochkeit eines dualen §- Zerfalls von Kalium. Phys. Zeitschr. 38, 623-624.

Wells, S.G., McFadden, L.D., Renault, C.E., and Crowe, B.M., 1990, Geomorphic assessment of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: Implications for the proposed high-level radioactive waste repository: Geology, v. 18, p. 549-553.

Wells, S.G., Crowe, B.M., and McFadden, L.D., 1992, Measuring the Age of the Lathrop Wells Volcanic Center at Yucca Mountain": Science, v. 257, p. 555-558.

York, D., and Berger, G.W. (1970). ⁴⁰Ar/³⁹Ar age determinations on nepheline and basic whole rocks. Earth Planet. Sci. Lett. 7, 333-336.

York, D., Yanase, Y., and Berger, G.W. (1971). Determination of geological time with a nuclear reactor and a mass spectrometer. In Activation analysis in geochemistry and cosmochemistry (ed. A.O. Brunfelt and E. Steinnes), pp. 419-422. Universitetsforlaget, Oslo.