Current Version: 1.0 Last updated: 26 Dec 1994
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.
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.
Aldrich and Neir
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