17 K-Ar and Ar-Ar Dating Simon Kelley Department of Earth Sciences The Open University Milton Keynes MK7 6AA, United Kingdom [email protected] INTRODUCTION — A BIT OF HISTORY The aim of this chapter is to present the K-Ar and Ar-Ar dating techniques in the context of noble gas studies, since there are already several recent texts on K-Ar and Ar- Ar dating (Dickin 1995; McDougall and Harrison 1999). The focus of this section will be aspects of argon transport and storage in the crust, which affect K-Ar and Ar-Ar dating including Ar-loss from minerals by diffusion and Ar-gain by minerals or “excess argon.” The K-Ar dating technique was one of the earliest isotope dating techniques, developed soon after the discovery of radioactive potassium, and provided an important adjunct to U-Pb and U-He dating techniques. The ease of measurement and ideal half-life (1250 million years; see Table 2 below), for dating geological events has made this the most popular of isotopic dating techniques. Aldrich and Nier (1948) first demonstrated that 40Ar was the product of the decay of 40K, and soon after K-Ar ages were being measured in several laboratories most often using an absolute method such as a McLeod gauge to measure argon concentrations. The first published K-Ar results by such a technique were those of Smits and Gentner (1950) who analyzed sylvite from the Buggingen Oligocene evaporite deposits, obtaining an age of 20 million years. Mass spectrometers, which simultaneously measured very small amounts of gas, and the isotope ratios necessary to make corrections for atmospheric contamination, quickly replaced manometric techniques. Crucially the use of static vacuum techniques, pioneered by John Reynolds at the University of California-Berkeley, meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals. Although the earliest mass spectrometers were built ‘in house’, the introduction of the commercially available MS10 (Farrar et al. 1964), a small 180° metal mass spectrometer built for leak testing, made K-Ar dating generally available. Complete descriptions of early K-Ar development and techniques can be found in Schaeffer and Zähringer (1966) and Dalrymple and Lanphere (1969). Although Thorbjorn Sigurgeirsson proposed the principles of Ar-Ar dating in an unpublished Icelandic laboratory report in 1962, he never succeeded in publishing or testing the idea. The Ar-Ar dating technique as it is practised today originated in the noble gas laboratory of John Reynolds in Berkeley, where Craig Merrihue and Grenville Turner were working on neutron irradiated meteorite samples using the I-Xe dating technique. Merrihue recognised that a 39Ar signal seen in the chart recorder traces was the result of neutron irradiation and published the idea in an abstract (Merrihue 1965). The publication of Merrihue and Turner (1966) saw the birth of the Ar-Ar dating technique. Written by Turner after the untimely death of Merrihue, this paper unusually describes a fully formed isotope dating technique (compare this with the slow emergence of the full K-Ar technique), possibly because the Berkeley Laboratory had been recording the full traces of all noble gases for some time, allowing Merrihue and Turner rapid access to a considerable database of measurements. The advantage of the Ar-Ar technique is that potassium and argon are effectively measured simultaneously on the same aliquot of sample, providing greater internal precision and also the ability to analyse very small and 1529-6466/02/0047-0017$05.00 DOI:10.2138/rmg.2002.47.17 786 Kelley heterogeneous samples. Ar-Ar dating proved to be an ideal technique for dating meteorites because it made the best use of the extremely limited number of samples and also provided thermal histories. Indeed when lunar samples were returned from the Apollo 11 mission, Ar-Ar provided a crucial dating technique. Some samples were dated using K-Ar and yielded ages in the broad range 3 to 4 Ga, testifying to the antiquity of the lunar surface, although this much had been estimated from crater densities. In contrast, the Ar-Ar dating technique provided a wealth of precise ages and thermal histories. Using very small samples, Grenville Turner was able to unravel the crystallization histories, thermal histories during post-eruption heating and the cosmic ray exposure histories in a classic series of papers (Turner 1970b,c; 1971b, 1972). Turner applied quantitative diffusion concepts to stepwise argon release and recovered information from partially outgassed samples, establishing techniques and protocols that are still used to interpret stepwise heating Ar-Ar spectra today. This work and much of the early history of the Ar-Ar dating technique are set out in detail in McDougall and Harrison (1999). Although this chapter describes both K-Ar and Ar-Ar techniques, it should be noted that K-Ar dating is now important in only limited situations including standardization (i.e., first principles dating of standards), dating fine grained clay samples, dating young basalts and obtaining dates in rapid turnaround times. Ar-Ar dating is now used in a very wide range of geological applications, dating samples as old as lunar basalts and primitive meteorites, and volcanic rocks erupted only 2000 years ago. Ar-Ar dating has been applied to many areas of Earth Sciences for dating igneous, metamorphic and sedimentary events. In recent years the introduction of laser techniques for single spot and laser heating analysis has widened the range of applications for Ar-Ar dating and the introduction of more sophisticated models for stepwise heating continue to provide ever more detailed thermal histories from K-feldspars. THE K-AR AND AR-AR DATING METHODS Introduction Both K-Ar and Ar-Ar dating techniques are based upon the decay of a naturally occurring isotope of potassium, 40K to an isotope of argon, 40Ar (Fig. 1). The decay of 40K is by a branching process; 10.48% of 40K decays to 40Ar by β+ decay (Beckinsale and Gale 1969, also proposed gamma-less electron capture decay but this has never been verified), followed by γ decay to the ground state, and by electron capture direct to the ground state, and 89.52% decays to 40Ca by β- to the ground state (Fig. 1). 40K-40Ca dating using the more common branch is also possible (e.g., Marshall and DePaolo 1982), but its application is generally restricted to old potassium-rich rocks since 40Ca is the most abundant naturally occurring isotope (96.94%), making the small amounts of radiogenically produced 40Ca very difficult to measure. Argon, in contrast, is a rare trace element and radiogenically produced 40Ar generally exceeds the levels of trapped 40Ar (although this is not always the case—see later). The naturally occurring isotopes of argon are measured by mass spectrometry for K-Ar dating (36Ar, 38Ar and 40Ar). The 36Ar/38Ar ratio is almost constant (see Table 1), although cosmogenic 38Ar can be detected in some Ca-rich samples (Renne et al. 2001). Absolute argon concentrations, required for the K-Ar technique, are measured as a ratio against a known amount of 38Ar tracer gas. Mass spectrometry for Ar-Ar dating requires only isotope ratios between naturally occurring isotopes and also reactor produced isotopes 39Ar and 37Ar which have half lives of 269 years and 34.95 days respectively. As we shall see later, the irradiation procedure produces not only the radioactive isotopes but also small amounts of stable isotopes of argon, and it is thus important to measure all argon masses precisely by mass spectrometry in order to correct for neutron-induced interferences. K-Ar and Ar-Ar Dating 787 Figure 1. Branching diagram showing the decay scheme for 40K, showing decay to 40Ar and 40Ca (after McDougall and Harrison 1999). Table 1. Naturally occurring iso- topes The essential difference between K-Ar and of argon and potassium. Ar-Ar dating techniques lies in the Isotope Abundance (%) measurement of potassium. In K-Ar dating, 40Ar 99.600 potassium is measured generally using flame photometry, atomic absorption spectroscopy, or 38Ar 0.632 isotope dilution and Ar isotope measurements 36Ar 0.336 are made on a separate aliquot of the mineral or rock sample. In Ar-Ar dating, as the name 39K 93.2581 suggests, potassium is measured by the 40K 0.01167 transmutation of 39K to 39Ar by neutron 41K 6.7302 bombardment and the age calculated on the basis of the ratio of argon isotopes. After Steiger and Jäger (1977). Assumptions The “date” measured by both K-Ar and Ar-Ar techniques reflects the time since radiogenic argon produced by decay of 40K, became trapped in the mineral or rock. This may be the “age” of the rock or the most recent cooling event and in some samples may even reflect an integrated cooling age for a range of sub-grains. However, like all isotope- dating techniques, there is a set of assumptions that must be valid if the number measured is to be interpreted as the age of a geological event: 1. The decay of the parent nuclide, potassium must be independent of its physical state. This is the standard assumption that must be valid for any isotope dating technique. 2. The 40K/K ratio must be a constant at any given time (Table 1). Most recently 788 Kelley (Humayun and Clayton 1995a; Humayun and Clayton 1995b) measured a range of samples and found less than 0.05% variation in the 39K/41K ratio, even in samples where previous studies had measured some variation. 3. All radiogenic 40Ar measured in the sample results from 40K decay. The occasional presence of contaminating 40Ar from various sources can make determining the actual radiogenic content difficult but these are not strictly speaking radiogenic argon (see below). 4. Corrections can be made for any non-radiogenic argon. This is a simple procedure in terrestrial samples where there is generally some contaminating argon from the atmosphere (0.934% argon), but with a constant 40Ar/36Ar ratio of 295.5 (Table 1). Such corrections are less simple in extra-terrestrial samples where the initial 40Ar/36Ar ratios are not constant, and are generally achieved using an isochron plot. Cosmogenic contributions are considered elsewhere in this volume (Wieler 2002; Niedermann 2002). 5. The sample, whether mineral or whole rock, must have remained a closed system since the event being dated. This includes gain or loss of either argon or potassium. This assumption is sometimes invalid, particularly in systems with complex geological and thermal histories. However, Ar-Ar stepwise heating and laser spot techniques can often be used to extract thermal history information from partially opened systems, taking advantage of the manner and extent of argon loss. Table 2. Decay constants for K-Ar and Ar-Ar dating. After Steiger and Jäger (1977). Decay Decay factor Value 40K→40Ca by β- λ - 4.962 × 10-10 a-1 β 40K→40Ar by electron capture and γ λ 0.572 × 10-10 a-1 e 40K→40Ar by electron capture λ' 0.0088 × 10-10 a-1 e combined value λ = λβ- + λec+ λ'ec 5.543 × 10-10 a-1 present day 40K/K 0.0001167 CALCULATING K-AR AND AR-AR AGES The age equation for the K-Ar isotope system is: 1 ⎡ λ 40Ar*⎤ t= ln⎢1 + ⎥ (1) λ ⎣ λ +λ' 40K ⎦ e e where t is the time since closure, λ is the total decay of 40K, and (λ + λ′ ) is the partial e e decay constant for 40Ar (Begemann et al. 2001) (Table 2). 40Ar*/40K is the ratio of radiogenic daughter product (shown conventionally as 40Ar* to distinguish it from atmospheric 40Ar) to the parent 40K. Since there is no common natural fractionation of potassium isotopes (Humayun and Clayton 1995a,b), the modern ratio of 40K/K is a constant (Table 1), and thus measurement of potassium and argon concentrations together with isotope ratios of Ar, enable an age to be calculated. The Ar-Ar technique, first described by Merrihue and Turner (1966), is based on the same decay scheme as K-Ar, but instead of measurement on a separate aliquot, potassium is measured by creating 39Ar from 39K by neutron bombardment in a nuclear reactor. The reaction induced is: 39K(n,p)39Ar (2) 19 18 The ratio of 39K to 40K is effectively constant (see above) and thus the critical 40Ar*/40K K-Ar and Ar-Ar Dating 789 ratio is proportional to the ratio of the two argon isotopes 40Ar/39Ar. Although 39Ar is radioactive, decaying with a half-life of 269 years, this effect is small for the period between irradiation and analysis (generally less than 6 months) and is easily corrected for. Mitchell (1968) showed that the number of 39Ar atoms formed during irradiation can be described by the equation: 39Ar = 39K Δ∫ ϕ(ε)σ(ε)d(ε) (3) where 39K is the number of atoms, Δ is the duration of the irradiation, ϕ(ε) is the neutron flux density at energy ε, and σ(ε) is the neutron capture cross section of 39K for neutrons of energy ε for the neutron in/proton out reaction shown in Equation (2). Rearranging Equation (1) in terms of 40Ar* yields: λ +λ' [ ] 40Ar* =40K e e (eλt)−1 (4) λ Combining Equations (3) and (4) for a sample of age t yields: 40Ar* 40K λ +λ' 1 [(eλt)−1] = e e (5) 39Ar 39K λ ΔT ∫ ϕ(ε)σ(ε)d(ε) This can be simplified by defining a dimensionless irradiation-related parameter, J, as follows: 39K λ J= ΔT∫ ϕ(ε)σ(ε)d(ε) (6) 40K λ +λ' e e The J value is determined by using mineral standards of known age to monitor the neutron flux. Substituting Equation (6) into Equation (5) and rearranging, yields the Ar-Ar age equation: 1 ⎡ 40Ar*⎤ t = ln 1+J (7) ⎢ ⎥ λ ⎣ 39Ar ⎦ The ratio of the two isotopes of argon, naturally produced radiogenic 40Ar and reactor-produced 39Ar is thus proportional to the age of the sample. For terrestrial samples, the 40Ar peak measured in the mass spectrometer most often has two components (neglecting the 40Ar interference reaction), radiogenic and atmospheric. The K 40Ar/36Ar ratio of the atmosphere was determined by IUGS convention as 295.5 (Table 1; Steiger and Jäger 1977), though Nier determined a value of 296 (Nier 1950). When the 40Ar/36Ar ratio of contaminating argon components is >295.5, the extra argon is known as extraneous argon. The term extraneous argon includes both excess and inherited argon following the terminology of Dalrymple and Lanphere (1969) and McDougall and Harrison (1999). Excess argon is the component of argon incorporated into samples by processes other than in situ decay, generally from a fluid or melt at the grain boundary. Inherited argon results from the incorporation of older material in a sample, such as for example grains of sand caught up in an ignimbrite eruption. However, in the simple case, assuming that all the non-radiogenic argon is atmospheric, the daughter/parent ratio (40Ar*/39Ar) can be determined from the equation: 40Ar*/39Ar = [40Ar*/39Ar] – 295.5[36Ar/39Ar] (8) m m where subscript m denotes the measured ratio. This equation is always a simplification; in most terrestrial samples surface contamination ensures that some atmospheric argon is present, though fluids at depth rarely have atmospheric ratios (see below). However, in 790 Kelley extraterrestrial samples atmospheric argon is recognised as a modern contamination or the result of weathering. In this light, it might seem strange to assume that all contaminating argon in terrestrial samples has an atmospheric isotope ratio, given that many trap argon at depth, not in equilibrium with the atmosphere. This puzzle will be discussed in some detail in the later section on argon diffusion and solubility. Sample irradiation for Ar-Ar dating induces not only Reaction (2) but also a series of interfering reactions caused by neutron bombardment of potassium, calcium, chlorine and argon. The complete series of interfering reactions is detailed in Table 3, but most have low production rates relative to the reaction in Equation (3) and can be ignored. The most important reactions are those involving calcium and potassium. The corrections are generally small, though they are critical for samples younger than 1 Ma when the inter- fering reactions producing 40Ar from 40K are important, and for samples with Ca/K > 10, when reactions producing 36Ar and 39Ar from isotopes of Ca become important. The magnitude of the interference from these reactions varies with the irradiation time and neutron flux energy spectrum. The range of measured interference factors for many of the world's reactors are listed in McDougall and Harrison (1999). The 42Ca(n,α)39Ar and 40Ca(n,nα)36Ar production ratios do not vary a great deal, because they are caused by fast neutrons and the energy spectrum of fast neutrons in most reactors is fairly similar. The far larger variation in the interference in the 40K(n,p)40Ar reaction is caused by its higher sensitivity to the ratio of fast to thermal neutrons in the reactor. This ratio varies between reactors and also between different irradiation positions within a reactor. In fact samples are often shielded with cadmium foil to reduce the thermal neutron flux and lower the efficiency of the 40K(n,p)40Ar reaction. The precise correction factors can be determined by irradiating pure salts of Ca and K (often CaF , 2 KCl and K SO ). An additional correction must also be made for the decay of 37Ar 2 4 (produced by neutron bombardment of calcium) which has a half-life of 34.95±0.08 days (Renne and Norman 2001). The short half-life of 37Ar means that all Ca-rich samples must be analyzed within about 6 months of irradiation otherwise the precision determin- ing the original 37Ar concentrations may be affected, compromising the corrections to 36Ar and 39Ar for Ca irradiation. Another factor affecting the accuracy of Ar-Ar dating is 39Ar recoil. This effect is crucial when studying very fine scale argon distributions or fine grained minerals such as clays, but 39Ar recoil from mineral surfaces can also affect high precision dating. Turner and Cadogan (1974), calculated the likely distances of 39Ar recoil during irradiation to be a mean of 0.08 μm, a study which was refined by Onstott et al. (1995) and measured directly by Villa (1997). The effects are most obviously detected in measurements of fine grained clays (e.g., Foland et al. 1983), but are commonly cited as a reason for variable ages produced from altered minerals (Lo and Onstott 1989), and basaltic rocks (e.g., Baksi 1994; Feraud and Courtillot 1994). The Ar-Ar technique is able to achieve higher levels of internal precision than K-Ar dating since it does not depend upon separate absolute measurements but instead requires only the ratios of Ar isotopes and can achieve precision of better than 0.25%. However, external precision and accuracy are affected by the uncertainty in the age of mineral standards, as we will see in the following section. In order to achieve optimum precision in the mass spectrometric measurements, the neutron flux (which affects the magnitude of the J value) must be carefully selected. The flux must be sufficient for precise measurement of 39Ar and a 40Ar*/39Ar ratio within the dynamic range of the mass spectrometer (generally less than 100 for good precision). Further, at higher flux levels the interfering reactions on Ca and K also become more important, degrading the precision and accuracy with which the 40Ar*/39Ar ratio may be determined. Therefore, for K-Ar and Ar-Ar Dating 791 Ar Ar The 36 38 n. → → o r. -→β -→β missi single borde Cl 36Cl(n,)Clγ 38Cl(n,)Clγ he resulting e with a 35 37 d b is t n an ow Ar Ar cle 3 ons on Ca, K, Ar and Cl. 39main Ar-producing reaction is sh Ar 3637Ar(n,)Ar γ 4038-38Ar(n,nd)Cl→β→ Ar 3839Ar(n,))Ar γ 4039-39Ar(n,d)Cl→β→ radiation where a is the incident parti- particle and = a positron. β ctie 38 g ir ma e 3. Interfering reawn in bold type. Th K 3937K(n,nd)Ar 3938K(n,d)Ar 138-K(n,)Clα→β→ 3939K(n,p)Ar 4039K(n,d)Ar 4040K(n,p)Ar 4140 K(n,d)Ar ons taking place durin pha particle, = a gamγ TablImportant interfering reactions are sho Ca 4036Ca(n,n)Ar α 4037Ca(n,)Ar α 4238Ca(n,n)Ar α 4 4239Ca(n,)Ar α 4339Ca(n,n)Ar α 4340Ca(n,)Ar α 4440 Ca(n,n)Arα ogy (a,b) used here refers to nuclear reacti neutron, p = proton, d = deutron, =and alα ol = Argon isotope 36Ar 37 Ar 38Ar 39Ar 40Ar he termin rms are n T e t 792 Kelley each sample there is an optimum flux level and given that many samples are irradiated together, each package sent for irradiation is a compromise. Turner (1971a) calculated the fields for optimum J value, and correspondingly integrated neutron flux, which were upgraded by McDougall and Harrison (1999) in the light of higher sensitivity, higher resolution mass spectrometers (Fig. 2). Figure 2. A Figure for optimizing irradiation parameters, taking account of age and Ca/K. The irradiation parameter is plotted against Age (Ma) and zones of optimum irradiation level are highlighted (after McDougall and Harrison 1999; Turner 1971a). The availability of five argon isotopes provided by the Ar-Ar technique facilitates isotope correlation plots, the most common of which is the three isotope plot 36Ar/40Ar vs. 39Ar/40Ar (Fig. 3). Samples containing a mixture of radiogenic and atmospheric Ar plot along a line with negative slope between the 39Ar/40Ar ratio representing the age and the atmospheric 36Ar/40Ar ratio of 0.003384 (= 1/295.5) (Fig. 3a). The correlation plot also allows Ar-Ar ages to be calculated for samples with contamination other than modern air, since the age can equally be determined from lines passing through the 36Ar/40Ar axis at values other than the atmospheric ratio (Fig. 3b). However, a mixture of contaminating phases with more than one isotope composition in a sample results in a scatter of points not defining a line, and no age can be calculated. In many cases atmospheric ‘blank’ argon released from furnaces during heating is the only detected contaminating argon component. In cases where the contaminating argon is not homogeneous, physical techniques such as stepped heating, in vacuo crushing and laser spot dating have been used to separate components (see below). The values of constants and estimation of errors As the internal precision of Ar-Ar ages has improved over the years, the following have been the focus of debate: 1. The commonly accepted values for the K decay constants (Steiger and Jäger 1977) 2. The inter-laboratory and inter-standard calibration of Ar-Ar ages. K-Ar and Ar-Ar Dating 793 Figure 3. (a) An argon isotope correlation diagram, showing a correlation between atmospheric and radiogenic argon components which form an isochron. Any pure 40Ar component would lie at the origin and thus any excess argon component tends to pull the point B towards the origin. (b) Two samples of amphibole analyzed by laser spot technique. The upper line intercepts within errors of atmospheric argon, the lower line yields a very similar age yet intercepts at a much lower 36Ar/40Ar ratio and contains excess argon. K-Ar can be regarded as an absolute dating technique, dependent only on the value of the decay constant, and calibration of 38Ar spike. However, all Ar-Ar ages are derived relative to the age of mineral standards, which are irradiated at the same time as the sample. The external precision of Ar-Ar ages is thus limited by the external precision of the age of the mineral standard as determined by the K-Ar method. The most widely used international standards are the hornblendes Hb3gr and MMHb1; biotites GA1550, GHC- 305 and B4B, muscovite B4M, and sanidines from the Fish Canyon Tuff, Taylor Creek and Alder Creek (widely accepted ages for these standards are found in McDougall and Harrison 1999). Many other pure mineral samples are used as internal standards and several have been proposed as international standards but are not mentioned here since they are not in wide use. The advantage of using mineral standards is that they are freely available but since they are natural, series errors can be introduced if the various standards are not inter-calibrated. This has been an area of particular controversy in recent years, somewhat masking the improvements in internal precision. Fish Canyon Tuff is a prime example of the problems that are faced by those attempting to achieve accurate, and high precision Ar-Ar ages. Fish Canyon sanidine was proposed as an international standard by Cebula et al. (1986), who reported an age of 27.79 Ma but this was relative to another standard, MMHb1 with an age of 518.9 Ma (Alexander et al. 1978). When the age of the MMHb1 was revised to 520.4 Ma (Samson and Alexander 1987), the age of Fish Canyon sanidine became 27.84 Ma, though some workers used a value of 27.55 Ma, based on a different value for the age of MMHb1. In 1994, Renne et al. determined the age of Fish Canyon sanidine to be 28.03 Ma, an age later confirmed by 794 Kelley cross calibration (Renne et al. 1998b) with biotite standard GA1550. However, subsequent to 1994, many workers continued to use the value 27.84 Ma possibly because this yielded ages in agreement with the spline-fitted magneto-stratigraphic timescale, and in particular, the Cretaceous/Tertiary boundary of 65.0 Ma. Using the value recommended by Renne et al. (1998b) yields an age of 65.4 Ma for tektites from the K/T boundary. In addition, Lanphere and Baadsgaard (2001), maintain that a value of 27.51 Ma is the best age for Fish Canyon sanidine, based on Rb/Sr and U/Pb dates on Fish Canyon Tuff. The problem with this approach is that it requires cross calibration between dating methods, something that is even more fraught with problems. The decay constant of 87Rb is no better constrained than 40K (see below) and meteorite cross calibrations indicate ages may be as much as 2% too low (Renne 2000). In addition, the 27.52±0.09 Ma bulk U/Pb age of Lanphere and Baadsgaard (2001) differs strongly from the U/Pb age of 28.476±0.064 Ma obtained by Schmitz and Bowring (2001) on single grain and small multi-grain fractions of zircon, which confirmed an earlier determination of 28.41±0.05 Ma by Oberli et al. (1990). The work to improve the inter-calibration of standards has been accompanied by parallel discussions of the accuracy and precision of accepted decay constants of several important parent isotopes including 40K (e.g., Begemann et al. 2001). It is notable that the decay constant quoted in most of the physical sciences literature is not the same as the one generally accepted for K-Ar and Ar-Ar dating. In 1977, based mostly on work by Beckinsale and Gale (1969) and Garner et al. (1975), Steiger and Jäger (1977) recommended the use of the decay constants in Table 2. However, Endt and Van der Leun (1973) later compiled the same data as Beckinsale and Gale (1969) to produce different results mainly by different selection criteria and statistical techniques. In a recent summary of 40K decay constants, Audi et al. (1997) report a total decay constant of 5.428 × 10-10 a-1, as previously cited in the nuclear physics literature, a branching ratio of 89.28% and 40K/K = 1.17 × 10-4. Min et al. (2000) showed that a better correlation could be achieved between Ar-Ar and U-Pb ages using the decay constants of Endt and Van der Leun (1973) together with modern physical constants resulting in a total decay constant of 5.463±0.054 × 10-10 a-1 which corresponds to a half-life of 1269±13 Ma. Renne (2000) further demonstrated that in one of the oldest rapidly cooled meteorites, called Acapulco, nuclear physics decay constants (Audi et al. 1997) produced Ar-Ar ages within errors of U-Pb ages, in line with the petrologic interpretation of these samples as rapidly cooled, whereas the existing Ar-Ar ages indicated slow cooling. Note that the interpretation of this important result is controversial (Trieloff et al. 2001; Renne 2001) and final resolution may await a more complete characterization of the thermal history of the Acapulco meteorite. (UTh)/He studies of Acapulco phosphates (Min et al. 2000) appear to validate the rapid cooling which supports the Audi et al. (1997) constants, particularly - for the β decay (Min et al. 2001). Reading this section may leave those new to K-Ar and Ar-Ar dating bemused by the current controversy over decay constants and the inter-calibration of standards in such a mature isotope dating technique. It must be emphasised that this controversy has arisen recently as the precision on age determinations has improved, and attempts are made to correlate with U-Pb ages, where decay constants are better constrained. Ages in the literature other than those cited above are still calculated using the decay constants recommended by Steiger and Jäger (1977), and Renne et al. (1998b) represents the most precise inter-calibration of standards combined with a precise K-Ar date on the GA1550 biotite standard using isotope dilution for the K analysis. Analytical errors on the Ar-Ar age have generally been calculated using the simple error propagation of Dalrymple et al. (1981):
Description: