8.2 The Re–Os and Pt–Os decay schemes

8.2.1 The Re decay constant

¹⁸⁷Re decays to ¹⁸⁷Os by $ decay, but the decay energy of 2.65 keV is extremely low, even compared with that of ⁸⁷Rb (275 keV). This makes measurement of the decay constant by direct counting very difficult. Accurate counting of solid samples is almost impossible, due to absorption of $ particles by surrounding Re atoms. An alternative technique is to use either a gaseous Re compound to replace the gas filling of a proportional counter, or a liquid Re compound in a scintillation detector. In both cases it is difficult to find compounds with suitable properties, but Brodzinski and Conway (1965) obtained a ¹⁸⁷Re half-life of 66 " 13 Byr by the former method, while Naldrett (1984) obtained a value of 35 " 4 Byr by the latter.

The difficulty with counting determinations has encouraged alternative measurements of the ¹⁸⁷Re half-life based on growth of the ¹⁸⁷Os daughter product, either in the laboratory or in geological samples. One of the most successful was by Lindner et al. (1986), who used the ‘laboratory shelf’ technique to make an independent half-life determination. A 1 kg sample of purified perrhenic acid (HReO₄) was spiked with two different non-radiogenic Os isotopes (190 and 192), set aside for two years to allow radiogenic Os growth, and then sampled for Os isotope composition over a further two-year interval. Os isotope measurements by LAMMA and ICP-MS were of comparable precision and in good agreement, although the two spikes gave results differing by 2%. Results for ¹⁸⁷Os/¹⁹⁰Os by ICP-MS are shown in Fig. 8.2.

Fig. 8.2. Least squares growth line of ¹⁸⁷Os/¹⁹⁰Os as a function of time ‘on the shelf’ for a Re stock solution. Note non-zero initial ratio. After Lindner et al. (1986).

Unfortunately, the starting material used by Lindner et al. had a non-zero level of initial radiogenic Os, as indicated by the positive intercept in Fig. 8.2. Hence the first two years of storage were effectively wasted. Despite this setback, a precise half-life of 43.5 " 1.3 Byr was determined. This was refined by Lindner et al. (1989), using five times as much data as the earlier work, to a half-life of 42.3 " 1.3 Byr, equivalent to a decay constant of 1.64 H 10^!11 yr^!1.

Geological measurements of the half-life are an attractive alternative to laboratory experiments, but these have also encountered technical difficulties. In view of the low concentrations of Re in normal rocks, early attempts at age determination (e.g. Hirt et al., 1963) were made on molybdenite (MoS₂), which strongly concentrates Re at contents of ca. 10 ) 50 ppm. Since molybdenites effectively incorporate no initial osmium, only the total abundance of the daughter need be measured, but this still involves mass spectrometry, since isotope dilution is the only method with enough sensitivity. Unfortunately, the data scattered significantly, yielding an imprecise half-life of 43 " 5 Byr.

Luck and Allegre (1982) made further studies of the potential for Re)Os dating with molybdenite. They selected samples of known age over a wide range of geological time and analysed Re and Os concentrations by isotope dilution. Since insignificant amounts of common osmium were found, no isotope ratio determination was necessary. High-precision results were obtained, sometimes in good agreement with published ages, but often giving older ages. This suggested that Re)Os dating of molybdenite is an unreliable geochronometer. However, more recent work has shown that the method can work, given careful sample selection (section 8.2.3).

8.2.2 Meteorite isochrons

In view of these problems, other attempts at geological half-life determination were focussed principally on iron meteorites. These have moderately large Re and Os contents, commonly in the high ppb (parts-per-billion) to low ppm range, and also display the good range of Re/Os ratios necessary for a precise age. Since these samples contain initial Os, the age must be calculated on an isochron diagram. Luck et al. (1980) ratioed radiogenic ¹⁸⁷Os against ¹⁸⁶Os, following Hirt et al. (1963). However, most other workers have normalised osmium isotope ratios with respect to ¹⁸⁸Os, because ¹⁸⁶Os can itself exhibit small variation in nature (see below). This leads to the following Re–Os isochron equation:

(¹⁸⁷Os) (¹⁸⁷Os) ¹⁸⁷Re

()))) = ()))) + ))) (e^8t ! 1) [8.1]

(¹⁸⁸Os)_P (¹⁸⁸8s)_I ¹⁸⁸Os

Luck et al. (1980) determined a good isochron fit for ‘whole-rock’ (bulk) samples of five iron meteorites, suggesting that iron meteorites of different types were all formed during a narrow time interval. Good isochron fits were also obtained in subsequent work on iron meteorites by several workers. These isochrons can be used to calculate values for the Re half-life by substituting a value of t into equation [8.1]. Because group II and III iron meteorites represent the cores of differentiated planetessimals, their age is constrained to be slightly younger than chondrites. However, iron meteorites cannot be more than about 20 Myr younger than chondrites, since they incorporated the short-lived nuclide ¹⁰⁷Pd with a half-life of 6.5 Myr (section 15.5). Hence t must be near 4.55 Byr.

Unfortunately, early meteoritic half-life determinations were also dogged by analytical problems, principally involving spike calibration. For example, Luck et al. (1980) determined a half-life of 42.8 " 2.4 Byr (Fig. 8.3), in close agreement with the estimate of Hirt et al. (1963), but Luck and Allegre (1983) retracted this value on the grounds that isotope dilution analysis of Os in their 1980 data set was upset by a change of osmium species in the spike solution subsequent to its calibration. However, subsequent work has shown the original determination to be more nearly correct.

Fig. 8.3. Re)Os isochron diagram showing best-fit regression for five iron meteorites (notation indicates sub-group) and one chondrite ( " ). For T = 4.55 Byr, 8 = 1.62 " 0.08 H 10^!11 yr^!1 (t_1/2 = 42.8 Byr). After Luck et al. (1980).

Problems with spike calibration were also encountered by Walker and Morgan (1989), but this time involving an over-estimate of the Re content of the spike, as later demonstrated by Morgan et al. (1992). However, Walker and Morgan also encountered problems with inadequate homogenisation between sample and spike. For example, analysis of seven chondrites gave results that did not lie on the isochron through iron meteorites (using the same spike calibration for both types of meteorite). This discrepancy was probably caused by the very different chemistries of the two types of meteorite during dissolution, since more recent work places the chondrites on the iron meteorite isochron, although with a large degree of scatter attributed to later disturbance of Re–Os systems in chondrites (Walker et al., 2002).

To avoid problems of inadequate spike homogenisation, the Carius tube technique was used in subsequent studies aimed at more accurate dating of meteorites. In the most detailed of these studies, Smoliar et al. (1996) determined Re–Os isochrons for four different classes of iron meteorites, of which the IIIA group is the oldest. Evidence from extinct nuclides (section 15.5.1) suggests that these (group IIIA) iron meteorites have ages within 5 Myr of the age of angrite meteorites (4558 Myr). Therefore, assuming this age for IIIA meteorites, the Re–Os isochron slope was used to determine a Re decay constant of 1.666 H 10^!11 yr^!1, equivalent to a half-life of 41.6 " 0.4 Byr. The error estimate was based on the possibility of a 1% error on the spike calibration. Although the error estimate itself is uncertain, 41.6 Byr is the best current value for the Re half-life.

Using this half-life, other iron meteorite groups (except IV-B) yield ages and initial ratios on a common osmium evolution curve (Fig. 8.4). The slope of this evolution curve represents the Re/Os ratio for the iron meteorite source, and is within error of the H chondrite Re/Os ratio. Hence, Smoliar et al. suggested that this source is probably the evolving solar nebula. The offset of the IV-B initial ratio from the evolution line was later found to be caused by instrumental bias (Smoliar, pers. comm). Shen et al. (1996) also made a precise isochron determination on group IIAB irons, yielding a slope which was identical to the IIAB isochron of Smoliar et al. (within analytical error). The spread of Re–Os ratios in the other groups did not allow precise ages to be determined.

Fig. 8.4. Osmium isotope evolution diagram for ages and initial ratios of iron meteorite isochrons, used to reconstruct the evolution of the early solar system. After Smoliar et al. (1996).

8.2.3 Dating ores and rocks

Very few dating methods have shown much success in the dating of ore deposits, despite the importance of such studies for Economic Geology. This is because most dating schemes involve lithophile elements that are not stable in ore minerals. In contrast, Re, and to some extent Os, display chalcophile chemistry, so both parent and daughter should occupy relatively stable lattice sites in sulphide minerals. Hence, many attempts at Re–Os dating of ore deposits have been made. However, following the problems encountered by Luck and Allegre (1982) in dating molybdenite, there has been continuing disagreement over the susceptibility of sulphide minerals to open system behaviour.

Suzuki et al. (1993) claimed that recent Re)Os dates on molybdenite were usually concordant with other methods, and suggested that errors in the earlier work might be due to poor sample)spike homogenisation rather than geological disturbance. However, other workers such as McCandless et al. (1993) maintained that open-system behaviour is a major problem, at least in old molybdenites. They suggested that a combination of microprobe analysis, electron back-scatter imaging and x-ray diffraction should be used to screen samples for alteration prior to analysis.

More success has been achieved in dating young ore deposits with molybdenite. For example, Selby et al. (2002) achieved perfect Re/Os isochrons (MSWD <1) for molybdenites associated with Mesozoic gold deposits from Alaska. The ages were close to the time of igneous intrusion (from U–Pb dating) and older than Ar–Ar dates on hydrothermal muscovite and sericite.

In an attempt to widen the usefulness of the Re)Os method as a geochronometer, Luck and Allegre (1984) applied it to the dating of a Ni)Cu sulphide ore from the Cape Smith komatiite of NE Quebec. This material has moderate concentrations of both Re and Os (ca. 0.4 ppm each), and can be used to calculate a model Re)Os isotope age in a manner analogous to model Nd ages. Using the then-current decay constant, Luck and Allegre calculated a model Os age of 1740 " 60 Myr for the sulphide ore, in reasonable agreement with the Sm)Nd isochron age of 1871 " 75 Myr. However, using the 1.666 H 10^!11 yr^!1 decay constant of Smoliar et al. (1996), the Re)Os model age is reduced to 1590 Myr, 250 Myr younger than the Sm)Nd age. This suggests that Ni)Cu sulphides are susceptible to re-setting in the same way as Mo sulphide. Similar evidence of open-system behaviour was found by Walker et al. (1989b) in Archean schists from India.

Laboratory experiments (Brenan et al., 2000) confirmed the susceptibility of pyrrhotite to re-setting by diffusional gain or loss of osmium. An Arrhenius relationship was observed in high temperature diffusion experiments. This allowed a pyrrhotite ‘blocking’ or closure temperature to be calculated (sections 3.3.2 and 10.5; equation 10.15). The resulting estimates of blocking temperature for a variety of grain size were between 300 and 400 ^oC, similar to the blocking temperature for Rb–Sr and K–Ar in biotite. This means that pyrrhotite as a dating tool is easily reset by metamorphic events. In contrast, Brenan et al. found diffusion rates over an order of magnitude lower in pyrite, implying a blocking temperature over 500 ^oC.

A recent study on sulphide-rich ores of the Sudbury nickel deposit in Ontario exemplifies the problems of trying to date pyrrhotite-rich ores (Morgan et al., 2002). Suites of sulphide ore from two different mines gave errorchrons with large MSWD values, although the resulting isochron ages were within error of the known age of the complex from U–Pb dating. Similarly, most of the Sudbury ore samples analysed by Dickin et al. (1999) appeared to be relatively undisturbed, but a few samples gave impossible (negative) initial ratios indicative of major disturbance. Hence, it is concluded that ages and initial ratios based on pyrrhotite must be treated with caution and substantiated by the analysis of large sample suites.

In contrast to these difficulties, several recent studies have generated good isochrons from sulphide-poor material. One example is a dating study on the Deccan basalts (Allegre et al., 1999). A suite of ten whole-rock basalt samples formed an excellent Re–Os isochron with a good spread of data points. Using a decay constant of 1.663 H 10^!11 yr^!1, the isochron gave a precise age of 65.6 " 0.3 Myr, in excellent agreement with previous K–Ar and Ar–Ar ages averaging 64.5 " 1.5 Myr. This study shows that young whole-rock suites are capable of generating Re–Os isochrons, although older material may be more problematical.

In another recent dating study, Kirk et al. (2002) successfully determined a Re–Os isochron age for gold samples from the Witwatersrand Supergroup of South Africa (Fig. 8.5). Gold from the Vaal Reef had moderate Re/Os ratios, and despite the authigenic appearance of some of the grains, gave an age of 3016 " 110 Myr (MSWD = 1.9) which was older than the maximum age of sedimentary deposition. This age provides powerful evidence that the gold is of detrital origin, and was not introduced by later hydrothermal fluid circulation.

Fig. 8.5. Re–Os isochron defined by gold samples from the Vaal Reef in the Witwatersrand Supergroup of South Africa. (The age and initial ratio on the diagram are erroneous. They should read: Age = 3016 +/- 110 Myr, I.R. = 0.109 +/- 0.013). After Kirk et al. (2002).

A final example of the potential of Re–Os geochronology is its application to dating the depositional ages of organic-rich sediments. Sedimentary deposition is one of the most difficult events to date, and attempts to apply the Rb–Sr and U–Pb methods have met with mixed success (sections 3.5 and 5.1.1). However, it was shown by Ravizza and Turekian (1992) that carbonaceous shales could be used to recover seawater osmium signatures (section 8.5), and more recent work (e.g. Cohen et al., 1999) has shown that this material can also be used for dating sedimentary deposition.

Evidence for open Re–Os systems in sulphides suggests that open system behaviour might also be a problem in dating black shales. However, a study of Paleozoic black shales from the Western Canada sedimentary basin (Creaser et al., 2002) suggested that Re–Os systems remain closed over a variety of degrees of hydrocarbon maturation. On the other hand, contamination of some samples with detrital osmium led to a large scatter on the isochron diagram (MSWD = 103). Therefore, Creaser et al. selected samples with total organic carbon contents over 5% in order to sample only seawater-derived (hydrogenous) osmium. These samples formed a much tighter array (MSWD = 1.8) with an age of 358 " 9 Myr. This suggests that the hydrogenous osmium component in the rock can be successfully isolated to determine depositional ages.

8.2.4 Os normalisation and the Pt–Os decay scheme

Hirt et al. (1963) established the convention of ratioing ¹⁸⁷Os data against ¹⁸⁶Os, and were followed in this practice by Luck and Allegre (1983), who normalised Os data for within-run fractionation to a ¹⁹²Os/¹⁸⁸Os ratio of 3.0827. However, ¹⁸⁶Os is itself the " decay product of the rare long-lived unstable isotope ¹⁹⁰Pt. This is not a significant problem in most geological applications, since ¹⁹⁰Pt makes up only 0.013 % of total platinum. However, in view of the growing importance of the Pt–Os decay scheme for understanding mantle evolution (section 8.3) it now seems best to use an alternative normalising isotope for both decay schemes, and most workers are now using ¹⁸⁸Os for this purpose. Hence, in this book ¹⁸⁸Os will be used in all cases as the normalising isotope, and isotope ratios previously quoted as ¹⁸⁷Os/¹⁸⁶Os ratios are now quoted in terms of ¹⁸⁷Os/¹⁸⁸Os. However, in such cases the original ¹⁸⁷Os/¹⁸⁶Os ratios are given in brackets and these ratios are shown where possible on figures (when ¹⁸⁶Os was the original normalising isotope). ¹⁸⁷Os/¹⁸⁶Os ratios can be converted to ¹⁸⁷Os/¹⁸⁸Os by multiplying by 0.12034.

The Pt–Os decay scheme was first applied by Walker et al. (1991), who analysed a suite of Pt-rich Fe–Cu–Ni sulphide ores from the Strathcona mine of the Sudbury nickel deposit. They constructed a Pt)Os isochron by substituting into the general decay equation as follows:

(¹⁸⁶Os) (¹⁸⁶Os) ¹⁹⁰Pt

()))) = ()))) + ))) (e^8t ! 1) [8.2]

(¹⁸⁸Os)_P (¹⁸⁸Os)_I ¹⁸⁸Os

By using the known age of the complex, Walker et al. were able to use the isochron for a rough determination of the ¹⁹⁰Pt decay constant, obtaining a value similar to the counting determination of 1 H 10^!12 yr^!1 (Macfarlane and Kohman, 1961).

A much more precise determination of the ¹⁹⁰Pt decay constant was made by Walker et al. (1997) by analysis of Fe–Ni sulphide ores with very high Pt/Os ratios from the Noril’sk complex in Siberia. Using the isochron shown in Fig. 8.6 and a published U–Pb age of 251.2 " 0.3 Myr, Walker et al. determined a precise value for the decay constant. After modification of the ¹⁹⁰Pt abundance to 0.01296 % (Brandon et al., 1999), this gave a decay constant of 1.477 H 10^!12 yr^!1, equivalent to a half-life of 469 Byr.

Fig. 8.6. Pt–Os isochron diagram for Fe–Ni sulphide ores from the Noril’sk complex, Siberia. Inset shows data for samples with low Pt/Os ratios. After Walker et al. (1997).

An alternative osmium isotope notation proposed by Walker et al. (1989a) is in the form of percentage deviations (() from a chondritic reference point. However, the Bulk Silicate Earth does not have an exactly chondritic Os signature (see below), so the CHUR reference point is less powerful for Os than it is for the Sm)Nd system (section 4.2). Nevertheless, if we wish to compare Os isotope data of different ages, we may need to use this notation, so it will be used here to a limited extent. The present-day average chondrite reference values chosen by Walker et al. were ¹⁸⁷Os/¹⁸⁸Os = 0.127 and ¹⁸⁷Re/¹⁸⁸Os = 0.40186 (after conversion to the new normalisation).

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