8 Osmium isotopes

Osmium is the least abundant member of the group of six elements called the platinum group elements (PGE). Like lead, osmium is an element with siderophile)chalcophile affinities, but unlike lead, osmium appears to be a strongly ‘compatible’ element during melting in silicate systems (meaning that it is strongly retained in the mantle source mineralogy). These geochemical properties mean that osmium can be used as a dating tool and a tracer in different ways from lithophile isotope systems such as Sr, Pb and Nd, providing unique evidence which complements these other systems.

Osmium has seven naturally occurring isotopes, two of which (¹⁸⁷Os and ¹⁸⁶Os) are the decay products of long lived radioactive isotopes, ¹⁸⁷Re and ¹⁹⁰Pt. Of these two decay schemes, the Re–Os method has been used as a dating tool and geochemical tracer for over twenty years. ¹⁸⁷Re has a half life of ca. 42 Byr and makes up 62% of natural rhenium, a chalcophile element which behaves like molybdenum. The Pt–Os method has only recently been applied because the radioactive parent, ¹⁹⁰Pt, has an extremely long half life of ca. 470 Byr and makes up only 0.013 % of natural platinum. This means that the natural variations in ¹⁸⁶Os are extremely small and hard to measure. However, in combination with the Re–Os couple, the Pt–Os system provides unique information that justifies the effort of its analysis. Technically, ¹⁸⁶Os is itself radioactive, but the half-life is so long that it can be considered stable for geological purposes.

8.1 Osmium Analysis

Despite its great potential as a geochemical tool, analytical difficulties initially limited the application of the osmium isotope method. The chief of these difficulties is the high ionisation potential of Os (ca. 9 eV) which prevents the formation of positive osmium ions at temperatures attainable in conventional thermal ionisation mass spectrometry (TIMS). Alternative methods of excitation therefore had to be sought.

Hirt et al. (1963) analysed osmium isotopes as the gaseous species OsO₄, but precision was low (" 10% on a 200 ng sample of pure radiogenic osmium). This was probably due to dissociation of OsO₄ during thermal ionisation of the molecule. Consequently, this method was not pursued for over 25 years. Instead, subsequent work focussed on the enhanced production of atomic osmium ions using more energetic ion sources. One of the most successful of these methods was secondary ion mass spectrometry (SIMS). In this technique, a beam of light negative ions (e.g. O^!) is used to bombard and sputter a purified solid concentrate of osmium metal to yield a positive Os ion beam which is analysed in a double focussing mass spectrometer (section 5.2.3). Other high energy excitation methods used with success were ICP-MS (Russ et al., 1987) and RIMS, involving laser (resonance) ionisation (Walker and Fassett, 1986).

All of these excitation methods for atomic osmium ions were rendered largely obsolete by the discovery that a solid osmium sample could yield negative Os-bearing molecular ions by conventional thermal ionisation (Volkening et al., 1991). This N-TIMS method allows levels of precision over an order of magnitude better than the positive ion techniques described above.

In the N-TIMS method Os is measured as the species OsO₃^!, using platinum filaments. These are coated with a barium salt to lower the work function of the filament, which enhances the emission of negative ions relative to electrons. The formation of the oxide species may also be enhanced by bleeding oxygen into the source (Walczyk et al., 1991). The same N-TIMS method may be used to perform isotope dilution analysis of other PGE, as well as rhenium, which forms the ReO₄^! species (Fig. 8.1). This method can generate beams large enough for analysis by Faraday detector from a few nanograms of osmium, while multiplier analysis allows picogram size samples to be analysed (Creaser et al., 1991). This technical advance has now brought the osmium isotope system to the same wide range of applications as the Sr, Nd and Pb isotope methods.

Fig. 8.1. Mass spectrum of Re and Os molecular ions produced from a Ba-doped Pt filament at 770 ^oC (ca. 2 A), loaded with 5 ng Os and 3 ng Re. After Creaser et al. (1991).

In addition to the difficulties of osmium ionisation, another major problem with Re–Os analysis has been the chemical behaviour of osmium in solution, due to the existence of multiple oxidation states, including the volatile tetroxide species. The volatility of osmium tetroxide allowed Luck et al. (1980) to establish a chemical extraction method in which samples were oxidised after dissolution, allowing separation by distillation. However, the variable oxidation states of osmium have continually plagued the isotope dilution analysis of osmium, by preventing complete homogenisation between sample and spike osmium.

This problem was finally resolved by the introduction of the somewhat hazardous ‘Carius tube’ digestion method (Shirey and Walker, 1995) In this technique, samples are dissolved in sealed glass ampoules under high temperature and pressure. Reagents are typically either aqua regia or a mixture of sulphuric acid and chromium trioxide, and the samples are heated to 240 ^oC. Outer metal safety jackets are used, but the pressure is retained entirely by the sealed glass tube, which may quite often break! After a successful reaction, the products are frozen before the vial is broken to release the sample.

The development of multiple collector ICP-MS has reopened the possibility of performing high precision osmium isotope analysis with the ICP source. However, the principal advantage of MC-ICPMS is the ability to perform in situ analysis by laser ablation. Hirata et al. (1998) made the first demonstration of this method by performing in situ analysis of Os–Ir alloys. The rarity of this type of material limits the usefulness of the technique. However, Pearson et al. (2002) showed that laser ablation MC-ICPMS could also be used to make in situ measurements of osmium isotope ratio and Re/Os ratios in sulphide inclusions within mantle olivines. Since most of the osmium inventory from mantle rocks is probably in sulphide inclusions, the ability to perform in situ analyses on this material offers a powerful technique for understanding the behaviour of the Re–Os system in the mantle.

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