Published online by Cambridge University Press: 01 February 2018
Osmium is the least abundant member of the group of six elements called the PGE (platinum group elements). 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 insights that complement 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 widely used as a dating tool and geochemical tracer, since its parent, 187Re, has a half-life of ca. 42 Ga and makes up 62% of natural rhenium. Rhenium is a typical chalcophile element which behaves like molybdenum in magmatic and ore-forming systems.
The Pt–Os method was developed more recently, since the radioactive parent (190Pt) makes up only 0.013% of natural platinum, and has an extremely long half-life of ca. 470 Ga. This means that natural variations in 186Os abundance 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 to be stable for geological purposes.
In spite of its great potential as a geochemical tool, analytical difficulties have hindered the development of the osmium isotope method. The chief of these difficulties is the high ionization potential of Os (ca. 9 eV) which prevents the formation of positive osmium ions at temperatures attainable in conventional thermal ionization 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 ionization 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.