Rhenium/Osmium isotope analysis
Application
Osmium is a highly siderophilic element that is almost completely absorbed by ferrous metal during iron smelting and, unlike lead and strontium, there is hardly any risk of contamination from charcoal or furnace material because it only occurs in extremely low concentrations in the earth’s crust. Osmium is therefore ideal for determining the origin of iron.
Just like the isotope ratios of strontium and lead, the 187Os/188Os ratio in natural osmium is variable, depending on the geological age and type of rock as a result of the radioactive decay of 187Re to the stable 187Os. However, this variability is many times higher than in the strontium or lead isotope system.
The special feature of the rhenium-ismium isotope system is due to the very different geochemical behaviour of rhenium compared to osmium. Rhenium is the most incompatible (not suitable for incorporation into the crystal lattices of the rock-forming minerals) element of the period during the melting processes in the Earth’s mantle, i.e. during the formation of continental crust an extremely large Re/Os ratio results and creates the prerequisite for the formation of a very large variability of the 187Os/188Os ratio in rocks of the continental crust.
Basics
The decay of 187Re to the stable 187Os forms the basis for the well-established Re-Os dating method in geochemistry and geology. In recent decades, osmium isotopy has also been used for the (geological) origin of rock material and for chronostratigraphy, and the rhenium-osmium system has been used in ore deposit science to date ore formation processes.
During chemical reactions, such as the smelting of iron ores or the corrosion of iron objects, the isotope ratios of osmium are not fractionated by kinetic effects as is the case with light elements due to their high mass and low mass difference of the individual osmium isotopes. This makes it possible in principle to use the 187Os/188Os ratio to investigate the provenance of archaeological iron objects, even if they are heavily corroded.
In addition to the use of osmium isotopes to determine the origin of iron objects, it can also be used to determine the origin of ceramic objects. To do this, the raw material (clay) must be compared with the product (ceramics). Osmium is particularly suitable as an indicator for determining the origin of archaeological objects because the Os concentration on the earth’s surface is extremely low and osmium is also very immobile in nature. This means that the probability of contamination of archaeological samples by osmium is very low if they are stored for a long time in soil, for example.
Powdered and homogenised samples or iron samples are digested with acids in a melted quartz tube at high temperature and high pressure. The osmium is then separated by microdistillation and the rhenium by ion exchange. The measurement is carried out using a thermal ion mass spectrometer, which is set to negative ions because osmium forms negative ions more easily than positive ions, as is usually the case. In this way, even extremely small quantities of osmium in the picogram range can be analysed isotopically.
In the course of investigations, the isotope ratios of the DTM standard, which has also been certified by other institutions (e.g. Monash, Australia), are regularly measured. Overall, we estimate the total error of the determination of 187Os/188Os (including the calibration of the spiking solution) to be 0.4 % (2σ). In addition to the determination of reference standards, blank value measurements are also carried out at regular intervals.
Limits
In order to minimise the entry of dust by people, special work clothing is required while in the cleanroom laboratory. In addition, all chemicals used must also fulfil special requirements in terms of purity.
Sample composition
The sample size depends on the osmium content, which can be very variable! We recommend 10 g of sample material for iron ore, 0.5 g for ferrous metal, 10 g for clay and 10 g for ceramics.