Re and Os are both siderophile “transition metals”. Re is moderately incompatible in mantle minerals whereas Os is highly compatible. Therefore, the Re/Os ratio is fractionated during melting and melt extraction, with Re/Os increased in melt and lowered in the residue (similar behaviour to Rb/Sr). As a consequence, the Earth’s continental crust contains radiogenic Os.
Os isotopic analysis is difficult because of its high ionisation potential. However, new techniques have recently been developed to analyse Os isotopic compositions.
It is also possible to convert 185Re and 187Re to 186Os and 188Os in a reactor and determine the Re/Os ratio during the isotopic analysis of Os (analogous to K/Ar via 40Ar/39Ar).
Re-Os has been applied to the investigation of basaltic melt extraction and Earth differentiation, as with the Sm/Nd method. The method can also be used to date mafic rocks such as basalts and komatiites (difficult to date with other methods) under favourable circumstances.
Palaeoenvironmental applications of the Re-Os method : seawater Os isotopic evolution
- Os in very low concentration in seawater, but can obtain enough Os to analyse by leaching marine sediments
- Os in marine sediments found to be very radiogenic (from continental crust)
- Rapid change in Os isotopic composition observed in Tertiary to Recent marine sediments
- Cosmic dust in sediments contributes unradiogenic Os
- Os isotopic composition in marine sediments correlated with Sr isotopic compositions