Re-Os Dating of Black Shales

Black organic-rich shales also have a tendency to concentrate Re over Os, albeit not to the same extent as molybdenite, so isochron rather than single sample dating needs to be performed. The ages obtained are interpreted as the depositional age of the sediments and the initial Os/ Os ratio as that of seawater at the time of deposition [116]. 

Re-Os dating of black shales needs to be done after digestion, spiking, and isolation of the spiked Re and Os fractions. For these isochrons, it is important to avoid inclusion of detrital Os-bearing material as much as possible, as their composition may vary from one sample to the next, giving scatter around the 

The Re and Os concentrations of the shales are variable, between 0.3-1500 ng kg for Re and 0.04—1.2 ng kg for Os, with Re/ Os ratios of 50-1000 [117-120]. Again, the small amounts of Os makes it more difficult to carry out the analysis by MC-ICP-MS using Faraday cups, but the rather high Os/ Os ratios make it possible to obtain meaningfid ages with single-collector ICP-MS [119]. 

As Os is compatible in the mantle, whereas Re has an incompatibility similar to that of A1 or Lu [121], melting of the mantle will lead to a very strong decrease in the Re/Os ratio. Although this could be used to obtain isochrons from variably melt-depleted mantle rocks (peridotites), this is generally unsuccessful, as a result of the relatively high mobdity of Re, resulting in Re addition ( refertilization ) after melt depletion. 

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