One of our key goals is to provide absolute ages and chronostratigraphic correlations. To this end we have developed unrivalled in-house expertise in the Rhemium – Osmium (Re-Os) isotopic dating technique as well the more established Uranium – Lead (U-Pb) geochronology methods.
Re - Os Isotopes
The Re-Os isotope system provides a means of absolute dating in carbon-rich sediments and therefore fits perfectly into our stratigraphic portfolio. However, it also provides oil generation timing and timing of sulphide mineral formation.
Geochronology of mudrocks
Re and Os become concentrated in the organic, hydrogenous fraction of organic rich shales and by analysis of between six and eight samples from approximately the same stratigraphic interval, an absolute age for the deposition of the shale is generated. Typical uncertainties are between 1% and 5%. The technique can be applied to any organic rich shale older than ~10 Ma and can include lacustrine and Proterozoic shales for which biostratigraphical information is lacking. The 187Os/188Os value (Osi) can also provide information on whether deposition occurred in a lacustrine or an open marine environment.
Establishing dates for the generation of oils
When oil is formed from a source rock, a proportion of the Re and Os from the source rock is transferred to the oil. Therefore, using the Re – Os geochronometer the timing of oil generation can be calculated. Furthermore, Os isotopes can be employed to correlate link oil and source. Re – Os dating and oil – source rock correlation is most effective if the oils are highly biodegraded, e.g., oil sands, tar mats and bitumen-stained sandstones (where traditional techniques such as biomarker identification are disrupted).
Establishing the formation date of sulphide minerals
The Re – Os technique can also be utilised to date the formation of sulphide minerals
By determining the U-Pb isotopic composition of a zircon crystal, the age of the crystal can be calculated by utilising a ‘concordia diagram’. If the analysed zircon crystal has not suffered either Pb loss or U gain, it will plot on the concordia line from which its age can be deduced. In the stratigraphic context U-Pb zircon geochronology is applied to tuffaceous intervals to determine the age of the tuff. Chemostrat also use U-Pb zircon geochronology for sediment provenance studies.