Peridotite xenoliths which are thought to sample the upper mantle sometimes contain supra-chondritic Os-isotopic ratios.Radioactive dating is a method of dating rocks and minerals using radioactive isotopes.Isochrons are created by analysing several samples believed to have formed at the same time from a common source.
For example, the boundaries of many Phanerozoic systems, epochs, and stages are marked by the deposition of black shale units, several of which are coincident with mass extinction events.
Suitable lithologies for Re-Os geochronology include black shales and organic-rich carbonates.
In addition to geochronology data, the Re-Os geochronometer provides the Os isotope composition (Os = 0.12) derived from cosmic dust and hydrothermal alteration of oceanic crust.
As such, the Re-Os geochronometer can be used to directly date some stratigraphic boundaries.
Proterozoic sedimentary successions often record enormous portions of geological time, yet many of them lack volcanic horizons and are bereft of fossils suitable for relative age dating.