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Rhenium osmium dating

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Here's a look at what we can do with two of Earth's rarest elements, rhenium and osmium.

Until the 1990s, our instruments couldn't even measure these elements reliably.

The Re-Os geochronometer can be used to date sedimentary successions lacking suitable horizons for traditional geochronology techniques such as Ar-Ar or U-Pb zircon geochronology.

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.

Osmium has two isotopes that are variable in nature (Os isotopic method has been fraught with analytical difficultes, which hampered development after the pioneering efforts of Herr and co-workers.The better our instruments are, the more things we can see.This is an old truism in science—think of telescopes for the most well-known example—and it continues to be true today.Today almost all of the planet's rhenium and osmium are in the iron core, leaving the silicate mantle with just a whisper, less than one part per billion.Rhenium, which occurs at about 200 parts per trillion outside the core, comes in two isotopes, of atomic weight 185 and 187.As such, the Re-Os geochronometer can be used to directly date some stratigraphic boundaries.