Introduction to Re-Os

The Re-Os Geochronometer

The Rhenium-Osmium (Re-Os) geochronometer is a novel tool for dating ancient sedimentary successions. Analytical and instrument developments over the past 25 years have improved the reliability and accuracy of the method such that precision now routinely approaches ~1% uncertainty. 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 (187Os/188Os [Osi]) of seawater at the time of deposition. This Osi value represents a balance of radiogenic continental inputs (187Os/188Os = 1.40) derived chiefly from runoff with minor contributions from aeolian dust, and unradiogenic inputs (187Os/188Os = 0.12) derived from cosmic dust and hydrothermal alteration of oceanic crust.

Applications of Re-Os geochronology

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. 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. As a result, accurate and precise ages are difficult to determine for these successions, which hampers attempts to correlate successions across different regions. Recent studies with the Re-Os geochronometer are beginning to improve our understanding of the biological, geochemical, atmospheric, climatic and tectonic transformations recorded in the Proterozoic rock record. The Re-Os geochronometer can aid in establishing a robust chronometric framework that is essential to fully understand the timing of events and processes through geologic time.

In addition to geochronology data, the Re-Os system yields a wealth of information on the weathering fluxes into a particular basin or ocean at a given time in Earth history. The Osi data can be used in a complementary way to Sr isotopes (but with a shorter residence time: ~10 kyr vs. 2 Myr) to provide high-resolution chemostratigraphy data that can reveal the effect of paleoenvironmental processes such as volcanism, tectonic reorganization, or extra-terrestrial impacts on geochemical and biological systems.

Although not as precise as U-Pb zircon CA-ID-TIMS ages, the Re-Os geochronometer can be viewed as a complementary technique that is unique in its ability to provide depositional ages for sedimentary units and provide useful information on the Os isotopic composition of paleoseawater.

Contact details of some of the laboratories offering Re-Os geochronology are listed below: Below is a short and by no means exhaustive list of examples of the application of Re-Os geochronology and Osi chemostratigraphy in the Earth sciences.

Re-Os geochronology studies Os isotopes for investigating paleoweathering Tracking extra-terrestrial-impacts in the geological record using Os isotopes Alan Rooney | Harvard University

EARTHTIME is supported by the National Science Foundation.