S12: Geologic Time Through Re-Os Dating

Geologic Time Through Re-Os Dating of Organic-Rich Shales

Holly Stein and Judith Hannah,
AIRIE Program, Colorado State University, USA Department of Geosciences, University of Oslo, Norway,

Web Lecture on May 12,  2022 at 3:00 PM (Paris Time)

Re-Os dating of shales is now a highly successful means to organize the sedimentary record in absolute time.  This is achieved by isolating the organic (hydrogenous) component from the detrital component in black shales and determining Re-Os  concentrations and the Os isotopic composition of the organic matter.  This, in turn, describes marine (or lacustrine) water-sediment conditions in ancient seas and lakes.  As with all radiometric dating, the integrity of the Re-Os clock is based on fundamental assumptions: (1) all samples had the same starting Os isotopic ratio, acquired from the water column; (2) the samples have variable Re/Os ratios, controlled by variable types of organic material in each sample, an essential requirement to form a 187Re/188Os versus 187Os/188Os isochron; and (3) the rocks must remain in their original reduced state, as oxidation – even incipient –  will mobilize Re and Os and disturb the clock.  Sampled shales often include syn-sedimentary sulfide, usually pyrite (FeS2), which also sequesters Re and Os and extends the range of Re/Os ratios for an isochron plot.  Stirring and mixing of sediment or introduction of foreign bottom or pore water pre-lithification may introduce scatter on a Re-Os isochron.  An important hurdle in Re-Os dating of shale is the application of a thoughtful sampling strategy.  Each sample for an isochron should be a stratigraphically small interval of several millimeters to truly query the stability of the initial Os isotopic ratio.  Samples spanning a stratigraphic interval of a centimeter or more risk “homogenizing or averaging” Os isotopic variations during sedimentation, which may result in unjustly precise isochrons and incorrect Os initial ratios.  Correct interpretation of paleo-environmental conditions relies on assessment of Os isotopic variation at all scales.  Because Re-Os dating is based on only 6-10 samples whose isotopic integrity must be nearly perfect, Re-Os isochron uncertainties are often larger than reported zircon ages for volcanic tuffs where scores of zircons are analyzed by laser ablation and the U-Pb zircon age is based on an average whose precision is enhanced by dividing by a large number of analyses (n).  Re-Os isochron ages nevertheless are shown to be accurate, and with good to excellent precision.  Uncertainties in isochron ages can be reduced by applying Bayesian statistics to a dated sequence where the stacking of younger on older sedimentary layers is indisputable.  Bayesian-refined precision for Re-Os ages may rival U-Pb ages, depending on geologic circumstances for both.  In sum, Re-Os dating of shales provides the sedimentary record with a dating tool that tests biostratigraphic correlations and provides timelines that can be combined with U-Pb dating of volcanic tuffs.