Re-Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods

Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re-Os date obtained. To minimize the influence of any nonhydrogenous Re and Os, we evaluate analysis of isolated organic matter from the whole rock, together with whole rock analysis using a CrO3-H2SO4 digestion medium instead of inverse aqua regia, for a black shale unit of the Exshaw Formation, Canada. This unit previously returned a whole rock Re-Os date of 358 +/- 10 Ma (Model 3) [Geochim. Cosmochim. Acta (2002)] using inverse aqua regia dissolution. Organic matter isolated from the whole rock matrix using the HF-BF3 technique [Org. Geochem. 20 (1993) 249] yields scattered data and a Re-Os date of 449 220 Ma (Model 3, MSWD = 616). The organic matter analyses show similar Os-187/Os-188 values, but significantly lower Re-187/Os-188 values in comparison to the whole rock analyses. We show that the Re-Os systematics of organic matter are altered during chemical isolation, and as such we suggest that the HF-BF3 method should not be used for Re-Os analysis of organic matter. Whole rock Re-Os analysis using a CrO3-H2SO4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method, and the Re-Os data identify two distinct initial Os-187/Os-188 values for the sample set. Separate regressions of these data yield precise dates [366.1 +/- 9.6, MSVvD = 2.2 and 363.4 +/- 5.6 Ma, MSWD = 1.6 (Model 3)], which are indistinguishable from the age constraints for this formation (363.4 +/- 0.4 Ma, U-Pb monazite). Comparison of the Re-Os dates obtained from aqua regia and CrO3-H2SO4 methods suggests that the former may contain nonhydrogenous Re and Os, whereas the CrO3-H2SO4 method dominantly liberates hydrogenous Re-Os from organic matter, allowing for better stratigraphic age determinations and evaluation of the Os isotope composition of seawater. (C) 2003 Elsevier B.V. All rights reserved.