Re-Os dating of algal laminites : reduction-enrichment of metals in the sedimentary environment and evidence for new geoporphyrins
Miller, Christian A.
The principal aim of this study is the evaluation of algal laminites as appropriate substrates for the application of the Re-Os chronometer. Devonian Keg River algal laminites produced an isochron date (382.2 ± 6.9 Ma) consistent with the expected age of about 388 Ma, and showed a lower degree of sample scatter than comparable dating studies of black shales. Ordovician kukersites from the Yeoman Fm. also produced a consistent age (459 + 14 Ma, compared to the expected age of 450 Ma) but exhibited a much higher degree of scatter. The scatter was the result of blank over-and under correction due to the extremely low concentrations of Re and Os in these organic-rich rocks. The unexpectedly low metal concentrations require an oxic interpretation of the kukersite depositional environment. Kukersites were likely rapid, shallow water deposits protected from grazing and burrowing by hypersalinity, rather than deeper anoxic deposits. ID-NTIMS investigation of the association between various metals and specific organic ligands known as porphyrins revealed very good circumstantial evidence for the existence of Re porphyrins in the sedimentary environment, something never reported before. The overall lack of equivalent Os porphyrins indicates that sedimentary organic matter possesses distinct organic matter fractions with differing 187Re/188Os ratios, which could allow the construction of an organic matter isochron if these separate reservoirs remain closed with respect to Re and Os. An Os blank correction performed on the solvent extracts indicates that most of the Os extracted from the rock is 187OS - to be expected on the basis of the Re-support, and their age. However, fewer 187OS porphyrins were observed than could be expected based on the abundance of 187Re.