OSMIUM ISOTOPIC EVIDENCE AGAINST AN IMPACT AT THE FRASNIAN-FAMENNIAN BOUNDARY

Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and Os-187/Os-188 ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and. ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial Os-187/Os-188 of about 0.49, does not show the characteristic meteoritic Os imprint with an Os-187/Os-188 of about 0.13. The integrity of the section with regard to Re and Os retention is indicated by the construction of an isochron with an age of 388 +/- 41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (Os-187/Os-188 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. These possibilities prevent the reconstruction of an initial Os-187/Os-188 value for the boundary for this site. Assuming reasonable Re concentrations prior to loss, the Os isotopic composition is inconsistent with a large meteoritic component. This inconsistency emphasizes that platinum group element (PGE) concentrations are not reliable indicators of extraterrestrial addition.