PLATINUM-GROUP ELEMENT ENRICHMENTS AND POSSIBLE CHONDRITIC RU-IR ACROSS THE FRASNIAN-FAMENNIAN BOUNDARY, WESTERN NEW-YORK-STATE

The Frasnian-Famennian boundary is recognized as the culmination of a global mass extinction in the Late Devonian. In western New York state the boundary is a distinct horizon within a pyritic black shale bed o f the upper Hanover Shale defined by the first occurrence of Palmatole pis triangularis in the absence of Frasnian conodonts. The boundary is characterized by a minor disconformity marked by a lag concentration of conodonts. Iridium at the boundary is 0.11-0.24 ng/g, two to five t imes background levels of <0.05 ng/g; other Ir enrichments of 0.38 ng/ g and 0.49 ng/g occur within 50 cm of the conodont-constrained boundar y. Numerous Ir enrichments in the boundary interval suggest extraterre strial accretion and platinum group element (PGE) concentration at dis conformities, or mobilization and concentration in organic-rich/pyriti c-rich laminations from cosmic or terrestrial sources. PGE ratios of P t/Pd and Ru/Ir at the boundary horizon approximate chondritic ratios a nd are suggestive of an unaltered extraterrestrial source. These value s do not conclusively establish a single extraterrestrial impact as th e ultimate cause of the Frasnian-Famennian mass extinction, especially given the presence of similar Ir enrichments elsewhere in the section and the absence at the boundary of microtektites and shocked mineral grains. (C) 1997 Elsevier Science B.V.