Re-Os isotopic composition and PGE contents of proterozoic carbonaceous argillites, Virginia Formation, Northeastern Minnesota

Carbonaceous, sulfidic argillites From the Proterozoic Virginia Formation i n Minnesota have been investigated in terms of platinum group elements (PGE ) distribution and Re-Os isotopic signatures. Kerogen from protoliths and s lightly metamorphosed equivalents is characterized by Re and Os enrichment factors relative to whole rock concentrations that range from 4 to 272. Re- Os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1.85 Ga reference isochron. This age is cons istent with that estimated for the Virginia Formation, and suggests that da ting of organic-rich Precambrian sedimentary rocks and the retrieval of dat a pertinent to the Os isotopic evolution of seawater may be possible using the Re-Os isotopic system. (C) 2001 Elsevier Science Ltd. All rights reserv ed.