RE-OS ISOTOPIC COMPOSITIONS OF MIDCONTINENT RIFT SYSTEM PICRITES - IMPLICATIONS FOR PLUME-LITHOSPHERE INTERACTION AND ENRICHED MANTLE SOURCES

Picrites and tholeiites from the Mamainse Point Formation, a 5.3 km th ick section of Keweenawan (1100 Ma) volcanic and sedimentary fill on t he eastern flank of the central portion of the Midcontinent rift syste m, contain a nearly continuous record of rift magmatic activity. Picri tes occur primarily in the lowermost two units of the formation. In th is study, they are compared to rarely exposed, slightly older Keweenaw an basalts from the North Shore Volcanic Group and the Powder Mill Gro up to constrain mantle source compositions during early phases of rift magmatic activity. The most primitive picrites analyzed have low Re c ontent (0.069-0.18 ppb), high Os content (0.8-2.1 ppb), and low Re-187 /Os-188 (0.28-1.18). A Re-Os isochron with an age of 1128 +/- 54 Ma an d an initial Os-187/Os-188 of 0.1267 +/- 0.0013 (gamma(Os) = +5.7) was obtained from a 24-point isochron an all but two analyzed samples. Th e Re-Os data, regressed separately for the older basalts, and the grou ps 1 and 2 samples from the Mamainse Point Formation, have barely reso lvable initial Os-187/Os-188 that decrease up-stratigraphy from initia l gamma(Os)(1100) of +12.2 to +6.2 and +4.2, respectively, and couple with changes in initial Nd isotopic composition. These data can be exp lained by mixing of melts of an enriched mantle plume and unradiogenic continental lithospheric mantle. A radiogenic initial Os isotopic com position (gamma(Os) of +8 or higher) for the Keweenawan plume marks th e first known appearance of demonstrably radiogenic plume-derived magm as on Earth. Plume-derived magmas with radiogenic Os signatures are mo re common later. The radiogenic Os signatures of Keweenawan plume magm as may mark the appearance of melts derived from mantle containing rec ycled slab components from late Archean subduction.