OS-187 ENRICHMENT IN SOME PLUMES - EVIDENCE FOR CORE-MANTLE INTERACTION

Calculations with data for asteroidal cores indicate that Earth's oute r core may have a rhenium/osmium ratio at least 20 percent greater tha n that of the chondritic upper mantle, potentially leading to an outer core with an osmium-187/osmium-188 ratio at least 8 percent greater t han that of chondrites. Because of the much greater abundance of osmiu m in the outer core relative to the mantle, even a small addition of m etal to a plume ascending from the D '' layer would transfer the enric hed isotopic signature to the mixture. Sources of certain plume-derive d systems seem to have osmium-187/osmium-188 ratios 5 to 20 percent gr eater than that for chondrites, consistent with the ascent of a plume from the core-mantle boundary.