186Os-187Os systematics of Hawaiian picrites

Primitive Hawaiian picrites have Os-187/Os-188 as high as similar to 0.145 and are more radiogenic than the depleted upper mantle. reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision Os-186/Os-188 measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has Os-186/Os-188 that range from chondritic mantle values of similar to 0.119834 to more radiogenic values as high as similar to 0.119848. The higher Os-186/Os-188 reflects long-term suprachondritic Pt/Os and is coupled with higher Os-187/Os-188 in all but the Koolau picrites. The latter have near-chondritic Os-186/Os-188 but With radiogenic Os-187/Os-188. Th, Pt/Re of crustal materials that may make up ancient recycled slabs ranges from similar to 0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1-3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88-100, however. are required for the ancient recycled crust to generate the coupled enrichments of Os-186/Os-188 and Os-187/Os-185 in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os > Re > Pt, during metal crystallization from an initially chondritic molten core (Pt/Re similar to 21-24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic Os-186/Os-188 and Os-187/Os-188 over rime. Small amounts of outer core metal (less than or equal to 1.2%) mixed into the Hawaiian plume source can explain the coupled Os-186/Os-188 and Os-187/Os-188 enrichment in some of the Hawaiian picrites. In addition, the most radiogenic Os-186/Os-188 in the Hawaiian picrites is correlated with higher He-3/He-4, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary. (C) 1999 Elsevier Science B.V. All rights reserved.