186Os–187Os systematics of Hawaiian picrites

Primitive Hawaiian picrites have 187Os/188Os as high as ∼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 186Os/188Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186Os/188Os that range from chondritic mantle values of ∼0.119834 to more radiogenic values as high as ∼0.119848. The higher 186Os/188Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187Os/188Os in all but the Koolau picrites. The latter have near-chondritic 186Os/188Os but with radiogenic 187Os/188Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from ∼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 186Os/188Os and 187Os/188Os 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 ∼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 186Os/188Os and 187Os/188Os over time. Small amounts of outer core metal (≤1.2%) mixed into the Hawaiian plume source can explain the coupled 186Os/188Os and 187Os/188Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186Os/188Os in the Hawaiian picrites is correlated with higher 3He/4He, 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.