Platinum group elements in Kostomuksha komatiites and basalts: Implications for oceanic crust recycling and core-mantle interaction

We report precise PGE (Os, Ir, Ru, Pt, and Pd) concentrations for komatiite -basalt lava sequences of the Kostomuksha greenstone belt, and for the koma tiite standards KAL-1 (Abitibi) and WITS-1 (Barberton). The flowtop breccia komatiites (MgO = 27.5%) from the Rostomuksha sequence, which represent th e composition of the primary komatiite liquid, have Os = 1.5 +/- 0.1 ppb, c hondritic (Os/Ir), = 1.06 +/- 0.05, and moderately fractionated PGE pattern s, (Pd/Os)(N) = 6.5 +/- 0.4. Based on the KAL-1 standard, the Alexo primary liquid had Os = 1.8 +/- 0.1 ppb, (Os/Ir), = 1.05 +/- 0.05, and (Pd/Os), = 8.5 +/- 1.0. PGEs in the Kostomuksha komatiites exhibit incompatible behavi or during magmatic differentiation, with bulk D values of Os, Ir = 0.75, Pt = 0.52 and Pd = 0. PGE abundances from Kostomuksha komatiites and basalts were used to calcula te the Pt/Os and Re/Os composition of a magnesian Archean oceanic crust. Th e Pt-Re-Os isotopic evolution of this crust was modeled, indicating that co upled (OS)-O-186,187 enrichments observed in some modern plumes require a n on-crustal source. A previously proposed model for core mantle interaction by physical admixture was evaluated and was shown to produce mantle sources with high PGE abundances (Os = 6 to 14 ppb, Pd = 140 ppb for 1% outer core addition) and (Os/Ir)(N) greater than or equal to 1, depending on the assu mptions made. These features were not observed in the Kostomuksha komatiite s (gamma Os-187(T) = +3.6), shown here to be derived from a reservoir with -25% lower absolute PGE abundances than primitive mantle. If the radiogenic initial Os-187/Os-188 in the Kostomuksha komatiite is an outer core signat ure, then core-mantle interaction cannot have involved physical admixture o f outer core material into mantle plume sources. Instead, it must have occu rred in the form of isotopic equilibration between liquid outer core and so lid mantle at the core-mantle boundary. Copyright (C) 2000 EIsevier Science Ltd.