Os-187-enriched domain in an Archean mantle plume: evidence from 2.8 Ga komatiites of the Kostomuksha greenstone belt, NW Baltic Shield

The Re-Os data on Archean komatiites from the Kostomuksha greenstone belt i n the Baltic Shield are presented. This greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacemen t of an ancient plume head [Puchtel et al., Earth Planet. Sci. Lett, 155 (1 998) 57-74]. Samples of flowtop breccia, spinifex-textured and cumulate kom atiites and a chromite separate, all collected from the core of a 300 m dee p diamond drill hole, yielded a Re-Os isochron with an age of 2795 +/- 40 M a and an initial Os-187/Os-188 of 0.1117 +/-0.0011 (gamma Os-187 = +3.6 +/- 1.0). The high positive gamma Os-187(T) implies that the komatiites wire d erived from a mantle source with a time-integrated suprachondritic Re/Os ra tio. Recycling of oceanic lithosphere to produce the enriched Os-187 isotop e signature is considered unlikely. as 15-25% crustal component is required to be incorporated into the plume source as early as 3.5-4.3 Ga. Such a su bstantial proportion of mafic material in the source would likely destroy t he major and trace element characteristics of the komatiites. Our tentative interpretation is that the Os-187-enrichment in the Kostomuksha plume repr esents an outer core signature. If confirmed by the ongoing Pt-Os isotope s tudies, the results would provide evidence for the existence of whole-mantl e convection in the late Archean, and might: place constraints on the timin g of core differentiation in the early Earth. (C) 2001 Elsevier Science B.V . All rights reserved.