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

摘要

The Re-Os data on Archean komatiites from the Kostomuksha greenstone belt in the Baltic Shield are presented. This greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head [Puchtel et al., Earth Planet. Sci. Lett. 155 (1998) 57-74]. Samples of flowtop breccia, spinifextextured and cumulate komatiites and a chromite separate, all collected from the core of a 300 m deep diamond drill hole, yielded a Re-Os isochron with an age of 2795 ± 40 Ma and an initial ~(187)Os/~(188)Os of 0.1117 ± 0.0011 (γ~(187)Os = + 3.6 ± 1.0). The high positive γ~(187)Os(T) implies that the komatiites were derived from a mantle source with a time-integrated suprachondritic Re/Os ratio. Recycling of oceanic lithosphere to produce the enriched ~(187)Os isotope 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 substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites. Our tentative interpretation is that the ~(187)Os-enrichment in the Kostomuksha plume represents an outer core signature. If confirmed by the ongoing Pt-Os isotope studies, the results would provide evidence for the existence of whole-mantle convection in the late Archean, and might place constraints on the timing of core differentiation in the early Earth.