OS ISOTOPE SYSTEMATICS OF KOMATIITIC BASALTS FROM THE VETRENY BELT, BALTIC SHIELD: EVIDENCE FOR A CHONDRITIC SOURCE OF THE 2.45 GA PLUME

Abstract

The Vetreny belt in the southeast Baltic Shield is a large volcano-sedimentary basin containing a 4- to 8-km-thick sequence of basaltic to komatiitic lavas, which were erupted 2.45 Ga ago in a continental rift setting during the interaction of a mantle plume and the Archean continental crust of the Karelian granite-greenstone terrane. Re-Os isotope data for olivine cumulate samples and chromite separates from Victoria's lava lake and Golets flow 3 define isochrons with ages of 2387+/-57 and 2432+/-34 Ma, respectively. These ages are in good agreement with the previously reported, and new, Nd-Pb and U-Pb zircon ages. These data, coupled with the evidence for immobile behavior of the Re and Os, indicate that the Re-Os system remained closed since the lava eruption. The weighted average initial gamma187Os values range between -0.43+/-0.10 for Golets flow 3 and -0.07+/-0.13 for the lava lake. A single chromite separate from Golets flow 1 has a gamma187Os(T) of -0.06+/-0.15. Nd-Pb isotope and lithophile trace element data for the Vetreny belt komatiitic basalts were used to monitor the effect of crustal contamination on the Os isotope system and to estimate the initial Os isotope composition of the Vetreny plume source to be approximately chondritic with a gamma187Os(T) of -0.9. This implies that the mantle source evolved with a long-term nearly chondritic Re/Os ratio. The data provide further evidence that, by the end of the late Archean, the Earth's upper mantle was well homogenized with respect to the highly siderophile elements, added during the accretion of a late veneer, on a time scale of 1 Ga.