Os isotope systematics of komatiitic basalts from the Vetreny belt, BalticShield: evidence for a chondritic source of the 2.45 Ga plume

The Vetreny belt in the southeast Baltic Shield is a large volcano-sediment ary basin containing a 4- to 8-km-thick sequence of basaltic to komatiitic lavas, which were erupted similar to2.45 Ga ago in a continental rift setti ng during the interaction of a mantle plume and the Archean continental cru st of the Karelian granite-greenstone terrane. Re-Os isotope data for olivi ne cumulate samples and chromite separates from Victoria's lava lake and Go lets flow 3 define isochrons with ages of 2387 +/- 57 and 2432 +/- 34 Ma, r espectively. 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 rem ained closed since the lava eruption. The weighted average initial gamma Os -187 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 g amma Os-187(T) of -0.06 +/- 0.15. Nd-Pb isotope and lithophile trace elemen t data for the Vetreny belt komatiitic basalts were used to monitor the eff ect of crustal contamination on the Os isotope system and to estimate the i nitial Os isotope composition of the Vetreny plume source to be approximate ly chondritic with a gamma Os-187(T) of -0.9. This implies that the mantle source evolved with a long-term nearly chondritic Re/Os ratio. The data pro vide further evidence that, by the end of the late Archean, the Earth's upp er mantle was well homogenized with respect to the highly siderophile eleme nts, added during the accretion of a late veneer, on a time scale of simila r to1 Ga.