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

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 2387ᇍ and 2432ᆶ 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 %¹⁸⁷Os values range between -0.43ǂ.10 for Golets flow 3 and -0.07ǂ.13 for the lava lake. A single chromite separate from Golets flow 1 has a %¹⁸⁷Os(T) of -0.06ǂ.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 %¹⁸⁷Os(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.     

The Vetreny Poyas (Windy Belt) subprovince of southeastern Fennoscandia: An essential component of the ca. 2.5–2.4 Ga Sumian large igneous provinces

Ca. 2.5–2.4 Ga Sumian magmatism is widespread in the Karelia and Kola cratons of Fennoscandia and probably represents at least two intermixed large igneous provinces (LIPs). It is distinct from other Paleoproterozoic LIPs (Jatulian 2.22–2.1 Ga and Ludicovian 2.06–1.96 Ga) elsewhere in the Fennoscandian Shield. A poorly understood portion of Sumian magmatism is the Vetreny Poyas (Windy Belt) subprovince, which covers ∼75,000 km² in southeastern Fennoscandia. This subprovince consists of four genetically related complexes which developed at different levels in the crust: a volcanic complex (komatiitic basaltic lava flows on Golets, Levgora and Myandukha hills, and Victoria lava lake on Levgora hill), a subvolcanic complex (mafic–ultramafic sills and lopoliths including Ruiga, Kirichgora, Kozhozero and Undozero), plutonic complexes (Burakovsky and Vyzhiga) and a dyke complex (gabbronoritic Avdeyevo and Shala dykes and peridotitic Vinela and Koppalozero dykes). Similar patterns are present in other Sumian belts elsewhere in Karelia, for instance in southern Lapland and the Kola Peninsula.     

Elemental composition of suspended particles from the surface waters of Lake Baikal in the zone affected by the Selenga River

The elemental composition of suspended particles in surface waters of Lake Baikal has been studied by ICP MS along a transect of the zone affected by the Selenga River. The amount of terrigenous suspension in water was estimated from Al content, which decreases in a saltatory manner along the transect: 6.5–8.3 km offshore it decreases by an order of magnitude; 11–14 km offshore, by other 20 times; in the pelagic zone it remains almost unchanged. During the study period (late June 2001), the distribution limit of suspension from the Selenga River in the lake’s surface waters lays 11–14 km offshore.It has been found that pelagic suspension is more than 20 times richer in Ca, P, Zn, Cu, Ni, Sn, Mo, Bi, S, and Cd than littoral one (6.5 km offshore) in the zone affected by the Selenga River. This is mainly due to fine suspension (<1.2 μm), which was considerably richer in Ca, P, Zn, Cu, Ni, Sn, Mo, Sb, and Cd than coarser ones in the river.Fine suspension in Lake Baikal, both in the zone affected by the river and in the pelagic zone, is considerably (2–35 times) richer in most elements, except for typical terrigenous ones, than river one. This suggests that element enrichment takes place in the lake, probably owing to bioaccumulation and chemisorption. These processes may be affected by additional factors, because fine suspension from the zone affected by the river differs significantly from pelagic one in elemental composition.