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Valery Gorozhanin, Institute of Geology, Ufimian Scientific Center, Russian Academy of Science (Russian Federation)
Sergey Michurin, Institute of Geology, Ufimian Scientific Center, Russian Academy of Science (Russian Federation)
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Sulfur isotopic composition in ancient sulfates is a sensitive indicator of the changing of sea water chemistry and biomasse activity during geologic history. Our knowledge of the sulfur isotopic composition of seawater is more complete for widely developed Phanerozoic and fragmentary for scarce Neo- and Mesoproterozoic occurrence of evaporites.
We represent isotopic data for sulfates and sulfides from the Lower (1.65-1.35 Ga) and Middle Riphean (1.35-1.0 Ga) Mesoproterozoic deposits in the eastern Volga-Uralian part of the East-European platform and Southern Urals. In the Mesoproterozoic both of regions belonged to Baltica continent.
The Lower Riphean terrigenous-carbonate Sarapul formation uncovered by deep boreholes in the Volga-Uralian area, contains numerous lithological features of the origin in a tidal flat environment and at arid climate: microbial and fenestral structures, MTS, dolomite composition of carbonates and presence of embedded anhydrites. Among sulfates, synsedimentary and epigenetic types are discriminated. For the first, δ34S variations are 18.6-24.0 ‰, for the latter - 24-43.8 ‰. The pyrite was generated completely by sulfate reduction (δ34S 20 ‰) is also described.
The Lower and Middle Riphean deposits of the Southern Urals which are well correlated with those of the Volga-Uralian area were affected by anchimethamorphism and the most of evaporite features had been lost (Krupenin et. al, 2008). However our isotopic data for dispersed sulfides from sediments of Bolshoi Inzer, Suran, Satka Formations (δ34S from 1.1 to 39‰, mean 16.3‰, n=38) also prove their origin as a result of lower temperature sulfate-reduction of evaporites.
The Middle Riphean Avzyan Formation of the Urals contains barite associated with stratiform Pb-Zn deposits and epigenetic gypsum veins. The limits of δ34S variation for them are more wider: from 21.5 to 42.3 ‰ (n=45), according T.Shirobokova (1992) and our data. The minimum values which do not exceed ∼25 ‰, are often met, that also indicates at participation of sulfur from sulfates in their genesis.
It is importantly to point out the similarity of δ34S values for synsedimentary sulfates from Lower Riphean Sarapul Formation and barites and gypsum from Middle Riphean Avzyan Formation which both cases are close to 20‰. That value is characterstic for modern ocean water and for the most of Phanerozoic evaporitic formations (except Permian). Thus ocean water sulfate perhaps kept a δ34S value close to 20 ‰ during not only in the Phanerozoic, but also in Mesoproterozoic. In such way the circle of sulfur in ocean-continent system was constant during these times. As a result we can approve an idea of a principal similarity of salt composition in ocean and oxygen content in atmosphere in the Mesoproterozoic and modern time (Vinogradov,2007; Huston,Logan,2004 etc.)
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