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The U-Pb isotope system and uranium isotope composition (235U/238U and 234U/238U) were studied in a number of samples from the vertical section of the uranium ore body at the Vershinnoe sandstone-type deposit, Vitim uranium ore district, Russia. These parameters were determined to broadly vary. Deviations of the 234U/238U ratio from the equilibrium value indicate that the uranium ore was not completely conserved during the postore stage, and uranium was determined to continue migrating at the deposit. Comparison of the U−Pb isotope age value and 234U/238U isotope ratio provides an insight into the migrate direction of uranium in the ore body. The broad variations (137.377–137.772) in the 238U/235U ratio over the vertical section of the ore body can be explained by the different settings of the samples relative to the ore deposition front and changes in the redox conditions when this front shifted. The fact that the δ238U and K234/238 values are correlated indicates that the transfer of the 234U isotope into the aqueous phase may have been coupled with isotope fractionation in the 238U−235U system during the postformation uranium migration within the orebody. [ABSTRACT FROM AUTHOR]

It has been suggested that the carbonated mantle reflected by Mg−Zn isotopic anomalies of Cenozoic intraplate basalts from East Asia coincides with the stagnant West Pacific slab in the mantle transition zone. However, the northern boundary of such carbonated domain beneath East Asia is uncertain. Late Mesozoic-Cenozoic intraplate basalts are widespread in far eastern Russia and thus provide an opportunity to examine this issue. Here we report major-trace element contents and Sr−Nd-−Mg−Zn isotopic compositions for 9 Late Mesozoic-Cenozoic basaltic samples from the Khanka Block and Sikhote-Alin accretionary complex. They are characterized by large variations in SiO2 contents (41 wt.% to 50 wt.%) and CaO/Al2O3 (0.50 to 0.97), enrichments of large-ion lithophile elements (LILE), positive Nb−Ta anomalies and strongly negative K, Pb, Zr, Hf, Ti, Y anomalies in primitive mantle-normalized trace element spider diagram. Furthermore, the rocks show good correlations of Ti/Ti* with Hf/Hf*, La/Yb, Fe/Mn and trace element contents (e. g., Nb). In addition, they have lighter Mg and heavier Zn isotope compositions than the BSE estimates, coupled with depleted Sr−Nd isotope compositions. These elemental and isotopic characteristics cannot be explained by alteration, magma differentiation or diffusion, but are consistent with the partial melting of carbonated peridotite. By and large, the Late Mesozoic-Cenozoic basalts from far eastern Russia bear very similar geochemical characteristics as those Na-series Cenozoic basalts from eastern China. The extended region of Mg−Zn isotopic anomalies is roughly coincident with the stagnant West Pacific slab beneath East Asia, and all of these alkali basalts can be generated from mantle sources hybridized by recycled Mg-carbonates from the Pacific slab stagnant in the mantle transition zone. We infer that (1) the carbonated big mantle wedge extends to the NE edge of the West Pacific slab and may have also appeared in the Late Mesozoic due to the effect of the Paleo-Pacific slab beneath this region, and (2) decarbonation of stagnant slabs in the mantle transition zone is a key mechanism for carbon outgassing from deep mantle to surface via intraplate alkali melts. [ABSTRACT FROM AUTHOR]