U-depleted versus U-enriched signatures in complementary crust-mantle sources of volcanic rocks from Asia

The Nb/U~47 and Th/U~4 ratios are considered as indicative for the OIB source referred by some authors to lower mantle plumes that in fact have no specific geochemical signatures but HIMU component. The Th/U ratio may vary because of the different garnet–melt and/or clinopyroxene–melt partition coefficients of U and Th. Anomalously high or low Th/U values in rocks can also be related to the input or removal of U, the migration of which is controlled by its mobility under oxidizing conditions owing to the formation of water-soluble uranyl  compounds with hexavalent U. These variations definitely distinguish non-plume magmatic sources. The Th/U ratio decreases to 2.5 in the MORB source and increases to 6 in the continental lower crust one. We describe anomalous behavior of uranium in sources of Cenozoic basalts and basaltic andesites from Primorye, Lesser Khingan, Tunka Valley, as well as similar Cretaceous-Paleogene rocks from Tien Shan. Significant deviations of the Th/U and Nb/U ratios from the OIB values are characteristics mostly of garnet-free sources. The U-depleted and U-enriched signatures are used as sensitive indicators for deciphering crust–mantle transitional processes.

This work is supported by the RSF grant 18-77-10027.