U-series disequilibria in minerals from Gandak River sediments (Himalaya)

The impact of mineralogical sorting on U-Th-Ra variations observed for the Ganga sediments in the Himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the U-series isotopic system in Himalayan minerals. For this purpose, U, Th, and Ra concentrations, along with 238U-234U-230Th-226Ra radioactive disequilibria, were measured in several pure mineral fractions, i.e. monazite, zircon, titanite, ilmenite, rutile, garnet, magnetite, quartz, biotite and muscovite, separated from one bank sediment of the Gandak River (Ganga Basin). The data, obtained through a double digestion protocol in high-pressure and high-temperature vessels, confirm that U and Th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in U and Th, namely, monazite, titanite and epidote, as well as major minerals with lower U and Th concentrations, such as quartz. More importantly, the data clearly indicate that, similarly to whole rock samples, the U-Th-Ra systematics of individual minerals have likely recorded complex U-Th-Ra fractionations. In particular, the 226Ra enrichment observed in the biotite and clay fractions likely results from the affinity of Ra for phyllosilicate minerals, while the 230Th and, to a lesser extent the 234U enrichment observed for the other minerals might result from a nuclide adsorption process on the Fe-oxide coatings present on the grains or directly on the mineral surface. The linear trend outlined by the minerals phases in the (226Ra/230Th) vs (230Th/234U) diagram suggests that 230Th is probably mobilized from zircons, the only fraction presenting (230Th/234U) < 1, and from monazites, and redistributed among the all minerals during weathering. Such a scenario suggest that zircon weathering, evidenced in this study, must have been very recent, estimated < 2000 years by a modeling approach, including simulation of nuclides mobility by alpha-recoil ejection and loss and gain processes.