~(238)U series isotopes and ~(232)Th in carbonates and black shales from the Lesser Himalaya: implications to dissolved uranium abundances in Ganga-Indus source waters

摘要

~(238)U and ~(232)Th concentrations and the extent of ~(238)U-~(234)U-~(230)Th radioactive equilibrium have been measured in a suite of Precambrian carbonates and black shales from the Lesser Himalaya. These measurements were made to determine their abundances in these deposits, their contributions to dissolved uranium budget of the headwaters of the Ganga and the Indus in the Himalaya and to assess the impact of weathering on ~(238)U-~(234)U-~(230)Th radioactive equilibrium in them. ~(238)U concentrations in Precambrian carbonates range from 0.06 to 2.07 μg g~(-1). The 'mean' U/Ca in these carbonates is 2.9 ng U mg~(-1) Ca. This ratio, coupled with the assumption that all Ca in the Ganga-Indus headwaters is of carbonate origin and that U and Ca behave conservatively in rivers after their release from carbonates, provides an upper limit on the U contribution from these carbonates, to be a few percent of dissolved uranium in rivers. There are, however, a few streams with low uranium concentrations, for which the carbonate contribution could be much higher. These results suggest that Precambrian carbonates make only minor contributions to the uranium budget of the Ganga-Indus headwaters in the Himalaya on a basin wide scale, however, they could be important for particular streams. Similar estimates of silicate contribution to uranium budget of these rivers using U/Na in silicates and Na* (Na corrected for cyclic and halite contributions) in river waters show that silicates can contribute significantly (～40% on average) to their U balance. If, however, much of the uranium in these silicates is associated with weathering resistant minerals, then the estimated silicate uranium component would be upper limits. Uranium concentration in black shales averages about 37 μg g~(-1). Based on this concentration, supply of U from at least ～50 mg of black shales per liter of river water is needed to balance the average river water U concentration, 1.7 μg L~(-1) in the Ganga-Indus headwaters. Data on the abundance and distribution of black shales in their drainage basin are needed to test if this requirement can be met. ~(234)U/~(238)U activity ratios in both carbonates and black shales are at or near equilibrium, thus preferential mobilization of ~(234)U from these deposits, if any, is within analytical uncertainties. ~(230)Th is equivalent to or in excess of ~(238)U in most of the carbonates. ~(230)Th/~(238)U > 1 indicates that during weathering, uranium is lost preferentially over Th. ~(232)Th concentrations in carbonates are generally quite low, <0.5 μg g~(-1), though with a wide range, 0.01-4.8 μg g~(-1). The variation in its concentrations seem to be regulated by aluminosilicate content of the carbonates as evident from the strong positive correlation between ~(232)Th and Al.