Chemical and strontium, oxygen, and carbon isotopic compositions of carbonates from the Lesser Himalaya: implications to the strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus rivers

Abstract

Samples of Precambrian carbonate (mostly dolomite) outcrops collected across the Lesser Himalaya have been analysed for their mineralogy, chemical composition, and isotope ratios of Sr, O, and C to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus. Their Sr concentrations range from 20 to 363 ppm, δ¹⁸O_PDB -1.4 to -12.8% and Mn 11-2036 ppm. The petrography of the samples, their low Sr concentrations, and wide range of δ¹⁸O values are suggestive of their postdepositional alteration. The ⁸⁷Sr/⁸⁶Sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0.7064 to 0.8935 and are generally more radiogenic than that of contemporaneous seawater. Comparison of the ⁸⁷Sr/⁸⁶Sr and Sr/Ca ratios among the carbonates and silicates from the Lesser Himalaya and the source waters of the Ganga, Ghaghara, and the Indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates. An upper limit of carbonate Sr in the various source waters is calculated to be between 6% and 43%, assuming that all the Ca in the rivers is of carbonate origin. The results show that on the average, weathering of the Precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters; however, they can be a dominant supplier of radiogenic Sr to some rivers on a regional scale. The silicate Sr component in some of the source waters of the Ganga (Bhagirathi, Bhilangna, Alaknanda, and Ganga), Ghaghara (Kali and Sarju), and the Indus (Sutlej) was calculated from the Ca/Na, Sr/Na ratios, and strontium isotope compositions of these rivers and the silicate endmember. These calculations suggest that 33-89% of Sr in the Bhagirathi, Bhilangna, Alaknanda, Ganga, and Sarju rivers is of silicate origin, whereas in the Kali and the Sutlej it is much lower, only ~8%. The remaining Sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites. This study underscores the importance of weathering of silicates, carbonates, and evaporites in contributing to the Sr mass balance and ⁸⁷Sr/⁸⁶Sr of the source waters of the Ganga, Ghaghara, and the Indus. The present day silicate and carbonate Sr contributions to the Sr budget of the rivers vary considerably, but among the major source waters of the Ganga, silicate Sr exerts a more dominant control on their Sr abundance and ⁸⁷Sr/⁸⁶Sr.