Geologic control of Sr and major element chemistry in Himalayan Rivers, Nepal

Abstract

Our study of the Seti River in far western Nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology. The Seti flows through four distinct terranes, starting with the Tethyan sedimentary series (TSS) and Greater Himalayan series (GHS). TSS/GHS waters display 87Sr/ 86Sr ratios of <0.73 and high Sr and Ca, consistent with the composition of limestone and marble common in these terranes. The 87Sr/ 86Sr ratio and Mg increase markedly as the river passes into the Lesser Himalayan series (LHS), where tributaries have 87Sr/ 86Sr ratios from 0.75 to 1.02 and high Sr, Ca, and Mg. The high Mg in LHS waters correlate with high 87Sr/ 86Sr ratios, which we attribute to weathering of highly radiogenic (0.71–0.82) dolostones. Tributaries to the Seti River draining the largely carbonate-free Dadeldhura thrust sheet (DTS) have ratios near 0.74, but low Sr, Ca, and Mg and therefore have little impact on Seti mainstem chemistry. Mass balance calculations and CaMg-weathering indices show that carbonate weathering accounts for >70% of total dissolved solids to the Seti River. Sr/Ca ratios of river waters provide a minimum estimate of the %-carbonate weathering contribution to Sr, due to partitioning of Sr and Ca during incongruent dissolution and reprecipitation of calcite. Overall, we attribute high 87Sr/ 86Sr ratios in the Seti River and its tributaries to the weathering of metacarbonates (especially dolostones in the upper Nawakhot Group) which have exchanged Sr with silicates during metamorphism. Our modeling of Sr fluxes in the Seti River indicates that the TSS/GHS accounts for 36–39% of the Sr, the LHS for 40–53%, and 8–23% for the DTS. Prior to exposure of LHS rocks at ∼12 Ma, TSS and GHS carbonates with low 87Sr/ 86Sr ratios dominated Himalayan rivers. We attribute the elevated 87Sr/ 86Sr ratios of Himalayan paleorivers during the late Miocene and Pliocene to exposure and weathering of LHS metacarbonates.