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

Our study of the Seti River in far western Nepal shows that the solute chem istry of the river and its tributaries is strongly controlled by geology. T he Seti flows through four distinct terranes, starting with the Tethyan sed imentary series (TSS) and Greater Himalayan series (GHS). TSS/GHS waters di splay Sr-87/Sr-86 ratios of <0.73 and high Sr and Ca, consistent with the c omposition of limestone and marble common in these terranes. The Sr-87/Sr-8 6 ratio and Mg increase markedly as the river passes into the Lesser Himala yan series (LHS), where tributaries have Sr-87/Sr-86 ratios from 0.75 to 1. 02 and high Sr, Ca, and Mg. The high Mg in LHS waters correlate with high S r-87/Sr-86 ratios, which we attribute to weathering of highly radiogenic (0 .71-0.82) dolostones. Tributaries to the Seti River draining the largely ca rbonate-free Dadeldhura thrust sheet (DTS) have ratios near 0.74, but low S r, Ca, and Mg and therefore have little impact on Seti mainstem chemistry. Mass balance calculations and CaMg-weathering indices show that carbonate w eathering accounts for >70% of total dissolved solids to the Seti River. Sr /Ca ratios of river waters provide a minimum estimate of the %-carbonate we athering contribution to Sr, due to partitioning of Sr and Ca during incong ruent dissolution and reprecipitation of calcite. Overall, we attribute hig h Sr-87/Sr-86 ratios in the Seti River and its tributaries to the weatherin g of metacarbonates (especially dolostones in the upper Nawakhot Group) whi ch 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 LH S rocks at similar to 12 Ma, TSS and GHS carbonates with low Sr-87/Sr-86 ra tios dominated Himalayan rivers. We attribute the elevated Sr-87/Sr-86 rati os of Himalayan paleorivers during the late Miocene and Pliocene to exposur e and weathering of LHS metacarbonates. Copyright (C) 2000 Elsevier Science Ltd.