Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara- Indus system is investigated from the available data. The contributions fro m silicate weathering are determined from the composition of granites/ gnei sses, soil profiles developed from them and from the chemistry of rivers fl owing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate t hat on an average similar to 77% (Na + K) and similar to 17% (Ca + Mg) in t hese rivers is of silicate origin. The silicate Sr component in these water s average similar to 40% and in most cases it exceeds the carbonate Sr. The observations that (i) the Sr-87/Sr-86 and Sr/Ca in the granites/gneisses b racket the values measured in the head waters; (ii) there is a strong posit ive correlation between Sr-87/Sr-86 Of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The gener ally low Sr-87/Sr-86 (less than or equal to 0.720) and Sr/Ca (similar to 0. 2 nM/ mu M) in the Precambrian carbonate outcrops rules them out as a major source of Sr and Sr-87/Sr-86 i, the headwaters on a basin-wide scale, howe ver, the high Sr-87/Sr-86 (similar to 0.85) in few of these carbonates sugg ests that they can be important for particular streams. The analysis of Sr- 87/Sr-86 and Ca/Sr data of the source waters show that they diverge from a low Sr-87/Sr-86 and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candi dates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimat ing the present-day silicate and carbonate weathering rates in the Himalaya and associated CO2 consumption rates and their global significance.