Isotopic composition of dissolved Mg in Himalayan river waters

The isotopic composition of a dissolved element is controlled by the isotopic composition of its source rock and any mass dependent fractionation occuring during dissolution. Analyses of Mg-isotopic compositions therefore have the potential application to help determine the relative proportions of Mg derived from carbonate and silicate sources and the extent to which incongruent dissolution govern its concentrations. This is of particular interest for the HimalyanTibetan region where there is debate about the amount of silicate weathering and where the Sr/Sr is not a straightforward proxy of the silicate contribution. We have measured the Mg from 13 tributaries of the Marsyandi watershed and the Langtang in the Nepal Himalaya by a Nu instruments MC ICPMS. The samples chosen for analysis represent a range of chemical compositions thought to correspond to tributaries which are dominated by either a silicate input or a carbonate input with a view to identifying a carbonate end-member and a silicate end-member for Mg. A 1.1‰ range in isotopic compositions is observed with waters in the range -0.7‰ to -1.8‰ relative to the DSM3 standard. The water is, therefore, 2.2 to 3.8 ‰ heavier than the Himalayan limestone [1] Based on the Mg of loess, a proxy for silcates [2], Himalayan tributaries have Mg close to the silicate rock. There seems to be little or no correlation between the Mg and the major cation chemistry of the tributary making silicate and carbonate end-members difficult to identify. Even in the most carbonate-like tributaries (where Mg is <2% of the bedrock), the Mg isotope budget is likely to be controlled by small amounts of silicate present in the bedrock dominating the even smaller amounts of Mg present in low Mg calcite. Simillar discrepencies beween the isotopic composition of low-Mg lithology and corresponding dissolved Mg have been previously reported [1].