Magnesium isotope systematics of the lithologically varied Moselle river basin, France

Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high Sr-87/Sr-86 ratios (0.7128-0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, Sr-87/Sr-86 ratios are lower, down to 0.70824. Magnesium in river waters draining silicates is systematically depleted in heavy isotopes (delta Mg-26 values range from -1.2 to -0.7 parts per thousand) relative to the value presently estimated for the continental crust and a local diorite (-0.5 parts per thousand). In comparison, delta Mg-26 values measured in soil samples are higher (similar to 0.0 parts per thousand). This suggests that Mg isotope fractionation occurs during mineral leaching and/or formation of secondary clay minerals. On the Lorraine plateau, tributaries draining marls, carbonates and evaporites are characterized by low Ca/Mg (1.5-3.2) and low Ca/Sr (80-400) when compared to local carbonate rocks (Ca/Mg = 29-59; Ca/Sr = 370-2200), similar to other rivers draining carbonates. The most likely cause of the Mg and Sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer. delta Mg-26 of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year (from -1.4 parts per thousand to -1.6 parts per thousand and from -1.2 parts per thousand to -1.4 parts per thousand, respectively), within the range defined for the underlying rocks. Downstream of Epinal, the compositions of the Moselle River samples in a delta Mg-26 VS. Sr-87/Sr-86 diagram can be explained by mixing curves between silicate, carbonate and evaporite waters, with a significant contribution from the Vosgian silicate lithologies (>70%). Temporal co-variation between delta Mg-26 and Sr-87/Sr-86 for the Moselle River throughout year is also observed, and is consistent with a higher contribution from the Vosges mountains in winter, in terms of runoff and dissolved element flux. Overall, this study shows that Mg isotopes measured in waters, rocks and soils, coupled with other tracers such as Sr isotopes, could be used to better constrain riverine Mg sources, particularly if analytical uncertainties in Mg isotope measurements can be improved in order to perform more precise quantifications. (C) 2008 Elsevier Ltd. All rights reserved.