Strontium isotope systematics used to decipher the origin of groundwaters sampled from granitoids: the Vienne Case (France)

摘要

Sr isotope data from surface, shallow and deep groundwaters from the granitoids of the Vienne District (France) are presented in this paper. In surface waters, the Sr contents in the rocks and groundwaters agree with previous data for groundwaters sampled from granitic and sedimentary rocks in France where a large range in Sr-87/Sr-86 ratios is observed. After correction for the Sr input from rain, the surface water samples plot within a mixing field that can be explained by three end-members, one anthropogenic (low Sr-87/Sr-86 and high Cl/Sr ratio) and two end-members characterised by low Cl/Sr ratios and a large range in Sr-87/Sr-86 ratios (from around 0.707-0.720). For deep groundwaters, the Sr-87/Sr-86 ratios and Sr contents are also determined by applying a correction to account for the influence of cleaning waters during drilling operations. The results are scattered amongst five different groups and the lack of a direct linear relationship between any of the samples implies that, as found for the shallow groundwaters, the results are due to mixing between more than two end-members. A model to determine the Sr-87/Sr-86 ratio (Irf) of groundwater after interaction with an actively weathering granite is developed. The results yield a low Irf value for waters associated with weathering of the tonalite (0.70463) and a higher one for waters associated with weathering of the monzogranite (0.70704). Given the much higher Irf values derived from the deep groundwater samples, these results indicate that the deep groundwaters analysed within the Vienne hydrosystem cannot be directly related to weathering of either tonalite or monzogranite. It is speculated that this high Sr-87/Sr-86 source Originated from marine incursions during the Jurassic and have been diluted by mixing with former groundwaters produced by water-rock interaction (WRI) with the granites. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 80]