Comparison of the Sr isotopic signatures in brines of the Canadian and Fennoscandian shields

摘要

A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fermoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1-75 g L-1 below 1500 m depth in the Fermoscandian Shield and from 10 up to 300 g L-1 below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fermoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of Sr-87/Sr-86 ratios for the deep brines in different selected sites. All sites show a specific Sr-87/Sr-86 signature and the occurrence of large Sr-87/Sr-86 variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the Sr-87/Sr-86 ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fermoscandian, and the well-constrained Sr-87/Sr-86 signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water-rock interaction. (c) 2004 Elsevier Ltd. All rights reserved.