We present a continuous Sr-isotope depth profile of waters, sampled in situ in the deepest borehole in Finland (∼1.1 km). The waters, all with a meteoric oxygen- and hydrogen-isotope signature, are compositionally stratified; a recent fresh-water zone (0.05 mg/L Sr, 87Sr/86Sr >0.730) ∼400 m thick is underlain by two saline-water zones, the upper with ∼25 mg/L Sr and 87Sr/86Sr = ∼0.724, the lower with 45 mg/L Sr and 87Sr/86Sr = ∼0.723. The Sr-isotope data show that the stratification is not due merely to upward dilution of saline water by fresh surface waters; two discrete saline waters are present. The two saline-water components are homogeneous in composition, apparently unaffected by local variations in rock composition either within the borehole or between neighboring boreholes. The implication is that the saline waters did not develop in isolated pockets but, rather, in large, well-mixed bodies. Most likely salinization mechanisms involve breakdown of low-Rb/Sr minerals (e.g., plagioclase feldspar) and/or leaching of fluid inclusions.

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