Calcite fracture fillings as indicators of paleohydrology at Laxemar at the Aspo Hard Rock Laboratory, southern Sweden

Isotopic compositions of C (??13C), O (??18O) and Sr (??87Sr) were determined for calcite fracture fillings in the crystalline rock penetrated by a 1.6 km drill hole at Laxemar, near the Aspo Hard Rock Laboratory (AHRL) in southern Sweden. These calcites precipitated from groundwater some time in the past, and their ??13C, ??18O and ??87Sr values reflect those of the source waters. The present-day groundwater system is hydrochemically stratified with highly saline water underlying more shallow brackish and fresh water. The origin of this stratified system is probably related to past glaciations although the ultimate origin of the deep, highly saline water is still problematical. None of the calcite fracture fillings sampled below 900 m could have precipitated from any of the present-day ground waters which in view of the glacial history of the region is not surprising. However, several shallow calcite fracture fillings are formed by precipitation from the present-day groundwater. Coupled variations in ??13C, ??18O and ??87Sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time-dependent evolution of groundwater composition.