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

Isotopic compositions of C (δ13C), O (δ18O) and Sr (δ87Sr) were determined for calcite fracture fillings in the crystalline rock penetrated by a 1.6 km drill hole at Laxemar, near the Äspö Hard Rock Laboratory (ÄHRL) in southern Sweden. These calcites precipitated from groundwater some time in the past, and their δ13C, δ18O and δ87Sr 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 δ13C, δ18O and δ87Sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time-dependent evolution of groundwater composition.