Chemical analysis of palaeogroundwaters: a new frontier for fluid inclusion research

Using a combination of fluid inclusion and mineralogical techniques, chemic al data have been obtained that help characterise the hydrogeochemistry of palaeogroundwaters in the onshore section of the Eastern Irish Sea Basin, n orthwest England. The study area, Sellafield, provided an excellent suite o f fracture-controlled carbonate cements deposited from shallow to deeply ci rculating late Triassic to Recent groundwaters. Methodologies developed for the analysis of hydrothermal fluid inclusions were extended to encompass a wide range of low temperature aqueous inclusions. These included single in clusion chemical analysis by UV laser ablation ICP mass spectrometry (Na, K , Mg, Sr, Li, Mn, Fe:) and high precision salinity measurements (+/-1000 pp m TDS) by microthermometric analysis. Closely integrated with these measure ments were calcite morphology and cathodoluminescence (CL) studies that wer e undertaken to provide a relative chronology 'stratigraphy' of carbonate c ementation and information on redox conditions and salinity with depth. The results demonstrate a clear distinction between the chemistry of the la te Triassic groundwaters and the present day, deep, saline groundwaters. Th ough both have TDS values >100,000 ppm, the former are Ca-Na-Cl brines with Na/Ca wt ratios of 2:1, whilst the latter are Na-Cl brines with Na/Ca wt, ratios greater than 20. For younger generations of calcite, attributable to deposition from groundwater during the Quaternary, TDS values are <100,000 ppm with salinities typically less than 20,000 ppm. Laser ablation analyse s of aqueous inclusions in these younger calcites for Na, Sr and Mg plot ex actly within the fields defined by the present day saline to brackish groun dwaters, and display similar trends. CL patterns for the Quaternary calcite s are primarily a function of trace element impurities (Fe and Mn), but all show a marked contrast at the fresh water-saline water transition. This co ntrast is also reflected in the morphology of the calcite crystals; from c- axis shortened 'nailhead' forms in the fresh groundwater zone to c-axis elo ngated 'scalenohedra' forms in the deeper saline zones. Implications for th e evolution of the palaeogroundwaters in response to Tertiary uplift with r espect to present day groundwater regimes are briefly discussed. (C) 2000 E lsevier Science B.V. All rights reserved.