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

Abstract

Using a combination of fluid inclusion and mineralogical techniques, chemical data have been obtained that help characterise the hydrogeochemistry of palaeogroundwaters in the onshore section of the Eastern Irish Sea Basin, northwest England. The study area, Sellafield, provided an excellent suite of fracture-controlled carbonate cements deposited from shallow to deeply circulating 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 inclusion chemical analysis by UV laser ablation ICP mass spectrometry (Na, K, Mg, Sr, Li, Mn, Fe) and high precision salinity measurements (±1000ppm TDS) by microthermometric analysis. Closely integrated with these measurements were calcite morphology and cathodoluminescence (CL) studies that were undertaken to provide a relative chronology ‘stratigraphy’ of carbonate cementation and information on redox conditions and salinity with depth.The results demonstrate a clear distinction between the chemistry of the late Triassic groundwaters and the present day, deep, saline groundwaters. Though both have TDS values >100,000ppm, 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 <100,000ppm with salinities typically less than 20,000ppm. Laser ablation analyses of aqueous inclusions in these younger calcites for Na, Sr and Mg plot exactly within the fields defined by the present day saline to brackish groundwaters, and display similar trends. CL patterns for the Quaternary calcites 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 contrast is also reflected in the morphology of the calcite crystals; from c-axis shortened ‘nailhead’ forms in the fresh groundwater zone to c-axis elongated ‘scalenohedra’ forms in the deeper saline zones. Implications for the evolution of the palaeogroundwaters in response to Tertiary uplift with respect to present day groundwater regimes are briefly discussed.