RARE-EARTH ELEMENTS IN CHLORIDE-RICH GROUNDWATER, PALO-DURO BASIN, TEXAS, USA

Rare earth element (REE) data for groundwater samples from the Deep-Basin Brine aquifer of the Palo Duro Basin, Texas, USA, illustrates the potential use of REE for inferring groundwater flow paths through different geologic materials. The REE content of the groundwaters range over 2.5 orders of magnitude and are depleted by 10(2) to 10(5) relative to aquifer materials. The shale-normalized REE patterns for groundwater that have primarily interacted with arkosic sandstones (granite wash) are flat with similar heavy REE (HREE) enrichments [(Lu/La)n = 0.60 to 0.80]. The samples with highest REE contents and REE patterns, which are enriched in the intermediate REEs (IREEs; Sm-Tb) reflect variable degrees of interaction with carbonate rocks. The IREE enrichment is the result of fluid interaction with Fe-Mn coatings on carbonate minerals and/or secondary minerals in fractures and vugs. The chloride complex, [LnCl2+], and free-ions are the predominant REE species, accounting for over 95% of the REEs. Carbonate and sulfate species account for the other 5% and have very little influence on the behavior of the REEs. Although this study indicates a potentially important role for the REEs in understanding geochemical transport and groundwater movement, it also indicates the necessity for developing a better understanding of REE speciation in high ionic strength solutions.