Origin of rare earth element signatures in groundwaters of circumneutral pH from southern Nevada and eastern California, USA

Concentrations of the rare earth elements (REE) were measured in circumneut ral pH groundwaters from southern Nevada and Death Valley, CA. Groundwaters from the regional lower Paleozoic carbonate-rock aquifer (Cambrian-Devonia n) have flat shale-normalized patterns that closely resemble the shale-norm alized patterns of the aquifer rock samples (principally Cambrian). Groundw aters associated with younger carbonate rocks (chiefly Permian) in the stud y region exhibit heavy REE (HREE) enriched, shale-normalized REE patterns w ith substantial negative Ce anomalies that also mimic these carbonate rocks . In addition, groundwaters from the felsic volcanic rock aquifers have the same flat to light REE (LREE) enriched shale-normalized patterns with larg e negative Eu anomalies as the felsic volcanic rocks. The similar REE patte rns of all the groundwaters and associated aquifer rocks studied suggest th at the groundwaters inherited REE signatures from the host rocks through wh ich they flow. Because negative Ce anomalies are not an uncommon feature of carbonate rocks of marine origin, the negative Ce anomalies reported here for these groundwaters may reflect a Permian marine Ce signature. Previousl y, we demonstrated that carbonate complexes dominate REE speciation in sout hern Nevada and Death Valley groundwaters. Moreover, solid-liquid partition ing coefficients (K-d) indicate that the affinity of LREEs to sorb to aquif er surface sites is substantially greater than for the HREEs in the souther n Nevada carbonate- and felsic volcanic-rock alluvial aquifers. Consequentl y, the HREEs enrichments reported here for groundwaters associated with you nger Paleozoic carbonate rocks compared to these source rocks is consistent with REE carbonate complexation and preferential removal of LREEs to aquif er surface sites. (C) 2000 Elsevier Science B.V. All rights reserved.