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

Abstract

Concentrations of the rare earth elements (REE) were measured in circumneutral pH groundwaters from southern Nevada and Death Valley, CA. Groundwaters from the regional lower Paleozoic carbonate-rock aquifer (Cambrian–Devonian) have flat shale-normalized patterns that closely resemble the shale-normalized patterns of the aquifer rock samples (principally Cambrian). Groundwaters associated with younger carbonate rocks (chiefly Permian) in the study region exhibit heavy REE (HREE) enriched, shale-normalized REE patterns with 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 large negative Eu anomalies as the felsic volcanic rocks. The similar REE patterns of all the groundwaters and associated aquifer rocks studied suggest that the groundwaters inherited REE signatures from the host rocks through which 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. Previously, we demonstrated that carbonate complexes dominate REE speciation in southern Nevada and Death Valley groundwaters. Moreover, solid–liquid partitioning coefficients ( K d) indicate that the affinity of LREEs to sorb to aquifer surface sites is substantially greater than for the HREEs in the southern Nevada carbonate- and felsic volcanic-rock alluvial aquifers. Consequently, the HREEs enrichments reported here for groundwaters associated with younger Paleozoic carbonate rocks compared to these source rocks is consistent with REE carbonate complexation and preferential removal of LREEs to aquifer surface sites.