Geochemistry of Rare Earth Elements in Groundwaters from a Rhyolite Aquifer, Central México

Abstract

Rare earth element (REE) concentrations were measured in groundwaters collected from wells finished in a fractured, rhyolitic (Cuatralba Ignimbrite) aquifer from the La Muralla region of the central Mexican State of Guanajuato. The study site is located within the Faja Volcanica Transméxicano (i.e., Trans-Mexican Volcanic Belt), an extensive region of active volcanism within central México. La Muralla groundwaters are relatively warm (32.2 ± 2.7C), dilute Na-Ca-HCO 3 waters (5.1 mmol/kg I 9.5 mmol/kg) of circumneutral pH (7.27 pH 8.01). Concentrations of REEs in La Muralla groundwaters are exceedingly low, as demonstrated by Nd values, which range from ~ 10 pmol/kg to 34 pmol/kg. La Muralla groundwaters exhibit enrichments in the heavy REEs (HREE) over the light REEs (LREE) compared to Average Shale, as well as volcanic rocks from the Trans-Mexican Volcanic Belt, including rhyolitic volcanic rocks similar to those of the Cuatralba Ignimbrite aquifer. Shale-normalized Yb/Nd ratios of La Muralla groundwaters range from 1.85 to 6.55, with a mean (± standard deviation) of 4.2 ± 1.2. Rare earth element concentrations for La Muralla groundwaters are normalized to the average REE values of 27 different calc-alkaline rhyolites (from the literature) from the Trans-Mexican Volcanic Belt. The average Trans-Mexican Volcanic Belt rhyolite-normalized Yb/Nd ratios for La Muralla groundwaters range from 1.57 to 5.55, with a mean (± standard deviation) of 3.52 ± 1. Speciation calculations predict that REEs occur principally as carbonate complexes in La Muralla groundwaters, with LREEs predominantly in the form of positively charged, carbonato complexes (LnCO 3 +), and to a lesser extent, free metal ions (Ln 3+), and HREEs chiefly in solution as negatively charged, dicarbonato complexes (Ln(CO 3) 2-). The speciation model predictions suggest that the HREE enrichment of La Muralla groundwaters originate from solution and surface complexation reactions within the system. Specifically, the preferential complexation of HREEs as negatively charged, dicarbonato complexes acts to stabilize HREE is solution owing to both the strength of these complexes and their low affinity for aquifer surface sites. Because La Muralla groundwaters are of circumneutral pH, surface complexation sites within the Cuatralba Ignimbrite are expected to predominantly be negatively charged. Therefore, because LREEs occur primarily as positively charged, carbonato complexes in La Muralla groundwaters, they are preferentially removed from solution owing to complexation to