REE fractionation during the gypsum crystallization in hyperacid sulphate-rich brine: The Poás Volcano crater lake (Costa Rica) exploited as laboratory

The critical role of rare earth elements (Lanthanides plus Yttrium; hereafter REE) in high-tech technologies and consequently their increasing demand from the industry, in addition to the capability of REE to trace water–rock interaction processes, boosted the study of REE in unconventional extreme environments. This study is focused on the geochemical behaviour of REE in the hyperacid sulphate-rich brine of the crater lake of Poas volcano (Costa Rica), where the precipitation of gypsum occurs. This system can hence be considered as a natural laboratory to evaluate the fractionation of REE between the lake water (mother brine) and the precipitating gypsum mineral. Total REE concentrations dissolved in waters range from 1.14 to 2.18 mg kg−1. Calculated distribution coefficients (KD) for REE between the gypsum and the mother brine indicate a preferential removal of the light REE (LREE) with respect to the heavy REE (HREE), with KD values mainly decreasing from La to Lu. During the observation period (2007–2009), the distributions of REE concentrations dissolved in lake water normalized to the average local volcanic rock show two different trends: i) LREE depleted patterns, and ii) flat patterns. The identification of the LREE depleted pattern is justified by the KD calculated in this study. We demonstrate that the precipitation of gypsum is able to strongly fractionate the REE in hyperacid sulphate-rich brine, inducing changes in REE concentrations and distributions over time. X-ray computed tomography imaging was performed on gypsum crystal (precipitated from the lake waters) to gain insights on crystal-scale processes possibly controlling the REE geochemistry, i.e. surface processes vs. structural substitution. Accordingly, the heavy metals and possibly the REE seem to be mainly located on the crystal surface rather than inside the crystal, suggesting that a surface process could be the major process controlling REE removal from the water to the crystal.