Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio

A study was carried out with a representative data set of volcanic lakes from the Azores archipelago. A total of 672 samples were collected during four field surveys conducted over the year and along the depth. Following water sampling, temperature, pH, and EC were measured, the dissolved CO2 and alkalinity were determined by titration, and aliquots were taken to perform analysis of major, minor and trace elements, as well as 18O/16O, 2H/1H and 87Sr/86Sr isotopic ratios. Waters are of meteoric origin and from the Na-HCO3 to Na-Cl types. The 87Sr/86Sr ranges between 0.709194 and 0.704294, and most of the lakes depict less radiogenic values than seawater, suggesting a potential contribution from rock dissolution. Along the reciprocal of the Sr vs. 87Sr/86Sr plot, most samples suggest a linear trend between rock values and rainwater. Samples display considerable variability in the ∑REE, ranging from 0.83 µg L−1 to 13.54 µg L−1, and when chondrite normalized, depict a negative slope, showing an enrichment in light REEs compared to heavy REEs. This pattern is consistent with the one from Azores rocks and bottom sediments from some lakes, and most lakes depict Eu anomalies, resulting from interaction between water and sediments or from incongruent mineral dissolution.