REE+Y patterns and positive europium anomalies recorded in limestones from zone XX of the Tlayúa Quarry Konservat-Lagerstätte (Tlayúa Formation, Lower Cretaceous; Tepexi de Rodríguez, Mexico)

Abstract

Major and trace element concentrations of thirteen limestone samples from zone XX of the Tlayua Quarry Fossillagerstatte were analysed with the aim of elucidating the source of rare earth elements plus yttrium (REE+Y) as well as some palaeoenvironmental conditions attributable to them. The concentrations of cortex-derived elements/element oxides (SiO2, Al2O3, Zr), as well as of phosphorus, show no significant contamination by means of terrigenous material and/or phosphates, which means that the REE+Y concentrations can be interpretable. REE+Y patterns display partly typical modern seawater signals, particularly positive La and Y anomalies, mostly superchondritic Y/Ho ratios (between 42.34 and 56.38) and moderate depletions of light rare earth elements (LREE) and middle rare earth elements (MREE) in comparison to heavy rare earth elements (HREE) (x PrSN/YbSN = 0.12, x NdSN/YbSN = 0.14 and x DySN/YbSN = 0.42, respectively). Negative cerium anomalies could also be identified [(Ce/Ce*SN) = 0.73], which indicate oxidising conditions. In addition, positive Eu anomalies [(Eu/Eu*SN) = 2.53] were detected. Since no convincing evidence for diagenesis, hydrothermal or aeolian input exists, the origin of these anomalies can, to some extent, be attributable to the influence of the dissolution of plagioclases in the water column, which can be inferred from the bivariate correlation of (Eu/Eu*)SN and Na/Al. Despite depicting modern seawater-like signals, an important difference between the REE+Y patterns from zone XX of the Tlayua Quarry and modern seawater REE+Y patterns can be determined. For instance, modern seawater does not display significant europium anomalies. Nevertheless, REE+Y patterns from zone XX of the Tlayua Formation can altogether be reliable palaeoenvironmental proxies for the Cretaceous. The inferred palaeoenvironment for zone XX from these proxies is represented by an open-marine setting, showing oxidising conditions, evidence of plagioclase dissolution and overall negligible terrigenous influence. The palaeoecological interpretation from these patterns does not exclude, however, the probability of monsoonal or tidally influenced sedimentation in other sediments from zone XX or from other zones in Tlayua, as previous works suggest.