REEs and U distribution in P-rich nodules from Gelasian Apulian Tethyan carbonate: A genetic record

Abstract

In the Tethyan realm the carbonate-dominated Meso-Cenozoic South Tethyan Phosphogenic Province is ofconsiderable economic importance since it represents the greatest accumulation of sedimentary phosphorites. Insouthern Italy, in the Salentine peninsula (the southern part of the Apulian Carbonate Platform, ACP), is welldocumented the occurrence of Cenozoic P-rich levels consisting of nodules and pebbles and showing a large P2O5fluctuation (4.07–22.07 wt%), due to variable calcite abundance. The mainly observed P-bearing minerals arehydroxyapatite and carbonatefluorapatite and U, together with Sr and Pb, preferentially acts as Ca substitutes inboth lattices. Minor authigenic monazite (LREE-bearing phosphate) and xenotime (HREE+Y-bearing phos-phate), likely formed during sediment burial compaction and diagenesis. The total REEs' abundances and theshape of the shale-normalized REE-patterns in the P-rich nodules are in the range of those typically observed inPaleocene-Eocene through Pleistocene-Recent P-rich sediments, supporting the idea of a broadly consistentocean chemistry in this span of geological time. The (La/Yb)Nproxy is within the modern seawater range,signifying early diagenetic adsorption played only a minor role in affecting the REEs' distribution. The Ce and Pranomalies suggest some P-rich nodules experienced very localized hypoxic to anoxic conditions promoting Ce/Ce* spurious results in a generally oxic to hypoxic environment causing real negative Ce anomaly. This scenariois reinforced by the lack of the coupled uranium-vanadium enrichment typically observed in an anoxic en-vironment. Since the Pliocene the eastern Mediterranean was variously affected by oxygenation and productivityand it is likely the ACP phosphate-rich sediments formed under low sedimentation rates and authigenesis in abottom current-dominated regime, as also indicated by the glauconite occurrence. Transgressions and sea levelsrising following major glaciations may have favored the deposition of phosphate sediments by creating newrestricted basin configurations and increased nutrient input promoted by upwelling processes.