REE+Y uptake and diagenesis in Recent sedimentary apatites

• Published in: Chemical geology 2019-10, Vol.525, p.268-281
• Main Authors: Lumiste, Kaarel, Mänd, Kaarel, Bailey, Jake, Paiste, Päärn, Lang, Liisa, Lepland, Aivo, Kirsimäe, Kalle
• Format: Article
• Language: English
• Subjects: Diagenetic overprinting; Early diagenesis; Geofag: 450; Geosciences: 450; Matematikk og Naturvitenskap: 400; Mathematics and natural science: 400; Mineralogi, petrologi, geokjemi: 462; Mineralogy, petrology, geochemistry: 462; Namibian phosphorites; Rare Earth Elements; REE+Y; Sedimentary apatite; VDP
• ISSN: 0009-2541; 1872-6836; 1872-6836
• DOI: 10.1016/j.chemgeo.2019.07.034

Rare Earth Elements and Yttrium (REE+Y) distribution in authigenic phases are frequently used as proxies for reconstructing past seawater conditions. Sedimentary apatite precipitates near the sediment-water interface and is therefore capable of recording the REE+Y composition of the overlying water column. While the overprinting of primary REE+Y signal during late stage diagenesis is a widely known phenomenon, less is known about early diagenetic effects. In this study, we investigate the REE+Y distribution in Recent sedimentary apatites found on the Namibian shelf using in situ mapping by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The Namibian phosphorite deposits consist of reworked and redeposited pelletal apatitic grains, ranging from Miocene to Pleistocene in age, and pristine concretionary apatitic grains that formed during Pleistocene to Recent. The results of this study show that the REE+Y signatures of pelletal and concretionary apatitic grains are different with both types showing intragranular variability and differences between grain centers and rims. The REE+Y are concentrated in the external part of the apatitic grains, forming a 10–20 μm wide “enrichment zone”. While the central parts of apatitic grains are low in REE+Y (∑REE+Y