Heterogeneous REE + Y distribution in Early Paleozoic shelly phosphorites: Implications for enrichment mechanisms

Abstract

Sedimentary phosphorites are important carriers of Rare Earth Elements and Yttrium (REE + Y). Primary biogenic and sedimentary apatite contain very low amounts of REE + Y and enrichment of these elements occurs during diagenesis. Although the influence of post-depositional processes on REE + Y concentrations in sedimentary phosphorites is well established, the processes controlling the degree of enrichment are poorly constrained. In this study, we examine the REE + Y composition of Early Paleozoic shelly phosphorites of the Baltica paleocontinent, using ICP-MS and LA-ICP-MS. Phosphate containing phases in these deposits are (i) phosphatic brachiopod shells, (ii) phosphatic clasts and (iii) phosphatic grain coatings. Measured REE + Y concentrations range from 162.7 to 2415.3 mg/kg. The distribution of REE + Y in phosphatic shells and clasts is heterogeneous, with the highest concentrations found in the outer margins. In the phosphatic shells, remnants of skeletal apatite contain higher REE + Y concentrations than authigenic apatite in baculate laminae. In addition, our results show significant locality-based variability, with up to 14 times differences in REE + Y concentrations from localities situated only a few tens of km apart. Based on (i) the heterogeneous distribution of REE + Y in apatite, (ii) LaN/YbN and LaN/SmN ratios pointing to diagenetic REE + Y uptake from sediment pore-fluids, as well as (iii) variable pyrite content, the geographic variations of REE + Y in sedimentary phosphorites of the Baltica paleocontinent are most parsimoniously explained by differences in local redox conditions during early diagenesis.