Tracing seawater- and terrestrial-sourced REE signatures in detritally contaminated, diagenetically altered carbonate rocks

Abstract

As two typical non-skeletal carbonates, ooids and calcified microbial structures have a potential for use in paleo-seawater rare earth element (REE) and yttrium (REY) reconstruction, but strong diagenetic alteration and detrital contamination may cover/overprint their primary carbonate REY signatures and significantly undermine their reliability and usability in recovering original aqueous information from carbonate rocks. In this study, we analyzed a series of Cambrian (Stage 3) oolites and microbialites with minor terrigenous admixtures in the Hannan-Micangshan area, South China using various fractions (bulk rock, acid soluble and insoluble, individual siliciclastic and heavy minerals), and solution- and laser ablation (LA)-based analytical techniques. The acetic acid digested samples exhibit enriched middle REEs (MREEs) and light REEs (LREEs) when normalized to shales, which may indicate the effects of carbonate and detrital diagenetic alteration. The LA-based analysis of the ooid cortices and calcified microbes revealed varying REE patterns from flat to heavy REE (HREE)-enriched features despite their average REY compositions being similar to those of the acetic acid digested samples. In contrast, the early isopachous cements exhibit seawater-like REY patterns with the least diagenetic and contamination characteristics, which potentially indicate seawater-sourced porewater fluids approaching the composition of the overlying surface ocean in the shore settings. In addition, we identified HREE-enriched detrital fractions (mostly distributed in the clay-sized particles) in all three study sections, which are likely sourced from the neighboring mafic igneous rocks in the Hannan-Micangshan area. This HREE-enriched signature would significantly mislead our judgement of the coexisting carbonate REY characteristics (e.g., Ce anomalies and Y/Ho ratios) if the contamination fraction could not be rigorously excluded. Although large quantities of contaminated (to varying degrees) LA-based data cannot be used for paleoceanographic reconstruction, their unique values have been demonstrated to indicate their terrigenous mineralogical sources based on multiple groups of contamination fingerprint elements, which conforms well to the elemental compositions of individual siliciclastic and heavy minerals. For detritally contaminated, diagenetically altered shallow-water carbonate rocks, in general, the LA technique can be applied not only to paleoceanographic reconstruction based on the most pure and least altered components (i.e., early marine cements) but also to provenance analysis based on fine-grained particles admixed in the ooid cortices and calcified microbes.