A NEW SAMPLE PROCESSING PROTOCOL FOR PROCURING SEAWATER REE SIGNATURES IN BIOGENIC AND ABIOGENIC CARBONATES

Rare Earth Elements are important proxies for tracing the evolution of the Earth’s hydrosphere. Many archives have been analyzed for their REE contents with qualified success, which may be due to alteration effects. Review of the database reveals a lack of a rigorous sample processing protocol and probably the leading cause for the spurious REE results reported in the literature.

We propose a ‘sample cleaning protocol’ for procuring reliable REEs from marine materials. To achieve this goal, valves from Liothyrella neozelanica recovered from the deep water of the Pacific Ocean were subjected to 5 different cleaning procedures (P). In P-1 valve fragments did not receive any cleaning other than rinsing with distilled water. In P-2 a set of fragments was immersed in 2.5% H₂O₂ for 3 days and then washed. In P-3 valve fragments were physically cleaned using a stainless-steel blade and then water washed. In P-4 a set of fragments was physically cleaned then immersed briefly in 10% HCl until they were deemed clean, and then washed. In P-5 the last set of fragments was processed using all cleaning protocols such as physical scraping, H₂O₂, HCl and washing.

Detritus adsorbed on the calcitic structure of the shells may lead to elevated ∑REE content, anomalous Ce/Ce* and elevated Mn, Fe and U concentrations as documented by the P-1 results. H₂O₂ immersion (P-2) eliminates primarily the organic tissue, which leads to partial depletion in the Ce/Ce* anomaly. Physical cleaning (P-3) removes adsorptive particulates and the periostracum, and leads to depleted ∑REE and lower Mn, Fe, and U concentrations with a slight Ce/Ce* anomaly. Physical cleaning followed by chemical cleaning (P-4 and P-5) removes the outer contaminant and the primary layer. These last two procedures produce drastic reductions in ∑REE, Mn, Fe, and U concentrations and normal Ce/Ce* anomalies with typical seawater REE signatures.

P-4 was tested with fossil brachiopods and enclosing whole rock as well as biogenic apatite. The brachiopods contain ∑REEs, Mn, Fe, and U concentrations and Ce/Ce* values comparable to their modern counterparts, while in the whole rock they are slightly to significantly enriched. In contrast, REE contents of conodonts give the typical ‘bell shape’ trend indicative of post-depositional alteration coupled with extraneous Ce anomaly values.