A review of retrieving pristine rare earth element signatures from carbonates

Rare earth element and yttrium (REY) concentrations of carbonates can be used as potential proxies to trace the geochemistry of precipitating waters and physico-chemical environments. However, it is difficult to retrieve pristine REY signatures of carbonates because they can be contaminated by non-carbonate phases and modified by post-depositional processes. In this paper, we review the controlling factors of the REY signatures, such as the total REE concentrations, REY patterns, as well as elemental anomalies (mainly La, Ce, Eu and Gd), and Y/Ho ratios from published information. The non-carbonate phases, including clastic detritus, Fe-Mn(oxyhydr)oxides and phosphates, and post-depositional modification are the important factors that contaminate pristine REY signatures. The useful proxies and relationships among element concentrations, anomalies, and ratios can be used to evaluate the non-carbonate contamination or post-depositional modification. The selection of suitable normalization standards is also critical in certain cases. We evaluate the appropriate analytical methods, dissolution processes and pre-leaches to minimize the influence of non-carbonate contamination and post-depositional modification. We conclude that partial dissolution with acetic acid may be a robust approach to retrieve pristine REY signatures from the bulk carbonates, whereas LA-ICP-MS technique is appropriate for in-situ analyses of pure carbonate microfacies.