Trace and rare earth element geochemistry of black shales from the Upper Ordovician Utica Shale magnafacies

Rare earth elements (REE) are well known tracers of geochemical processes in marine shales and have been used extensively in provenance studies and as markers of paleo-oceanographic conditions at the time of deposition. The application of REE hinges on the premise that REE are relatively immobile during sedimentary processes associated with sediment transport and deposition. However, recent studies have shown that diagenesis can substantially affect REE and trace element distribution patterns in sediments, leading to their non-conservative behaviour. This study investigates the patterns of diagenetic remobilization of REEs in the whole-rock and labile fraction of black shales of the Upper Ordovician Utica Shale magnafacies (USM) of Québec, Ontario, and New York. The time correlative shale units are of different thermal grades, ranging from thermally immature in Québec (T_max 20–50 °C), to mature oil-bearing in Ontario (T_max 50–140 °C), to post-mature gas-bearing in New York (T_max > 200 °C).

The results show that the whole-rock maintains a flat ‘continental-type’ rare earth element abundance pattern, whereas the labile fraction, comprising carbonate, phosphate, sulfide minerals, and organic matter, shows variable middle-REE (MREE) enriched patterns relative to the light and heavy rare earth elements (LREE + HREE). The MREE enrichment increases with level of thermal maturity, suggesting that the overall REE patterns in the labile fraction of these black shales may instead reflect diagenetic remobilization rather than paleo-seawater REE patterns. Correlation of trace element paleoredox indicators (Ni/Co, U/Th, and V/Cr) with MREE enrichment tracers (La/Sm and Gd/Yb), as well as Ce/Ce*, suggests diagenetic overprint of traditionally used paleoproxies due to mobilization of trace elements during thermal maturation.

The results reported here suggest that fractionation of trace elements including REE during thermal maturation of black shales have significant effects on REE patterns in whole-rock and labile shale fractions. This finding has important implications for the use of trace elements in paleoenvironmental reconstructions for sedimentary rocks that have undergone thermal maturation.