The influence of primary depositional conditions on Hg behaviour during early thermal maturation

Abstract

Mercury (Hg) behaviour in sedimentary rocks is important to understand both in terms of its potential volatilization during magmatic intrusions (e.g., those associated with large igneous province activity) and its redistribution during thermal maturation associated with burial and, in some cases, hydrocarbon generation. Mercury behaviour varies significantly across different lithologies, likely influenced by the amount and type of organic matter present and sulfur chemistry. This study investigates the effects of lithology on Hg dynamics during thermal maturation through pyrolysis experiments at isothermal conditions of 325 °C for various durations (24, 336, 500, and 840 h). To study Hg behaviour in different lithologies, representing different depositional environments and OM-types (organic-rich vs organic-lean), we analysed a relatively Hg-rich sample from the Lower Jurassic (Pliensbachian) Belemnite Marls, a marine carbonate-rich sequence exposed in southern England, and a coal sample from the Eocene Tanjung Formation, Indonesia composed of terrestrial organic matter. Before and after pyrolysis, we generated data on Hg concentrations, total organic carbon (TOC), hydrogen index (HI), and oxygen index (OI) and compared our results with existing data from the Posidonienschiefer, a marine, highly organic-rich, black shale of Toarcian age from Germany. Results indicate substantial Hg loss, with the coal and Belemnite Marls samples losing over 80 % of their Hg in the first 24 h, compared to a 50 % loss in the Posidonienschiefer. Thermal desorption profiles (TDPs) allow us to align the Hg losses in the isothermal heating experiments with the initial Hg speciation in the sedimentary rocks. Both the Belemnite Marls sample and Tanjung Formation coal are dominated by lower temperature Hg species, potentially bound to or associated with organic matter. These findings enhance our understanding of the interplay between lithology, thermal maturation, and Hg behaviour, which is critical for interpreting historical Hg cycling and the environmental impacts associated with the formation of large igneous provinces (LIPs).