Fig. 8
From: Continental weathering as the source of iron in Jurassic iron oolites from Switzerland
Schematic sketch for the formation of ancient and modern iron ooids. The formation of Jurassic iron ooids results from the coupling of sedimentary organic carbon enrichment (due to the delivery of carbon associated with detrital clays), microbially-mediated Fe cycling of sediment-sourced Fe, and sedimentary condensation processes related to agitated hydrodynamics. The variation in authigenic εNd between − 9 and − 7 of the studied iron oolites likely reflects variations in the weathered continental source areas. In contrast to ancient iron ooids, modern iron ooids form in a volcanic setting with intense submarine hydrothermal activity (Di Bella et al., 2019). Average continental and hydrothermal δ56Fe, the latter including mid-ocean ridge hydrothermal fluids, are from Beard et al. (2003). Continental εNd (− 10 to − 8) represents a range for four modern Swiss rivers (Rickli et al., 2013), which is similar to the average of detrital sedimentary rocks aged 0–200 Ma (− 10 ± 8, 1 SD; compiled in Garçon, 2021) and averaged world river suspended loads (− 11 ± 4, 1 SD; compiled in Garçon, 2021). Hydrothermal εNd represents averaged compositions of mid-ocean ridge basalts (± 1 SD; compiled in Gale et al., 2013). Middle Jurassic Tethyan Nd isotope composition cover the temporal variations given in Dera et al. (2015)