THE ORIGIN OF VANADIUM HYPER-ENRICHED BLACK SHALES

Vanadium is a critical transition metal utilized in the production of high-strength steel, chemical catalysts, and vanadium redox flow batteries, a promising technology for large-scale storage of electricity generated by renewable and intermittent sources such as wind and solar. Vanadium concentrations of 100 to 200 ppm are common in reduced sediments deposited throughout Earth’s history, but some Phanerozoic black shales are hyper-enriched, with concentrations ranging from 1,000 to 10,000 ppm. These concentrations are within the range of vanadiferous titanomagnetite deposits that are currently the principal source of vanadium. Vanadium is the second most abundant transition metal in seawater and the vanadium in hyper-enriched black shales is likely derived from normal seawater under a specific suite of environmental conditions. However, because vanadium hyper-enrichment does not occur in modern sediments, one or more crucial environmental variables must be absent. To identify the missing variables, we compare theoretical considerations of vanadium speciation and complexation to the geochemistry of modern marine environments and Phanerozoic black shales, with a focus on the Upper Mississippian Heath Formation. A survey of Phanerozoic black shales demonstrates that hyper-enrichments are associated with euxinic conditions. The absence of hyper-enrichment in the euxinic Cariaco Basin, Venezuela suggests that the accumulation of high concentrations of H₂S (>60 µM) in the water column is required to reduce V⁴⁺ to V³⁺, consistent with laboratory studies of vanadium reduction by H₂S. Because dissolved organic matter can form complexes with V⁴⁺, preventing further reduction, low concentrations of dissolved organic matter are also required. V³⁺ is primarily hosted by organic matter and/or authigenic clays, rather than sulfide minerals. Our detailed study of the Heath Formation suggests that a sharp and fluctuating redoxcline may also be required. Beyond the economic potential of hyper-enriched shales, their identification in the rock record has the potential to provide valuable information on environmental variables unique to the Phanerozoic.