Apportioning the molybdenum budget in shales to improve paleoenvironmental interpretations

Molybdenum (Mo) abundance in marine shales/mudstones has been widely used to reconstruct depositional redox conditions. Prior laboratory experiments have observed Mo association with iron-sulfide and organic matter in the presence of aqueous H2S (euxinic conditions). The results of these experiments imply that sulfide minerals and organic matter are the primary hosts of Mo in the rock record, and this notion has been corroborated in some individual measurements on shales. At the same time, a lack of correlation between bulk-rock Mo concentration and either pyrite or organic matter abundances in some mudstones has been observed, which challenges the experiment-based understanding of Mo enrichment mechanisms and therefore related paleoredox interpretations. Here, we use electron microprobe to analyze individual phases in a suite of mudstones to examine the application of bulk-rock Mo abundance as a redox proxy. The microprobe data reveal that pyrite, organic matter, and matrix (clay minerals) can all host Mo in our mudstone samples. We interpret the Mo distributions in these three hosts to reflect differences in early diagenetic conditions (e.g., H2S concentrations) and the aqueous Mo availability. We infer that pyrite and clay minerals host a large fraction of bulk-rock Mo concentrations when aqueous H2S and Mo are abundant. In contrast, with less available aqueous H2S or Mo, organic matter becomes more important as the Mo host. More generally, it appears that constraints on the partitioning of bulk Mo between sedimentary phases can improve paleoredox interpretations especially in conjunction with independent proxies (e.g., S to C ratios of organic matter). Lastly, comparisons of Mo abundance between organic matter from different mudstones indicate potential Mo preservation in the organic matter during thermal maturation, and variations in Mo concentrations between different pyrites qualitatively suggest variations in the porewater Mo inventory during diagenesis.