Micro-Scale Geochemical Analysis of Molybdenum Distribution in Shales Deposited Under Different Redox Conditions

To reconstruct the paleoredox conditions during shale deposition, the abundance of Mo has been widely used as a proxy given its lower mobility in the presence of aqueous H2S leading to sedimentary enrichment. While multiple laboratory experiments have reported Mo adsorption onto organic matter (OM) and/or iron-sulfide, a discrepancy between experiment results and bulk-rock data still exists. Specifically, bulk-rock Mo abundance is sometimes uncorrelated with OM and iron-sulfide concentrations in shales challenging our basic understanding of the proxy. Potentially, this is on account of other Mo hosts in sediments, varied Mo adsorption under different redox conditions, and/or diagenetic overprint. Testing these alternative hypotheses requires finer-scale research on individual phases to verify Mo accumulation in each component.

In this work, we apply micro-scale analyses including Time-of-Flight Secondary Ionization Mass Spectrometry (ToF-SIMS) and Field Emission-Electron Probe Micro-Analyser (EPMA) to investigate Mo distributions in shales. Our sample set includes shales from the Upper Devonian Duvernay Fm. (Canada), the Lower Permian Wolfcamp Fm. (USA), the Upper Devonian Woodford Fm. (USA), and mudstones from the Upper Carboniferous Ross-Tullig Fm. (Ireland).

We classify samples into low (~ 1 ppm) and high (> 10 ppm) Mo groups using bulk-rock ICP-MS analysis. Using EPMA, we observe OM, pyrite, and clay matrix as the three primary Mo hosts. Mo contents in the same host are varied between the two groups. Reconstructed Mo depth profiles based on such apportionment of Mo budget in each host are generally consistent with the original bulk-rock data despite apparent offsets at certain intervals with extremely abundant pyrite. OM-related Mo is significant for bulk-rock Mo content in the low-Mo intervals, while pyrite-related Mo becomes more important in Mo-rich intervals, the latter of which is qualitatively verified by ToF-SIMS. Between the samples from different basins with varied thermal maturity, we identify more concentrated Mo in more mature OM and in the clay matrix with higher specific surface area. We will perform density separation of powdered samples to separate OM, pyrite, and other minerals, and conduct elemental analysis of these purified aliquots to verify the reliability of the EPMA data.