STUDYING THE DISTRIBUTION OF MOLYBDENUM IN A BLACK SHALE WITH TOF-SIMS

The Mo content of black shales has found wide application as an indicator of euxinic (strong enrichment), anoxic (minor enrichment), or oxic (no enrichment) conditions of accumulation. Dissolved sulfide concentrations seem critical for Mo sequestration, but it is still unclear whether Mo co-precipitates with diagenetic sulfide minerals, such as pyrite, or whether it is adsorbed on organic matter. Mapping the distribution of Mo in black shales is essential to address this issue. Yet, Mo concentrations in black shales are too low for SEM-EDS or Electron Microprobe detection.

Time of Flight Secondary Mass Spectrometry (ToF-SIMS) allows simultaneous detection of atoms and isotopes from hydrogen to uranium. It has considerably higher sensitivity than SEM-EDS or Electron Microprobe, and in scanning mode acquires mass spectra for each pixel in the scanned area. These data are then processed for the production of chemical maps. Application to Devonian black shales from the eastern US enabled us to detect and map Mo isotopes, and to relate them to the different rock components. Low concentrations of Mo are irregularly scattered throughout, and elevated Mo coincides with diagenetic pyrite framboids. Highest concentrations occur in the periphery of framboids. Peripheral Mo enrichment in pyrite framboids suggests Mo addition following or towards the end of framboid formation, when porewater chemistry changed from anoxic/non-sulfidic to anoxic/sufidic. This suggests Mo diffusion from overlying seawater, and precipitation on pyrite grains when pore waters with elevated sulfide concentrations were encountered. In settings with very slow net accumulation, this can produce strongly Mo-enriched black shales without the “benefit” of euxinic bottom waters.