The formation of pyrite nodules in carbonaceous sediments as determined by in situ S isotope and trace element analyses

Abstract:

Pyrite nodules have been identified in many sedimentary rocks and their formation has attracted much interest. Recently, however, the mode and timing of nodule formation have become increasingly important as in situ pyrite compositions are now used frequently as proxies for trace element abundance in the oceans through time. For this method to be effective, the pyrite must form in pore waters that are in contact with the overlying water column. In this study we have taken steps toward a refined understanding of pyrite nodule formation by using SHRIMP-SI and LA-ICPMS to examine the in situ S isotope signature and trace element content of 27 pyrite nodules that range from the Archean to the Phanerozoic in age.

This study has revealed that there are three different ways in which pyrite nodules form. The first is a rapid precipitation of the entire nodule as indicated by an absence of significant δ³⁴S or trace element zoning in the nodule. The lack of zonation suggests that it formed early during sedimentation when the pore waters were still connected with the overlying water column. The second way results in a clear zoning of δ³⁴S and trace element compositions. This zonation indicates that the nodule formed deeper in the sediments, where the pore waters were not well enough connected to the overlying water column to avoid evolution of the pore fluid composition. The third is a combination of the first two modes of formation. These nodules initiated with rapid formation, as indicated by an initial lack of trace element and δ³⁴S zonation, followed by addition of later pyrite as revealed by the presence of zonation in the rims of the nodule. The identification of these different formation mechanisms may also have paleoceanographic utility as traditional nodule formation models suggest that pyrite nodules form relatively late as a product of diagenesis. Determining by which mechanism a nodule forms could give insight into the diagenetic history of pyritic black shale.