PNNL

Abstract

Pyrite framboids (spherical masses of nano-scale pyrite) are among the earliest textures of pyrite to form in sedimentary settings. It has been proposed that their trace element contents can be used to track the trace element composition of the water column in which they formed. However, it is not clear how these trace elements are associated with the framboidal pyrite grains. For instance, it is important to know whether they are incorporated uniformly or are enriched in different regions of the framboid. In this contribution, we use high-resolution scanning transmission electron microscopy to identify chemical zoning within pyrite framboids. We find that initial, nano-scale pyrite euhedra, which make up the volumetric majority of the framboids, is covered/infilled by later pyrite that templates on the earlier pyrite. Further, this later pyrite is enriched in chalcophile and siderophile elements, suggesting that many trace elements are incorporated in pyrite relatively late. This observation suggests that while chemical analyses of pyrite framboids may provide ocean water chemistry trends through time, the details are complex. Specifically, the trace elements found in pyrite can be linked to adsorption onto organic matter and detrital material followed by desorption in the sediments and incorporation into pyrite as overgrowths on the initial nano-scale euhedra that make up pyrite framboids. While the use of pyrite chemistry to understand past ocean conditions may still be viable, and even diagenetic additions may not preclude pyrite’s utility for reconstructing ancient ocean conditions, care must be taken in interpretations as the end concentration may be influenced by diagenesis.