PYRITE SULFUR ISOTOPIC ANALYSIS OF MAZON CREEK FOSSILS MAY PROVIDE CLUES ON NODULE FORMATION

The Mazon Creek deposits, collectively a late Carboniferous lagerstätte, is well-known for its preservation of marine and terrestrial fossils in siderite nodules. These nodules contain pyrite, iron oxides, and clays. There are three primary hypothesized methods of concretion formation: (1) concentric growth, in which siderite grows outward from the surface of the fossil; (2) inverted growth, in which siderite forms at a circumambient diffusion-precipitation front around the fossil; and (3) pervasive growth, where siderite precipitates throughout the nodule volume. To investigate the mode of concretion formation and pyrite association, pyrite in three woody plant Braidwood fossils were analyzed for spatial distribution of ³⁴S:³²S isotopic signatures through Secondary Ion Mass Spectrometry (SIMS). Before analysis, samples were embedded in epoxy, cut perpendicular to the plane of the fossil, and finely polished and coated in a 20nm Au layer. Vertical and horizontal transects, relative to the fossil surface, were collected on each section of pyrite. A transect of areal isotopic images were also collected on one sample, and isotopic data were analyzed on the regions with high ³⁴S and low ¹⁶O counts. Overall, no obvious δ³⁴S_py patterns were found within the transects. Even though there are several transitions from very heavy values to very light values, they seem to be randomly positioned with reference to the fossil material. Similarly, no gradual progression from light to heavy δ³⁴S_py values were found through the areal images. Throughout the transects and image regions, δ³⁴S_py values were heavier overall than expected, although this may be explained by partial closed-system distillation of porewaters through progressive sulfate reduction during pyrite formation and/or preferential diagenetic fractionation. Alternatively, the data collected here may show signs of multiple stages of pyrite precipitation, and a complexity of the concretion formation process that may be most similar to the pervasive growth model.