CHEMICAL CHARACTERISTICS OF CHLORITE AND MUSCOVITE IN TRILOBITE FOSSILS AND SURROUNDING MATRIX FROM THE BURGESS SHALE, BRITISH COLUMBIA, CANADA

One interesting feature of the exceptionally preserved fossils of soft-bodied fauna of the Burgess Shale, and the associated skeletal fossils, is that they are composed largely of sheet silicates. The purpose of this study is to further understand the general character and controls of sheet-silicate fossil replacement in Burgess Shale fossils through the examination of skeletal fossils in particular. The Walcott Quarry Shale Member of the Burgess Shale Formation is composed of calcareous siltstone and mudstone with discrete limestone beds. The shaley beds contain a typical lower greenschist facies metapelitic mineral assemblage dominated by chlorite and muscovite. From approximately 50 thin sections from the Walcott Quarry and Mt. Stephen Trilobite Beds, we identified 10 samples that contained well-defined trilobite fossils. For each of these samples, the composition of chlorite and muscovite were determined both within the fossil and in the surrounding matrix using electron microprobe analysis. In addition, we determined the major element composition of the matrix, and the relative abundance of minerals within each fossil. There is a strong correlation between chlorite Fe:Mg ratios in the trilobite fossils and the surrounding matrix. There is no correlation between ratios of chlorite to muscovite within fossils in comparison to those along the outside rim of the fossils, nor is there a correlation between chlorite:muscovite within fossils and the Fe+Mg:K ratios in the surrounding matrix. The coincidence of fossil and matrix chlorite compositions indicates that the fossil mineral assemblage equilibrated with the matrix during metamorphism. However, the lack of correlation between bulk composition of fossil and matrix suggests that initial formation of fossil-replacing phyllosilicates involved more than simple diffusion of matrix components during metamorphism. Edges of fossils are commonly composed of fine-grained, margin-parallel chlorite crystals that have been overprinted by coarser muscovite and chlorite. Microbial precipitation of minor amounts of Fe-bearing phases on the shell margin, and the subsequent development of a phase of chlorite growth during early diagenesis could account for these patterns.