UNRAVELLING NEOMORPHISM: RECRYSTALLIZATION PATHWAYS IN PROTEROZOIC MICROFOSSILIFEROUS CHERT

Proterozoic microfossiliferous chert provides a window into the emergence of early life on our planet, yet the mechanisms underpinning often remarkable preservation remains a key area of research. Petrographic data reveal that primary chert fabrics are commonly composed of radial-fibrous chalcedony spherules that have undergone varying degrees of neomorphism, resulting in the development of distinctive textures. The present study examines the pathways of, and extent to which, neomorphism occurs in Proterozoic microfossiliferous chert.

Initial chert fabrics consist of primary length-fast chalcedony spherules with crystal fibers radiating from the spherule center and exhibiting sweeping extinction. Under a gypsum plate, fibers of similar orientation result in optical identification of four wedge-shaped quadrants. Initial neomorphism of spherules commonly results in wedge-shaped or dumbbell morphologies that mimic these primary quadrants. We interpret these wedges to represent neomorphism wherein syntaxial cement fills space within primary crystal fibers bundles where fibers are similar in optical orientation. Elsewhere, crystal fibers show greater divergence reflecting potential dendritic growth. Here, the divergence of crystallographic axes appears too large to accommodate syntaxial growth, resulting in finer scale neomorphic crystals that represent only a small range of initial crystal divergence. Finally, more extensive neomorphism can result in amoeboid crystals with distinct extinction domains representing primary spherules.

SEM-EBSD permits us to define crystallographic orientations related to both primary spherule crystal growth and diagenetic recrystallization to explore the pathways neomorphism. Notably, in all cases, neomorphic chert demonstrates an exceptional ability to preserve intricate details of microfossils and other primary fabrics. Critically, because neomorphism involves dissolution and reprecipitation, this data suggests that neomorphism proceeded via low levels of fluid-rock interaction.