MOLAR-TOOTH MICROSPAR AS A WINDOW INTO PROTEROZOIC CARBONATE PRECIPITATION, STABILIZATION, AND DIAGENESIS

This abstract summarizes Kriscautzky et al. (2022). Molar-tooth structure (MTS) was first reported from the Belt Supergroup of NW Montana to describe a complex array of variously planar, folded, and broken sheets of lithologically pure calcite that occur as a distinct phase within generally argillaceous carbonate host rock. MTS is a sedimentary structure without modern analog that resists ready description and defies straightforward explanation. Its spatial distribution, its temporal restriction to the Precambrian, and its striking similarity around the globe all suggest an as-of-yet undetermined importance in our understanding of the early Earth. Specifically, the microcrystalline calcite that characterizes MTS has been inferred to be a synsedimentary precipitate that is little altered by diagenesis and is thus assumed to be an ideal target for geochemical analysis aimed at constraining ancient marine chemistry. The origin of molar-tooth voids and its void-filling microspar, however, remains poorly constrained, and a comprehensive understanding of MTS is hampered by a lack of fine-scale petrographic and geochemical analyses of these enigmatic carbonate textures.

Here we show examples of MTS and synthesize evidence for the mechanisms of void formation and subsequent carbonate precipitation. We then provide detailed petrographic analysis of MTS to illuminate the pathways of precipitation, neomorphism, and diagenesis recorded by the void-filling microspar. We argue that MT microspar shares a set of primary and early diagenetic petrographic features that are observed across both time and space. Recognition of this common microstructure enables us to trace potentially diverse diagenetic pathways and to evaluate the fidelity of geochemical signals. We ultimately argue that the void-filling microspar associated with MTS provides a unique window into the initial process of carbonate precipitation, stabilization, and its subsequent diagenetic alteration.