MINERALOGY AND CHEMISTRY OF PRECAMBRIAN MOLAR TOOTH STRUCTURES

Molar-tooth structure (MTS) is an enigmatic carbonate fabric that is known only from Precambrian carbonate platforms, with examples known from the late Archean (n=2), the Paleoproterozoic (n=4), the Mesoproterozoic (n=29), and the Neoproterozoic (n=36). MTS is composed of variously shaped cracks and voids filled with an unusual, equant calcite microspar that shows characteristic non-interlocking, micrometer-size calcite crystals. Cathodoluminescence microscopy highlights the two-phase, syntaxial nature of the microspar, consisting of inner non-luminescent cores surrounded by a luminescent rim. Microspar records crystal size distributions (CSD’s) showing lognormal distributions that vary toward normal distributions via Ostwald ripening; cores show clear features of Ostwald ripening. Petrographic observations are consistent with microspar precipitation as amorphous calcium carbonate (ACC) with hydration water forming a hydrogel that controlled the spatial distribution of the ACC cores. New elemental data from electron microprobe mapping exhibit sharp edges between cores and rims, with no indication of diffusion between cores and rims; a pattern that is qualitatively consistent to that observed under cathodoluminescence. In most cases, concentration of Fe, Mg, Mn, and Sr ions are relatively low in the cores but relatively high in the rims. We propose that such trace elements were displaced from the ACC cores to the gel phase during initial precipitation in supersaturated fluids near the sediment-water interface. Decreasing saturation state in the overall fluid resulted in growth of a syntaxial cement phase from the remaining gel rich in trace elements. This syntaxial phase nucleated on the surface of the previous cores as the ACC lost its hydration, converted to calcite, and resulted in secondary growth by surface-controlled processes. The rhombohedral shape of the nuclei of some samples suggests that carbonate precipitated directly as calcite; however, it is possible that rounded crystals precipitated first as an unstable polymorph that later transformed into calcite. Further research is needed to clarify whether ACC precipitates directly to calcite or through an unstable polymorph.