EXPLORATION OF PRECAMBRIAN MOLAR-TOOTH FABRIC WITH CATHODOLUMINESCENCE PETROGRAPHY: IMPLICATIONS FOR DIAGENESIS

Molar-tooth (MT) structure is an enigmatic carbonate fabric that consists of a variety of voids, ribbons, and blebs micrometer-filled with microcrystalline calcite. Evidence suggests that MT structures originated syndepositionally at, or near, the sediment-water interface. The microcrystalline calcite fill characteristically shows differences in morphology from the surrounding matrix, leading to an interpretation that molar-tooth microspar provides a pristine phase for recording geochemical conditions of the ancient ocean. Here we provide the first detailed petrographic analysis of molar-tooth microspar and its surrounding matrix using cathodoluminescence petrography. We compare samples from the USA, Canada, Russia, Mauritania and China, and explore the similarities or differences that will provide a better understanding of the origin and diagenesis of molar-tooth structures.

Cathodoluminescence (CL) is a petrographic technique that has long been used in the understanding of carbonate diagenesis. Cathodoluminescence activators (e.g. Mn²⁺) and quenchers (e.g. Fe²⁺) highlight critical differences in crystal structure that are not always apparent in standard light microscopy and can provide evidence of both postdepositional recrystallization and the composition of diagenetic fluids. Molar-tooth microspar occurs under CL as non-luminescent and non-interlocking spheroidal to semi-rhombic cores that range from 5 to 15 micrometer in diameter, coated by a luminescent syntaxial cement. These characteristics indicate that precipitated microspar cores remained isolated (i.e. not-touching) in a fluid or gel medium until precipitation of a secondary carbonate cement. Changes in luminescence also suggest the potential for rapid changes in a fluid composition during the early diagenetic filling of molar-tooth cracks, potentially by the segregation of trace elements during initial crystallization. Finally, the presence of characteristic MT microspar within matrix phases dominated by later diagenetic dolomite rhombs suggests that MT microspar remained stable even as matrix components record much more extensive diagenesis.