Investigating the Origin of Precambrian Molar-Tooth Carbonate

Molar-Tooth (MT) is an enigmatic carbonate structure that consists of a complex array of variously shaped voids filled with an unusual microspar cement. Most current research has been focused on the mechanism of formation of molar-tooth voids, and the leading model suggests a subsurface, soft-sediment, gas escape mechanism. However, little is known about the calcite microspar that fills MT voids. Brittle deformation of MT structures during sediment compaction indicates that lithification of MT voids is essentially synsedimentary. Under cathodoluminescence (CL) microscopy, MT microspar appears as small (3-12 µm), spheroidal to rhombic, non- luminescent cores surrounded by luminescent, isopachous overgrowths. Crystal size distributions (CSD) indicate that microspar cores originated in a spontaneous nucleation event, followed by varying degrees of Ostwald ripening.

Elemental analyses of void-filling microspar (core + rims) indicate that MT microspar is typically low-Mg calcite. SIMS ion imaging however shows that within the microspar, Mg is partitioned into the cores relative to the overgrowths. Laser-Raman spectroscopy confirms current calcite structure, and spectral peaks have half-widths consistent with low Mg-calcite. However, anomalous weak peaks are found in some MT samples. These peaks are located at 180, 200, 225 cm^-1 and may reflect remnants of a previously unstable mineral phase.

It has been suggested that vaterite may have been the original carbonate precipitate. Vaterite is a highly unstable calcium carbonate polymorph that commonly has a spheroidal crystal morphology and transforms rapidly to rhombic calcite. Penecontemporaneous precipitation of overgrowth cements that preserve spheroidal, transitional, and rhombic crystal forms is consistent with rapid mineralogical transformation. CSDs, SIMS trace element mapping, and Raman spectroscopy are consistent with original vaterite precipitation.