ORIGIN OF PRECAMBRIAN MOLAR-TOOTH MICROSPAR: A NEW LOOK AT AN OLD PROBLEM

Molar-tooth (MT) is an enigmatic structure that consists of a complex array of variously shaped voids, penecontemporaneously filled with an unusual carbonate microspar. Most current research has focused on the mechanism of formation of MT voids, and the leading model suggests a subsurface, soft-sediment, gas escape mechanism. However, little is known about the microspar that fills MT voids. MT microspar occurs petrographically as 7-15 µm, clear, non-interlocking, equant crystals that lack obvious evidence for neomorpism. Under cathodoluminescence (CL) microscopy, MT microspar appears as small (3-12 µm), spheroidal to rhombic, non-luminescent cores surrounded by luminescent, isopachous overgrowths.

Penecontemporaneous precipitation of overgrowth cements that preserve spheroidal, transitional, and rhombic crystal forms suggests rapid mineralogical transformation of an original, unstable carbonate phase. This interpretation is supported by crystal size distributions (CSDs) indicating precipitation in a single, spontaneous nucleation event, followed by varying degrees of Ostwald ripening, and by Laser-Raman spectroscopy, which reveals a calcite lattice overprinted by several anomalous peaks (at 180, 200, 225 cm^-1) that may reflect remnants of an unstable mineral phase. Combined, these observations, along with SIMS ion imaging that reveals a strong partitioning of Mg²⁺ into microspar cores, supports the hypothesis that MT microspar may have originally precipitated as vaterite, a poorly understood and highly unstable CaCO₃ polymorph that rapidly converts to calcite.

Laboratory precipitation of vaterite, however, typically occurs at carbonate saturations far greater than is expected in marine systems. By systematically varying pCO₂, pH, and trace element concentration of a synthetic seawater solution during laboratory precipitation of vaterite, we hope to constrain the conditions under which spheroidal vaterite most readily forms, and explore trace element signatures and changes in lattice structure as vaterite undergoes conversion to calcite. Ultimately, these investigations will aid in development of a detailed geochemical framework in which to examine the distribution of MT calcite and interpret the geochemical conditions of Earth's early marine environments.