LABORATORY PRECIPITATION AND GEOCHEMICAL INVESTIGATION OF UNSTABLE CACO3 POLYMORPHS: IMPLICATIONS FOR THE ORIGIN OF PRECAMBRIAN “MOLAR-TOOTH” CALCITE

A key to interpreting Proterozoic marine geochemistry lies in the investigation of carbonate fabrics exclusive to this period. One such fabric is molar-tooth (MT) calcite, which occurs as clear, 5-15μm, equant crystals within a complex array of variously shaped voids. The volumetrically significant and global occurrence of MT calcite in shallow marine strata suggests linkages between carbonate fabric development and the broad-scale geochemical evolution Earth's early oceans. Petrographic and geochemical analyses of MT calcite suggest that it may have originally precipitated as vaterite, a highly unstable CaCO₃ polymorph. Crystal size distributions suggest spontaneous nucleation followed by various degrees of Ostwald ripening. Cathodoluminescence analysis reveals spheroidal, non-interlocking, dull cores surrounded by luminescent, polygonal rims, similar to spheroidal crystal morphologies commonly exhibited by laboratory precipitated vaterite. High resolution SIMS analysis reveals that spheroidal cores of MT calcite are enriched in Mg²⁺ relative to polygonal rims; Mg²⁺ enrichment is consistent, as well, with an originally vaterite mineralogy.

Laboratory precipitation of vaterite typically occurs at carbonate saturations far greater than is observed in modern marine systems. By systematically varying pCO₂ of a synthetic, pH-buffered seawater solution during laboratory precipitation of vaterite, we are attempting to constrain the conditions under which quantities of spheroidal vaterite, sufficient to fill MT voids, most readily form. Furthermore, by varying the composition of the solutions, we evaluate the distribution of common trace elements into the precipitate, in order to produce additional tests for comparison of precipitated vaterite with Precambrian MT 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.