REASSESSING FORMATION OF PRECAMBRIAN MOLAR-TOOTH MICROSPAR: CONSTRAINTS FROM CARBONATE PRECIPITATION EXPERIMENTS

Molar-tooth (MT) fabric consists of a complex void network filled penecontemporaneously with an unusual carbonate microspar. Because MT microspar occurs globally, yet is restricted mainly to the later Proterozoic, understanding its genesis is critical to constraining the geochemical evolution of Proterozoic seawater. MT microspar consists of two phases (spheroidal cores and isopachous overgrowths) that occur in lattice continuity, yet differ in trace element composition. Crystal size distribution analysis indicates spontaneous nucleation of cores, rapid growth and modification via Ostwald ripening, followed by precipitation of overgrowth cements. Modern, and even hypersaline seawater, however, contains insufficient Ca²⁺ to precipitate observed microspar volumes without substantial fluid flow-through, yet fluid velocities required to maintain void integrity were likely too low to keep crystalline cores in suspension.

Here we present results of laboratory experiments and model considerations that may help constrain geochemical conditions of MT microspar formation. Spontaneous nucleation of spheroidal carbonate (calcite, vaterite, and amorphous phases) occurred with reaction of highly concentrated (0.2 M) CaCl₂ solutions with either Na₂CO₃ or NaHCO₃, in both MilliQ water and synthetic seawater. Initial amorphous Ca-Mg phases were associated with formation of a viscous colloid. Precipitate volumes were greatest when Na₂CO₃ was used as a carbonate source, when experiments were preformed in MilliQ water, and with addition of Ca-acetate. These experiments suggest that high saturation, elevated pH, absence of inhibitors (e.g. SO₄), and the presence of dissolved organic molecules as a catalyst nucleation may have been critical for formation of MT microspar. We suggest that in the Precambrian, when pCO₂ may have exceeded 10X PAL, acid weathering of terrestrial rocks would have increased delivery of divalent cations (Ca, Mg) to the ocean; hydration of these cations would support formation of viscous colloids that may have aided in retaining void structure during microspar formation; and increased alkalinity would maintain elevated pH, resulting in higher degrees of carbonate saturation that would significantly reduce fluid volumes necessary to precipitate observed quantities of MT microspar.