Laboratory Precipitation and Geochemical Investigation of Unstable CaCO₃ Polymorphs: Implications for the Origin of Precambrian “Molar-tooth” Microspar

• Main Author: Goodman, Emily Elizabeth
• Published: Trace: Tennessee Research and Creative Exchange 2007
• Subjects: Geology
• Online Access: http://trace.tennessee.edu/utk_gradthes/136

Molar-tooth (MT) microspar is a volumetrically significant, globally distributed carbonate fabric that occurs almost exclusively in Mesoproterozoic and early Neoproterozoic shallow marine successions. It occurs as uniform, equant crystals, averaging 9 –15 μm in diameter (Figure 1), that characteristically fill MT structures—interconnected networks of spheroidal, spindle, and ribbon-shaped voids that formed within unlithified sediment in shallow, subtidal environments. Cathodoluminescence (CL) analysis of MT microspar reveals two chemically distinct phases: a dully luminescent spheroidal core enclosed by a luminescent, polygonal rim. Crystal size distribution analysis of MT microspar indicates spontaneous nucleation, rapid growth and varying degrees of recrystallization by Ostwald ripening, followed by precipitation of overgrowth cements. Petrographic and isotopic characteristics suggest MT microspar is a primary precipitate and not a diagenetic or neomorphic phenomenon and that precipitation occurred in active communication with Proterozoic seawater. Therefore, constraining the genesis of MT microspar is critical to understanding the geochemical evolution of Proterozoic seawater. Morphological similarities have led to comparisons between MT microspar and experimentally precipitated vaterite, a metastable CaCO3 polymorph. Presented here are results of precipitation experiments designed to investigate the plausibility of vaterite as the precursor mineralogy to MT cores, and to explore the geochemical conditions under which MT microspar may have formed. Any hypothesis for the formation of MT microspar must account for the crystal size and morphologies observed in MT microspar cores, as well the volume of precipitate needed to fill and stabilize MT structures. These experiments show that (1) spheroidal morphologies are common in both vaterite and calcite, suggesting that MT microspar may have initiated as a polymineralic precipitate and (2) that even under greatly elevated saturations, insufficient precipitate is produced to account for observed volumes of MT microspar. Experiments performed at elevated pH with added Mg2+, however, produced a viscous colloid, suggesting that under conditions of high carbonate saturation an amorphous phase of significant volume may have filled, and thus stabilized, MT structures in the unlithified substrate.