INVESTIGATING THE ORIGIN OF PRECAMBRIAN MOLAR-TOOTH CARBONATE MORPHOLOGY

Molar-tooth (MT) structure is an enigmatic Precambrian carbonate fabric that consists of a complex array of variously shaped voids filled with an unusual carbonate microspar. MT microspar occurs predominantly within MT voids, in sharp contact with surrounding matrix, but also occurs interstitially within grainier lithologies. Relationships between filled cracks and surrounding matrix suggest rapid precipitation of MT microspar and complete cementation prior to compaction and lithification of the surrounding matrix. Although it has been suggested that MT microspar comprises up to 25% of precipitated carbonate in the Mesoproterozoic and early Neoproterozoic (James et al., 1998), most research has focused primarily on the mechanism of MT void formation, and little is known about this unusual calcite microspar fill.

A survey of MT samples of varying ages (~1450-700 Ma) from China, Russia, Mauritania, and the U.S. have been studied via transmitted light microscopy, cathodoluminesence (CL), ESEM imaging, and SIMS ion imaging. Analysis reveals a striking uniformity in MT microspar characteristics, regardless of sample origin. In CL and ESEM images, MT microspar appears as small (4-11 μm), spheroidal to subrounded to rhombic (non-luminescent) cores surrounded by (brightly luminescent) isopachous overgrowths. Construction of crystal size distributions (CSD's) from high-resolution images allows for interpretation of crystal growth mechanisms (Eberl et al., 2000). Microspar cores have CSD’s consistent with lognormal distributions, which suggest a homogenous nucleation event in an open system, that have become symmetrical to negatively skewed during early (pre-overgrowth) diagenesis. SIMS ion imaging of Ca, Mg, Fe, Mn, and Sr reveals significant enrichment of Mg within the microspar cores relative to the overgrowth cements. We suggest that initial precipitation sequestered Mg in microspar cores which then acted as a substrate for calcite cement overgrowths, preserving not only the unusual spheroidal core morphologies, but also core chemistry. Combined, unusual core/overgrowth structure, spheroidal core morphologies, and Mg enrichment suggest that precipitation of MT microspar may represent a common precipitation mechanism indicative of a particular set of geochemical conditions.