GEOCHEMISTRY OF MOLAR TOOTH STRUCTURE MICROSPAR FROM THE NEOARCHEAN MONTEVILLE FORMATION, SOUTH AFRICA

Molar tooth structures (MTS) are enigmatic, contorted mm- to dm-scale veins of carbonate microspar that formed during early diagenesis in the shallow sub-surface in Precambrian sediments. Uniformly-sized equant microspar 5-15 µm in diameter fills MTS veins and also commonly fills porosity in grainstone lenses and cements sheet cracks in stromatolites. A recent gas expansion hypothesis for MTS formation suggests that microbially mediated decay in sediments produced gas bubbles, which propagated through the sediment to produce cracks that were later cemented (Furniss et al, 1998). However, ductile deformation of MTS is inconsistent with thoroughly cemented MTS prior to deformation, and near-zero d¹³C values for MTS microspar do not support an organic decay model.

Observations from the ~2.6 Ga Monteville Formation, Campbellrand Subgroup, South Africa, can resolve these inconsistencies. Cathode-luminescent microscopy and electron microprobe analyses demonstrate that the microspar consists of euhedral to subrounded cores 4-10 µm in diameter, with polygonal overgrowths. Cores are slightly depleted in Mg, Mn, and Fe relative to the rims. The 2-phase precipitation may explain ductile deformation of MTS veins; granular cores may have precipitated and ductiley deformed before cementation by polygonal overgrowths. The uniform size range of microspar cores is consistent with Ostwald Ripening crystal growth. Kile et al (2000) experimentally formed crystal size distributions of calcite and vaterite consistent with modeled Ostwald Ripening curves by rapidly mixing highly concentrated reactants. Crystal size distributions were formed at the nano-scale during diffusion-controlled growth and maintained at the micron scale through surface controlled growth. MTS may have been produced similarly by rapid mixing of pore waters and open ocean waters of contrasting chemistries. Rather than chemically inducing precipitation, microbial decay may simply have provided fluid conduits and mixing sites. Gas expansion cracks, porous sand lenses, and voids in stromatolites would all then be loci of mixing for seawater and pore water. Near-zero d¹³C values suggest that microspar carbon was derived from open seawater. Ca may have been derived from the pore fluids through feldspar weathering to clay, smectite weathering to illite, or dolomitization.