GEOCHEMICAL CONSTRAINTS ON THE DEPOSITION OF MICROFOSSIL-BEARING EARLY DIAGENETIC CHERT IN THE PROTEROZOIC

Silicic acid, a product of silicate weathering, is the most common species of dissolved silica in seawater. Eukaryotes such as diatoms, however, readily remove dissolved silica from seawater, leaving the modern oceans undersaturated with respect to silicic acid. Prior to the evolution of eukaryotes, silica concentrations in the ocean were substantially higher (approximately 100-140 ppm; Holland, 1974), permitting different modes of silica sedimentation. In contrast to modern cherts, which largely reflect concentration of silica-secreting organisms, much chert in the Proterozoic formed from seawater as either a direct precipitate, or as an early diagenetic replacement phase after carbonate. Such early diagenetic chert precipitation was particularly common in Proterozoic peritidal and supratidal facies, and resulting permineralization of substrate microbial communities provides an unparalleled window through which to observe the silicification processes.

With no modern analog, it is critical to use permineralized facies to understand the timing and conditions of silica precipitation. Petrographic analysis shows that silica precipitation occurred largely prior to compaction of sediment, but after varying degrees of post-mortem alteration of the microorganisms present. Furthermore, early diagenetic chert is often associated with evaporitic facies (carbonate, gypsum, halite), suggesting that silica precipitation may have been associated with the increased ionic strength and salinity of ambient fluids. Organic matter may have also promoted silica precipitation by providing either nucleation sites, or changes in pore-water pH.

Possible seawater compositions were modeled with PHREEQC, using pH, pCO₂, pO₂ values estimated for the Proterozoic and elemental concentrations of seawater determined from published fluid inclusion data. Seawater and evaporative seawater models were mixed with a model freshwater. To better constrain the chemistry required to precipitate early diagenetic chert, resulting mixtures were evaluated with respect to their capability of precipitating mineral assemblages observed petrographically in early diagenetic chert.