LAB-CREATED ABIOTIC SILICA GELS: INSIGHTS INTO PROTEROZOIC SILICIFICATION PROCESSES

Chert deposits occur as lenses, nodules or beds in Meso- and Neoprotozoic carbonate strata. Microorganisms can be exceptionally preserved in these deposits and commonly show no evidence of a precursor phase, suggesting that silica is a primary mineral phase. The silicification process in low-temperature surface environments, however, is not yet understood. To enhance our understanding of this process, laboratory experiments were designed using an artificial seawater spiked with various concentrations of sodium silicate and magnesium chloride. The lab-made seawater was spiked with sodium silicate (Na2Si3O7) at concentrations of 35 ppm, 70 ppm, 100 ppm, and 120 ppm and allowed to evaporate over 24-48 hours.

Twelve experiments were conducted with the solutions made in the lab. Five of the twelve were spiked with magnesium chloride, immediately forming a solid evaporite phase. An additional 200 microliters of sodium silicate were added to the other seven tests. Of these, several samples resulted in the formation of a gel phase within 48 hours, although timing was dependent on the container used to hold the sample. Alternatively, some samples evaporated completely, in which case an additional 5 milliliters of its original artificial seawater and silica concentration was added in attempt to rehydrate the sample. Three of these experiments successfully formed a gel.

Preliminary analyses using the environmental scanning electron microscopy (ESEM) and x-ray diffraction spectrometry (XRD) were used to observe the morphology and determine the composition of the gel and evaporite phases formed during our experiments. Gel phases lacked surface textures and were, therefore, unable to be analyzed using ESEM without dehydration. The samples that formed an evaporite, revealed a magnesium-silica-rich phase associated with halite crystals. XRD confirmed the presence of halite and hydrated or amorphous phases in each sample.

We present the preliminary data on the evaporite phases and silica-rich gels that precipitated abiotically in these experiments and hypothesize how these experiments can be used to provide insights into the Proterozoic silicification process.