CRYSTALLIZATION OF OPAL TO LENGTH-FAST CHALCEDONY WITHIN PROTEROZOIC MICROFOSSILIFEROUS CHERT

The ~1.1 Ga Angmaat Formation, northern Baffin Island, Canada, contains abundant early diagenetic chert; exquisite preservation of microbial assemblages within this chert provides a critical window into organic preservation. Previous work has focused on the preservation of microfossils within the chert, and detailed petrographic evidence which highlights the dominance of a fabric composed of (10–200 µm) length-fast chalcedony spherules that comprise the bulk of the silica, associated with mimetically-preserved depositional fabrics. Length-fast spherules have both non-touching and compromise boundaries, discrete regions of different crystal size, and lognormal crystal size distributions that together suggest nucleation and growth within a nutrient-limited (i.e. closed) system, which we suggest was represented by a precursor silica gel phase.

Experimental and observational data show that silica precipitation typically initiates as nm-scale spherules of opal-A, which then convert to opal-CT and opal-C, and finally to fibrous chalcedony or granular quartz. This traditional diagenetic model involves substantial water loss, which is not consistent with the mimetic preservation of primary fabrics, as is observed in the Angmaat Formation. Early precipitation of opal-CT, however, would require far less water loss, and may represent a more likely pathway for preservation of primary microbial mat elements. Increased cross-linking of silanol, however, can result in a strongly polymerized gel that has reduced water content and a dense molecular framework. Polymeric linkages within the gel phase have been inferred to provide sufficient structure for precipitation of opal-CT, bypassing precipitation of opal-A, and resulting in minimal water loss associated with post-depositional recrystallization to chalcedony.

Here we propose that Angmaat chert precipitated under conditions favorable for the formation of a dense siliceous gel that could precipitate opal-CT without an opal-A precursor, resulting in a greater potential for mimetic preservation of depositional fabrics. Here we explore the use of Raman and/or FTIR to understand the water content of Angmaat chert and to test whether data can be used to trace chalcedony end members to their precursor opaline phase.