Latisha Brengman , Christopher M. Fedo , Martin Whitehouse , Iffat Jabeen, Neil Banerjee

The balance of the Archean silica cycle links fundamentally to large scale processes that govern the composition of the ocean. Shifting contributions from continental or hydrothermal inputs of silica to the ocean is commonly recorded in silica-rich chemical precipitates. Such information is critical to interpreting primordial conditions in early life environments but reading the archive of silica-rich chemical sedimentary rocks remains challenging because of diagenetic and metamorphic overprinting. Here, we utilize a robust geochemical tool – silicon isotopes, to track silica from source to sink within a single Mesoarchean basin – the ~ 3 Ga Buhwa greenstone belt in Zimbabwe. We take a source to sink approach and measure the silicon isotope composition of source material, associated clastic rocks, and three different types of chemical sedimentary rocks (Superior-type iron formation, Algoma-type iron formation, and chert) from the same basin to investigate mechanisms for silicon isotope heterogeneity. We observe that iron formation and chert possess isotopically distinct values, similar to previous studies. Chert is commonly thought to preserve geochemical attributes of seawater; however, we find that silicon isotope values of chert samples vary widely even in a single basin. This could indicate that the water-column was not well-mixed, or that over short time scales within the Buhwa basin, the silica cycle was not in balance. Overall, data from the Buhwa greenstone belt suggest that basin-specific trends may convolute the compiled silicon isotope archive.