FROM SOURCE TO SINK: SILICON ISOTOPE COMPOSITION OF BEDROCK, DETRITAL QUARTZ ARENITES, CHERT, AND BIF FROM THE ~3 GA BUHWA GREENSTONE BELT, ZIMBABWE

Disparate interpretations of silicon isotope values (δ³⁰Si) of quartz from Precambrian chert and banded iron formation (BIF) preclude the possibility of a unified explanation for any observed trend in the compiled silicon isotope archive. Attempting to resolve this problem and to develop a clear picture of silicon sources and sinks from one location, we used SIMS to measure the silicon isotope composition of different rock types (bedrock, sandstone, chert, BIF) from the variably metamorphosed ~3 Ga Buhwa Greenstone Belt, Zimbabwe. Supracrustal rocks have been divided into (1) an upward-deepening, marine-shelf succession of quartz arenites and shales capped by Superior-type BIF, and (2) a package of mafic-ultramafic rock, meta-chert, and Algoma-type BIF. A succession of shale intercalated with m-thick beds of chert and BIF crops out in between. Silicon isotopes show no evidence of metamorphic homogenization or preferential fractionation during recrystallization. When divided by rock type, samples possess distinct δ³⁰Si signatures, but display inter- and intra-sample heterogeneity. Crystalline bedrock and quartz arenites possess δ³⁰Si signatures near 0 ‰, due to the lack of significant fractionation during igneous crystallization and physical transport of quartz derived from igneous sources. Superior-type BIF possesses isotopically light values expected for seawater precipitates (δ³⁰Si = -3.3 – -0.2 ± 0.3 ‰; avg. -1.2 ‰), due to the kinetic fractionation observed during precipitation of a silica-rich phase from solution. Algoma-type BIF possesses slightly lighter isotopic values (δ³⁰Si = -3.6 – -1 ± 0.3 ‰; avg. -2.2 ‰) than Superior-type BIF, likely due to precipitation from rapidly evolving hydrothermal fluids. Chert is isotopically heavier (δ³⁰Si = -1.5 – 1.6 ± 0.3 ‰; avg. 0.3 ‰) than associated Algoma-type BIF, potentially indicating different formation mechanisms or fractionation pathways. We propose that these different rock types preserve primary silicon isotope values, and record signatures derived from distinct silicon reservoirs likely within a single marine basin.