Evaluating the geochemistry and paired silicon and oxygen isotope record of quartz in siliceous rocks from the ~3 Ga Buhwa Greenstone Belt, Zimbabwe, a critical link to deciphering the Mesoarchean silica cycle

Abstract

Rocks that make up the Mesoarchean (~3Ga) Buhwa greenstone belt (BGB) of the Zimbabwe craton divide into three associations: shelf, basinal, and transitional. Chert and iron formation exist within the three associations, allowing the unique opportunity to compare textural and geochemical attributes of iron- and silica-rich rocks from distinct positions within a single basin. We analyzed major-, trace-, and rare-earth-elements, and silicon and oxygen isotopes of chert and iron formation from the shelf, basinal, and transitional associations within the BGB. Samples possess elemental signatures consistent with formation from mixed seawater and hydrothermal fluids (LaSN/LaSN* > 1; PrSN/YbSN < 1; Y/Ho = 28.2–44.4; EuSN/EuSN* = 1.8–2.8). Oxygen isotope values broadly overlap for samples from different associations (ranging from δ18OV-SMOW = 9.41 to 15.10‰ (2σ = 0.8‰)), with isotopic variation linked to the presence of oxide inclusions in some samples. Silicon isotope values by contrast, vary widely. Iron formations possess 30Si-depleted silicon isotope signatures (δ30SiNBS-28 = −2.5 to −0.5‰ (2σ = 0.05‰)), in line with previous studies. Basinal and transitional cherts possess 30Si-depleted silicon isotope signatures (δ30SiNBS-28 = −1.1 to −0.5‰ (2σ = 0.05‰)) that overlap with values measured from iron formation samples. Within the transitional association, cherts are not 30Si-enriched compared to iron formation, while chert in the basinal association is 30Si-enriched compared to iron formation. Depleted silicon isotope signatures could result from fractionation during adsorption, re-equilibration, post-depositional processes, or possibly microbial reduction processes. Silicon isotope heterogeneity recorded in chert preserved in different assemblages across the BGB may in part reflect a seawater signature. The range of silicon isotope values preserved within broadly co-eval rocks from the same assemblage may indicate that samples preserve local isotopic signatures of a heterogeneous reservoir. Future work focused on identifying basin-position dependent trends may inform interpretations of the compiled Precambrian silicon isotope record.