New and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3.5–3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ18O values of these cherts indicate extremely high ocean temperatures of 55–85 °C. Previously, the large depletion in 18O shown by all Barberton cherts relative to their Phanerozoic counterparts has been attributed to low 18O in Archean oceans, chert formation during late diagenesis, wholesale loss of 18O during alteration, and/or regional silicification of sediments around hot springs. These alternative explanations are not compatible with the new results.

Cherts in the Onverwacht Group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying Fig Tree and Moodies Groups, demonstrating that the chert δ18O values were fixed prior to Archean uplift and erosion, which started at 3.26 Ga. The maximum δ18O value in Barberton cherts (+22‰) is lower than the minimum values (+23‰) in Phanerozoic bedded cherts, precluding late diagenesis as the explanation of the overall low δ18O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ18O values is also precluded by preservation of δ18O values across different metamorphic grades and by systematic δ18O differences among interbedded chert types, stratigraphic units, and conglomerate clasts.

The 7‰ δ18O variation of these Archean cherts is similar to that of Phanerozoic deep-sea cherts—formed when opal converted to microquartz during burial—but the actual Archean values are ∼10‰ lower. Marine opal was apparently converted to microquartz during burial to depths of <1 km. Cherts with δ18O < 15‰ reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and/or hydrothermal activity. Cherts with higher δ18O values formed during early diagenesis and indicate an extremely hot Archean ocean and surface environment.

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