High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa

摘要: New and compiled oxygen isotope data combined with the results of geological sedimentological studies demonstrate that enclaves synsedimentary to very early diagenetic cherts are widely preserved in 3.5-3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ 1 8 O values these indicate extremely high ocean temperatures 55-85 °C. Previously, large depletion shown by all relative their Phanerozoic counterparts has been attributed Archean oceans, chert formation during late diagenesis, wholesale loss alteration, and/ or regional silicification sediments around hot springs. These alternative explanations not compatible new results. Cherts Onverwacht Group display an isotopic stratigraphy is inversely repeated conglomerates overlying Fig Tree Moodies Groups, demonstrating 0 were fixed prior uplift erosion, which started at 3.26 Ga. maximum value (+22‰) lower than minimum (+23‰) bedded cherts, precluding diagenesis as explanation overall values. Regional metamorphic, hydrothermal, long-term resetting original also precluded preservation across different metamorphic grades systematic differences among interbedded types, stratigraphic units, conglomerate clasts. 7‰ variation similar deep-sea cherts-formed when opal converted microquartz burial-but actual ∼10‰ lower. Marine was apparently burial depths <1 km. < 15‰ reflect conversion deepest local areas enhanced geothermal gradient hydrothermal activity. higher formed surface environment.