History and geochemical signatures of hydrothermally silicified volcanic and sedimentary rocks from the Paleoarchean (3.5–3.2 Ga) Barberton Greenstone Belt

Seafloor-derived silicified volcanic and sedimentary rocks provide unique records of hydrothermal systems that operated at the top of the Paleoarchean submerged crust. Based on petrographic, thermometric, and geochemical analyses, we distinguish the signatures of Paleoarchean hydrothermal activity from those due to subsequent metamorphism and weathering in silicified volcanic and sedimentary rocks sampled from the 3.5–3.2 Ga Barberton Greenstone Belt. Measured 138La-138Ce and 147Sm-143Nd isotopic compositions indicate that weathering by post-Archean oxidised fluids modified LREE abundances in samples displaying Ce anomalies. Raman spectroscopy of carbonaceous material, chlorite thermometry and oxygen isotope thermometry provide evidence for mineralogical resetting by regional metamorphism at ∼350 ± 50 °C, which arguably did not modify the bulk-rock geochemistry. Oxygen isotope fractionation in a quartz-carbonate assemblage preserved from subsequent resetting provides a minimum temperature of ∼110 ± 50 °C interpreted as the highest possible temperature of the Paleoarchean silicifying hydrothermal fluids. Y/Ho and Zr/Hf ratios are chondritic in silicified volcanic and clastic sedimentary rocks, which differentiates them from Archean orthochemical cherts with suprachondritic Y/Ho and Zr/Hf ratios. Finally, silicified volcanic rocks that are free of Ce anomalies (mostly unweathered by oxidising fluids) display slightly lower Sm/Nd ratios and more variable Lu/Hf ratios than non-silicified counterparts, which we ascribe to differential REE mobilisation by silicifying hydrothermal fluids. The modification of Sm/Nd and Lu/Hf ratios during Paleoarchean hydrothermal activity should be integrated in future Sm-Nd and Lu-Hf isotopic investigations of hydrothermal inputs to Archean ocean chemistry and of recycled seafloor-derived rocks.