History and geochemical signatures of hydrothermally silicified volcanic and sedimentary rocks from the Paleoarchean (3.5-3.2 Ga) Barberton Greenstone Belt
Résumé
Seafloor-derived silicified volcanic and sedimentary rocks provide unique records ofhydrothermal systems that operated at the top of the Paleoarchean submerged crust. Basedon petrographic, thermometric, and geochemical analyses, we distinguish the signaturesof Paleoarchean hydrothermal activity from those due to subsequent metamorphismand weathering in silicified volcanic and sedimentary rocks sampled from the3.5–3.2 GaBarberton Greenstone Belt. Measured138La-138Ce and147Sm-143Nd isotopic compositionsindicate that weathering by post-Archean oxidised fluids modified LREE abundances insamples displaying Ce anomalies. Raman spectroscopy of carbonaceous material, chloritethermometry and oxygen isotope thermometry provide evidence for mineralogical resettingby regional metamorphism at∼350±50°C, which arguably did not modify the bulk-rockgeochemistry. Oxygen isotope fractionation in a quartz-carbonate assemblage preservedfrom subsequent resetting provides a minimum temperature of∼110±50°Cinterpreted asthe highest possible temperature of the Paleoarchean silicifying hydrothermal fluids. Y/Hoand Zr/Hf ratios are chondritic in silicified volcanic and clastic sedimentary rocks, whichdifferentiates them from Archean orthochemical cherts with suprachondritic Y/Ho and Zr/Hfratios. Finally, silicified volcanic rocks that are free of Ce anomalies (mostly unweatheredby oxidising fluids) display slightly lower Sm/Nd ratios and more variable Lu/Hf ratios thannon-silicified counterparts, which we ascribe to differential REE mobilisation by silicifyinghydrothermal fluids. The modification of Sm/Nd and Lu/Hf ratios during Paleoarcheanhydrothermal activity should be integrated in future Sm-Nd and Lu-Hf isotopic investigationsof hydrothermal inputs to Archean ocean chemistry and of recycled seafloor-derived rocks
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