Rare earth element variations in mid-Archean banded iron formations: Implications for the chemistry of ocean and continent and plate tectonics

Abundances of major and rare earth elements (REEs) are reported for mid-Arc hean (3.3-3.2 Gyr) sedimentary rocks including banded iron formations (BIFs ) and ferruginous/siliceous mudstone from the Cleaverville area in the Pilb ara craton, Western Australia. Geological, lithological, and geochemical li nes of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments, ranging from that typical of mid-ocea nic spreading centers to convergent plate boundary settings; a range of env ironments most likely caused by plate movements. Except for the mudstone, t he REE content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons. Europium anomalies decrease up-section (Eu /Eu* values normalized to NASC change from 3.5 to 1.1) as the REE contents and LREE/HREE ratios increase. The striking similarity in these REE signatu res of BIFs and modern hydrothermal sediments leads us to propose that the BIFs were in situ hydrothermal precipitates near a mid-ocean ridge. Signifi cant amounts of terrigenous materials contributed to the siliceous and ferr uginous mudstone of the uppermost horizon. The observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothe rmal to terrigenous suggests that plate tectonics, dominated by horizontal movement, was already operating in the mid-Archean. Distal hydrothermal sed iments without a Eu anomaly (when normalized to NASC) suggest that mid-arch ean seawater had already been strongly influenced by a riverine flux from a n upper continental crust and that this component bore no Eu anomaly (i.e, it had a negative Eu anomaly when normalized to chondrite). In addition to an absence of a Eu anomaly, mid-archean seawater did not have a Ce anomaly, suggesting less oxic conditions in the mid-Archean than in the modern ocea n. Copyright (C) 1998 Elsevier Science Ltd.