A new method for species-specific Fe-isotopes on Paleoproterozoic BIF

Susann.Henkel [ at ] awi.de


Banded iron formations (BIF) deposited during the Great Oxidation Event (GOE) at ~2.4Ga, potentially capture chemical changes in the Paleoproterozoic atmosphere and ocean. The Kuruman and Griquatown BIFs in the Griqualand West Basin, South Africa were deposited concurrent with the GOE. Continuous drill-cores thereof provide important insights into the Paleoproterozoic environment. Bulk-rock samples of these cores show an apparent relationship between modal mineralogy and Fe-isotopes: low δ56Fe-values (<-1.3‰) correlate with samples rich in Fe-carbonate, whereas positive δ56Fe-values correspond to magnetite-rich samples. To interrogate this further, an existing sequential extraction scheme was modified to accommodate the BIF mineralogy [1]. Since ferric oxides are insignificant in the rocks we studied, the hydroxylamine and dithionite leaches could be omitted. The remaining extractions were optimized for the dissolution of three main Fe-hosting mineral fractions (Fe-carbonates, magnetite and Fe-silicates). The final silicate residue was dissolved in HF-HClO4-HNO3. The tests established a 3-step sequential extraction procedure to quantitatively separate the Fe-mineral fractions. To measure the species specific Fe-isotopes a processing-protocol was applied to break down the organic leachates [2]. Combining the species specific data (δ56Fe and REE) with bulk-rock data (C-isotopes and mineralogy) it becomes apparent that the BIFs have recorded primary signals from Paleoproterozoic seawater. References: [1] Poulton & Canfield (2005) Chem. Geo., 214, 209-221. [2] Henkel et al., (2016) Chemical Geology, 421, 93-102

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Nederlands Aardwetenschappen Congres, 07 Apr 2016 - 08 Apr 2016, Veldhoven, The Netherlands.
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Oonk, P. , Tsikos, H. , Mason, P. , Henkel, S. , Staubwasser, M. and Williams, H. (2016): A new method for species-specific Fe-isotopes on Paleoproterozoic BIF , Nederlands Aardwetenschappen Congres, Veldhoven, The Netherlands, 7 April 2016 - 8 April 2016 .

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