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Stratigraphic changes of Ge/Si, REE+Y and silicon isotopes as insights into the deposition of a Mesoarchaean banded iron formation
Delvigne, C.; Cardinal, D.; Hofmann, A.; André, L. (2012). Stratigraphic changes of Ge/Si, REE+Y and silicon isotopes as insights into the deposition of a Mesoarchaean banded iron formation. Earth Planet. Sci. Lett. 355-356: 109-118. dx.doi.org/10.1016/j.epsl.2012.07.035
In: Earth and Planetary Science Letters. Elsevier: Amsterdam. ISSN 0012-821X, more
Peer reviewed article  

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Author keywords
    silicon isotopes; Ge/Si ratios; banded iron formation; Archaean; PongolaSupergroup

Authors  Top 
  • Delvigne, C., more
  • Cardinal, D., more
  • Hofmann, A.
  • André, L., more

Abstract
    In order to determine the origin of silicon (Si) in banded iron formation (BIF), we have undertaken a multi-tracer study combining REE+Y data, Ge/Si ratios and Si isotopes (d30Si) on stratigraphically resolved layers from a ~2.95 Ga BIF from the Pongola Supergroup, South Africa. Si in both Si-rich and Fe-rich layers has a common origin, represented by a seawater reservoir strongly influenced by continent-derived freshwaters (~10%) and very limited (<0.1%) high-T hydrothermal fluids as indicated by Eu anomalies and Y/Ho ratios. The coevolution of d30Si signatures of Si- and Fe-rich layers of the BIF coupled with similar Eu and Y anomalies in both types of layers is in accordance with a common silica precipitation promoted by Si adsorption onto Fe-oxyhydroxides from Archaean seawater. An increase in d30Si values from -2.27‰ to -0.53‰ stratigraphically upwards in the BIF is inferred to be the result of two successive isotopic fractionation processes during (1) silicon adsorption onto the Fe-oxyhydroxide precursor and (2) silica precipitation at the sediment–water interface from pore fluid triggered by the local silica saturation consecutive to an early diagenetic Si desorption from the precursor Fe-oxyhydroxide. The first fractionation process depleted the parental water in 28Si while the second released 30Si back into the parental water, resulting in an increase of the d30Si value of the parental water reservoir over time.

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