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471954 
Journal Article 
Source heterogeneity for the major components of similar to 3.7 Ga Banded Iron Formations (Isua Greenstone Belt, Western Greenland): Tracing the nature of interacting water masses in BIF formation 
Frei, R; Polat, A 
2007 
Yes 
Earth and Planetary Science Letters
ISSN: 0012-821X
EISSN: 1385-013X 
253 
1-2 
266-281 
English 
We report trace element, samarium (Sm)-neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the similar to 3.7-3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modem seawater with diagnostic cerium (Ce/Ce*), presodymium (Pr/Pr*) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 +/- 41 Ma, indicating that the uranium (U)-lead (Pb) system remained closed after the formation of this BIF. High Pb-207/Pb-204 relative to Pb-206/Ph-204 ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-mu (U-238/Pb-204) character of the source region, an essentially mafic Hadean protocrust. Sm-Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (epsilon Nd (3.7 Ga)similar to +3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass (epsilon Nd(3.7Ga)similar to+1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIT, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater. (c) 2006 Elsevier B.V. All rights reserved. 
Sm-Nd isotopes; Pb isotopes; Isua; BIF; Hadean; Greenland