Iron isotope, major and trace element characterization of early Archean supracrustal rocks from SW Greenland: Protolith identification and metamorphic overprint

The iron isotopic composition of subglacial streams draining the Greenland ice sheet

“…Dauphas et al, 2007) that may impact the d 56 Fe of the dissolved load. The GKU outflows have SMF values ranging from 0.3 to 0.53, suggesting in the context of the SFM, sulfide oxidation is linked to carbonate dissolution.…”

Metamorphic alteration, mineral paragenesis and geochemical re-equilibration of early Archean quartz–amphibole–pyroxene gneiss from Akilia, Southwest Greenland

“…The term ''quartz-pyroxene rock'' has been the most commonly used in the recent literature (Fedo and Whitehouse 2002a;Dauphas et al 2004Dauphas et al , 2007Lepland et al 2005;Manning et al 2006). However, the results presented here indicate that labeling this lithology as ''quartz-pyroxene rock'' is misleading.…”

“…The original inference of this lithology as chemically precipitated banded iron formation (BIF) was based on (1) magnetite layering, (2) similarity to Isua BIF (undisputed chemical sediment), (3) agreement with the criteria of James (1954) for BIF, and (4) its position between mafic and ultramafic metavolcanic rocks, an association that is often found in Archean supracrustal sequences (Nutman et al 1997). Additional support for a sedimentary origin of the quartzamphibole-pyroxene gneiss rock was gained from (5) inclusions of isotopically light graphite, interpreted as biogenic, in apatite crystals (Mojzsis et al 1996;McKeegan et al 2007), (6) sulfide minerals bearing signatures of mass independent fractionation (MIF) of sulfur (Mojzsis et al 2003), (7) Fe isotopic composition that is similar to several well characterized BIFs (Dauphas et al 2004(Dauphas et al , 2007, and (8) trace element compositions that have been argued to be comparable to Isua BIFs (Friend et al , 2008Mojzsis and Harrison 2002b;Nutman and Friend 2006;Manning et al 2006). Fedo and Whitehouse (2002a), on the other hand, have argued on the grounds of structural and geochemical characteristics that the Akilia quartz-amphibole-pyroxene gneiss is an intensively and multiply deformed and metasomatised (silica and carbonate) ultramafic rock.…”

“…Further sulfur isotope analyses have been unable to confirm the previously reported MIF component in samples from the quartz-amphibole-pyroxene gneiss (Mojzsis et al 2003); hence, a sedimentary protolith interpretation based on sulfur isotope data is rendered insecure. While most of the different lines of evidence that have been used to argue for either sedimentary or non-sedimentary protolith of the Akilia quartz-amphibolepyroxene gneiss have been challenged, a plausible nonsedimentary mechanism to explain the Fe isotope signature seen in some samples of the Akilia quartz-amphibolepyroxene gneiss (Dauphas et al 2004(Dauphas et al , 2007) has yet to be proposed.…”

In-situ iron isotope analysis of pyrites in ~ 3.7 Ga sedimentary protoliths from the Isua supracrustal belt, southern West Greenland

“…The positive δ 56 Fe values indicate that the oxidant(s) existed even in the Eoarchean, but were limited in their availability. In contrast, the iron isotopic signature of microbial activity on the early Earth is a matter of debate (Dauphas et al, 2007a;Whitehouse and Fedo, 2007;Craddock and Dauphas, 2011;Czaja et al, 2013). A δ 56 Fe value of Fe 2+ aq produced by microbial DIR is expected to have a lower value than that of the precursor, but negative δ 56 Fe values are scarcely found in sedimentary record prior to about 2.9 Ga (Fig.…”