Constant iron isotope composition of the upper continental crust over the past 3 Gyr

The Fe isotopic composition of twenty four glacial diamictite composites with depositional ages ranging from the Mesoarchean to the Palaeozoic serve as proxies of the average upper continental crust (UCC) and can be used to track how delta Fe-56 may have changed in the continental crust through time. The diamictites have elevated chemical index of alteration (CIA) values and other characteristics of weathered regoliths e.g., strong depletion in soluble elements such as Sr), which they inherited from their upper crustal source regions. The delta Fe-56 values in the diamictite composites range from -0.59 parts per thousand to +0.23 parts per thousand. Excluding three samples impacted by the incorporation of materials from Fe formations, the diamictites have an average delta Fe-56 of 0.12 +/- 0.13 parts per thousand (2 sigma), overlapping the recent estimated average delta Fe-56 of 0.09 +/- 0.03 %o (2 s.d.) in the upper continental crust (Dauphas et al., 2017, and references therein). There is no obvious correlation between delta Fe-56 of the glacial diamictites and the CIA. Our data suggest that the Fe isotope composition of the upper continental crust has been relatively constant throughout Earth history and that chemical weathering is not important in producing Fe isotope variations in the upper continental crust. Pre-Great Oxidation Event (GOE) anoxic weathering, when iron was soluble in its divalent state, did not generate different Fe isotopic signatures from the post-GOE oxidative weathering environment in the upper continental crust. Therefore, the large Fe isotopic fractionations observed in various marine sedimentary records are likely due to processes occurring in the oceans (e.g., biological activity) rather than abiotic redox reactions on the continents.