Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 331-342 Aleksandra M. Mloszewska, Ernesto Pecoits, Nicole L. Cates, Stephen J. Mojzsis, Jonathan O'Neil, ... The composition of iron formations in the ≥ 3.75 Ga yr old Nuvvuagittuq Supracrustal Belt in northern Québec provides a proxy for seawater composition of the Eoarchean, and perhaps Hadean oceans, as well as constraints on the types of nutrients available to Earth's earliest life forms. Integrated petrologic and geochemical relationships, mapped between mineral phases in thin section and whole-rock chemistry, provide a framework for interpreting bulk and micro-scale variations in these chemical sedimentary precipitates. Results show that there are two distinct chemical sedimentary units in the Nuvvuagittuq belt: i) a banded iron formation (BIF) consisting of alternating micro-bands of magnetite, Ca–Mg–Fe-silicates and quartz, and ii) a more silicate-rich (Fe-poor) unit, the banded silicate-formation (BSF), of alternating micro-bands of quartz and Ca–Mg–Fe silicates. Precursor BIF and BSF deposits were likely layered amorphous silica and ferric-oxyhydroxides, fine-grained carbonate oozes and/or Ca–Mg–Fe rich silicate gels deposited in a marine setting. Low Al2O3, TiO2and HFSE concentrations show that they are relatively detritus-free, with distinctively seawater-like REE + Y profiles and consistently positive Eu anomalies. These features suggest that the rocks preserved their seawater-like compositions despite metamorphic overprinting. The most significant trace elements in the sediments are Ni and Zn. Experimentally-derived partitioning coefficients show that Ni was enriched in Eoarchean seawater as compared to today (up to 300 nM), while Zn was fairly similar (up to 20 nM). Compositional resemblances between the Nuvvuagittuq sediments and those documented in the ca. 3.8 Ga Isua supracrustals (West Greenland) provide a plausible case that global ocean processes – in terms of trace metal abundances – had reached steady-state by the Eoarchean. Highlights ► We examine the composition of chemical sediments of the ≥ 3.75 Ga Nuvvuagittuq belt. ► These chemical sediments are relatively pure, with seawater-like REE + Y signatures. ► Thus, they can be considered as potential proxies for Eoarchean seawater composition. ► The most significant trace metals in these sediments are Ni and Zn. ► Our results imply that compared to modern seawater Ni was high and Zn fairly similar.