Neoarchean marine chemical sediments as archives of Hadean silicate differentiation

Abstract

Planetary differentiation had a profound influence on the geochemical signature of the Earth’s silicate reservoirs. Some of the early created complementary reservoirs dissipated with time (e.g., Bennett et al., 2007) and only remnants can be observed. Here, we apply the short lived isotopic system 146Sm-142Nd to an alternative archive—marine chemical sediments—and show that alternating Fe- and Si-rich bands from the 2.7 billion-year-old Temagami banded iron formation (BIF), Canada, display significantly different 142Nd isotopic compositions. The Fe-rich bands yield a depleted signature (expressed as deviation from the standard in μ notation) with an average μ142Nd of +7.02 ± 0.71, while the Si-rich bands display modern mantle-like signatures (average μ142Nd −2.83 ± 2.32) likely being the results of mixing between different sources. These complementary signatures reflect the dominant, locally derived source of Nd in the seawater at the time of deposition. Our results promote that layering in BIFs is a syn-depositional feature, and that BIFs are unique geochemical archives capable of recording silicate reservoirs that formed during the Hadean but were still extant during the Neoarchean.