Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater

Abstract

It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf-Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf-Nd isotope decoupling in aqueous systems.The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled εNd2.60Ga-εHf2.60Ga values of -2.3 and +9.48, respectively, and suggests that decoupled Hf-Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time.Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive εNd2.60Ga values, but still reveal decoupled εNd-εHf values relative to an associated schist that plots slightly below the 'terrestrial array'. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the 'zircon effect'. Alternatively, the less pronounced decoupling of εHf-εNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial εNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.

Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats

“…The pure BIF samples corroborate the Hf-Nd decoupling in Late Archean seawater that was also reported for the near-contemporaneous Late Archean Temagami BIF [8]. Although there may be potential hydrothermal Hf pathways into Archean seawater similar to the REE [9,68], the εHf 2.70Ga values of the Temagami BIFs are more radiogenic relative to the most positive εHf 2.70Ga value of mantle-derived rocks from the Abitibi Greenstone Belt, suggesting that not only hydrothermal Hf was responsible to decouple the Hf-Nd isotopes in Late Archean seawater, but also another process was needed to produce the radiogenic εHf 2.70Ga signature of Temagami seawater [8]. Hence, the very radiogenic Hf signature at two near-contemporaneous Late Archean BIF localities can only be explained by incongruent Hf weathering of emerged zircon-bearing uppermost continental crust and implies that Hf-Nd decoupling in Late Archean seawater was rather a global than a local phenomenon.…”

“…These fluids carried significant amounts of mantle-derived Nd, which had strong impact on the Nd isotopic composition of Archean seawater. Hafnium is also considered to have a hydrothermal pathway into Archean seawater [9], potentially affecting the Hf isotopic compositions of global Archean oceans.…”

“…Although analytically very challenging due to very low Hf concentrations (down to 20 ppb Hf [9]), the authors obtained Hf-Nd isotope compositions by isotope dilution technique and MC-ICPMS analyses of alternating chert and Fe-rich bands in ultra-pure BIF samples of the Temagami BIF (Canada) that are devoid of detrital contaminations. Sm-Nd as well as Lu-Hf isochron calculations of combined chert and Fe-rich bands of the Temagami BIF yield within the error the published depositional age of 2.7 Ga, suggesting that post-depositional alteration or reset of both isotope systems was negligible [8].…”

“…Large et al [7] showed with a comprehensive trace element study of Precambrian shales that the composition of the uppermost continental crust changed from predominantly ultramafic-mafic to more felsic at around 2.7 Ga ago. Viehmann et al [8,9] applied a novel geochemical approach using combined Hf and Nd isotopes in Late Archean marine chemical sediments to investigate weathering and erosion processes on Archean continents and their impact on Archean seawater chemistry. The authors showed that similar to modern oceans, Hf and Nd isotope systems were already decoupled in the suspended (>0.2 µm) and dissolved (<0.2 µm) fractions of Late Archean seawater.…”

“…The authors showed that similar to modern oceans, Hf and Nd isotope systems were already decoupled in the suspended (>0.2 µm) and dissolved (<0.2 µm) fractions of Late Archean seawater. Decoupled Hf-Nd isotopes in Banded Iron Formations (BIFs) indicate that incongruent Hf weathering and erosion of more evolved, zircon-bearing continental crust already occurred 2.7 Ga ago [8,9]. These findings in combination have major implications on the geodynamical evolution of the Archean continents and, of course, on the nutrient flux derived from the continents into the earliest marine habitats on Earth.…”