Hf-Nd Isotope Decoupling in Neoarchean Sea Water Likely From Weathering of Emerging Continents

Decoupled Hf-Nd isotopes in Neoarchean seawater reveal weathering of emerged continents

Authors:

Viehmann et al

Abstract:

Marine chemical sediments from the Temagami banded iron formation (BIF) in Canada exhibit nonchondritic Zr/Hf and Y/Ho ratios and seawater-like rare earth element patterns, indicating that their Hf and Nd are not detrital, but derived from seawater. This is confirmed by Sm-Nd and Lu-Hf isochron ages of 2605 ± 140 Ma (initial εNd +0.03 ± 4.1) and 2760 ± 120 Ma (initial εHf +7.2 ± 5.3), respectively, that overlap within error the 2.7 Ga U-Pb age of associated igneous rocks. The Temagami BIF is therefore an excellent archive of the Nd-Hf isotopic composition of Neoarchean seawater. Whereas εNd2.7Ga values cluster around +1, εHf2.7Ga values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an εHf-εNd distribution is typical of modern seawater, plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks. The only mechanism known to produce natural waters with decoupled Nd and Hf isotope compositions is the incongruent mobilization of Hf from continental crustal material. Therefore, input of such highly radiogenic Hf into seawater requires substantial amounts of evolved Neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering.