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

作者：Viehmann Sebastian^*; Hoffmann J Elis; Muenker Carsten; Bau Michael

来源：Geology, 2014, 42(2): 115-118.

DOI：10.1130/G35014.1

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 epsilon(Nd) +0.03 +/- 4.1) and 2760 +/- 120 Ma (initial epsilon(Hf) +/- 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 epsilon(Nd2.7Ga) values cluster around +1, epsilon(Hf2.7Ga) values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an epsilon(Hf)-epsilon(Nd) 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.

• 出版日期2014-2