The Hf isotopic composition of ferromanganese nodules and crusts and hydrothermal manganese deposits: Implications for seawater Hf

We present Hf and Pb isotopic data, and chemical compositions of the outermost layers of marine ferromanganese deposits of different types (hydrogenous and hydrothermal) with a worldwide distribution. The Hf isotopic compositions display a broad range and refine previously reported regional differences as follows: Atlantic Ocean epsilon(Hf) = -4 to +2, Indian Ocean epsilon(Hf) = +2 to +4, Pacific Ocean epsilon(Hf) = +3 to + 10. The most radiogenic Hf isotopic compositions in the Pacific samples are for hydrothermal manganese deposits that also have low Pb-207/Pb-204, demonstrating that this signature reflects a contribution from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing epsilon(Hf). The Hf and Pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and MORB. The range in epsilon(Hf) for ferromanganese crusts is narrower than it is for Pb-206/Pb-204 compared to the differences in isotopic composition of the sources of Hf and Pb. This is consistent with Hf having a longer residence time than Pb. The concentration of Hf in ferromanganese crusts has been found to co-vary with growth rate, and inversely correlates with Hf isotopic compositions. Hf isotope ratios may be used to determine not only the source of Hf, but possibly the source of Fe and Mn. Measurements of epsilon(Hf) and Hf concentrations in nodule tops, bottoms and associated sediments show that the epsilon(Hf) of nodules is sensitive to sedimentary oxic and sub-oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of Hf in seawater. (C) 1997 Elsevier Science B.V.