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 o utermost layers of marine ferromanganese deposits of different types ( hydrogenous and hydrothermal) with a worldwide distribution. The Hf is otopic compositions display a broad range and refine previously report ed 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 Pacifi c samples are for hydrothermal manganese deposits that also have low P b-207/Pb-204, demonstrating that this signature reflects a contributio n from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafniu m concentrations in the deposits increase from 20 ppb to 20 ppm with d ecreasing epsilon(Hf). The Hf and Pb isotopic compositions for ferroma nganese crusts define an apparent mixing trend between literature valu es of average continental crust and MORB. The range in epsilon(Hf) for ferromanganese crusts is narrower than it is for Pb-206/Pb-204 compar ed 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 c o-vary with growth rate, and inversely correlates with Hf isotopic com positions. Hf isotope ratios may be used to determine not only the sou rce of Hf, but possibly the source of Fe and Mn. Measurements of epsil on(Hf) and Hf concentrations in nodule tops, bottoms and associated se diments show that the epsilon(Hf) of nodules is sensitive to sedimenta ry oxic and sub-oxic diagenetic processes and thus most nodules may no t reliably reflect the isotopic composition of Hf in seawater. (C) 199 7 Elsevier Science B.V.