A Hf-Nd isotopic correlation in ferromanganese nodules

The Hf-176/Hf-177 ratio was measured on 34 ferromanganese nodules, mostly from the Atlantic ocean. The different ocean basins are isotopically distinct with the extreme compositions being less radiogenic in the Atlantic (epsilon(Hf) similar to +1) than in the Pacific (epsilon(Hf) similar to +9) A good correlation of epsilon(Hf) and epsilon(Nd) is observed amongst most samples which supports that Hf isotopic compositions in nodules reflect those of ambient seawater. For a given epsilon(Nd), epsilon(Hf) is more radiogenic in ferromanganese nodules than in rocks from either the mantle or the crust. This correlation makes the coupled Hf-Nd systems a potential paleoceanographic tool. It is argued that a zircon-free clayish component of probable eolian origin may account for the radiogenic Hf in nodules.