A Hf-Nd isotopic correlation in ferromanganese nodules

The ¹⁷⁶Hf/¹⁷⁷Hf 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 (є_Hf ∼ +1) than in the Pacific (є_Hf ∼ +9). A good correlation of є_Hf and є_Nd is observed amongst most samples which supports that Hf isotopic compositions in nodules reflect those of ambient seawater. For a given є_Nd, є_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.