Hf isotope ratios of marine sediments and Mn nodules: evidence for a mantle source of Hf in seawater

Hf isotope ratios are reported for 6 Mn nodules and 15 other sediments, including siliceous oozes, red clays and terrigenous sediments. Overall, ε Hf ranges from −19.8 to +3.5, but only from +0.4 to +3.5 in the Mn nodules. The Indian and Atlantic nodules have ε Hf values (+0.4 and +0.9) which are only slightly lower than values from Pacific nodules (+1.9 to +3.5). The positive ε Hf values of the Mn nodules indicate much, and perhaps most, of the Hf in seawater is derived from some “mantle” source such as ocean ridge hydrothermal activity, low-temperature alteration of the oceanic crust, or weathering of the young volcanic terranes of the circum-Pacific region. The dominance of “mantle” sources of Hf may result from a very low flux of Hf from the continents rather than a particularly high volcanic one. A low continental flux may result from retention of Hf in zircon, a mineral which is very resistant to weathering. The near uniformity of Hf in nodules from various oceans suggests a longer seawater residence time for Hf than for Nd, probably because Hf is not readily scavanged by particulate matter.