The Hf isotope composition of global seawater and the evolution of Hf isotopes in the deep Pacific Ocean from Fe-Mn crusts

This paper presents a revised chemical purification method of Hf for the me asurement of Hf isotope ratios of Fe-Mn crusts on a new generation of doubl e focusing multiple collector plasma source mass spectrometer (MC-ICP-MS). By measuring surface scrapings of hydrogenetic Fe-Mn crusts distributed ove r the three major ocean basins, the present-day Hf isotope distribution of oceanic deep water is assessed in detail. The results show an epsilon (Hf) provinciality correlated with those of other radiogenic isotope tracers suc h as Nd and Pb in agreement with previous studies. This supports the use of Hf isotopes as tracer of element source provenance and water masses. Fe-Mn crusts display the same present-day Hf-Nd isotope array as given before fo r Mn nodules. The smaller isotopic variability of Hf compared with Nd may e ither be caused by a more efficient mixing of Hf than Nd in the ocean due t o a longer residence time or may be a consequence of a systematically more radiogenic Hf than Nd isotope signature delivered to the oceans by weatheri ng of continental crust. A Hf isotope time series was measured on crust VA13/2 to assess the Hf isot ope compositions of the Central Pacific deep water over the past 26 Ma. No consistency is observed between the Hf and Pb isotope time series. In contr ast, Hf and Nd isotope time series display similar patterns which are, howe ver, apparently offset by 2 Ma prior to 14 Ma. Differential diffusion of Hf and Nd does not explain this offset. The smaller amplitude in the Hf isoto pe variations compared with the Nd isotopes rather argues for more efficien t mixing of I-if in the ocean. We suggest that both isotope systems have responded in a similar way to the processes affecting the dissolved radiogenic isotope composition of Pacifi c deep water during this time interval. The parallel increase in epsilon (H f) and epsilon (Nd) observed between 14 and 3 Ma may probably be attributed to the increased inputs of Hf and Nd into the Central Pacific Ocean derive d from the weathering of the Pacific Islands Arcs. Over the past 3 Ma an in creased aeolian continental input derived from Asia most likely caused nega tive shifts in epsilon (Nd) and epsilon (Hf) recorded by VA13/2. The fact t hat Hf and Nd isotope compositions plot along the present-day array for Fe- Mn crusts and Mn nodules over the entire past 26 Ma suggests that aeolian s upply of Hf to the Pacific Ocean has been a long-term important feature. (C ) 2001 Elsevier Science B.V. All rights reserved.