Nickel isotopes and rare earth elements systematics in marine hydrogenetic and hydrothermal ferromanganese deposits

International audienceAttention is now being given to Ni isotope systematics in hydrogenetic marine ferromanganese (Fe-Mn) crusts as paleoceanographic proxies. Previous work focused on identifying both mineralogy (post-depositional) and source effects (Gall et al. 2013; Gueguen et al. 2016), in particular regarding hydrothermal inputs in the oceans and the response of Ni isotope biogeochemical cycling through time. The most important sink for Ni in the oceans is the Fe-Mn oxides sink, but estimation of its Ni isotope composition is only based on hydrogenetic Fe-Mn crusts. In this study, we investigated a range of Fe-Mn deposits including Fe-Mn deposits variably affected by hydrothermal inputs, including hydrothermal deposits from the Lau back-arc basin (South West Pacific) and Lo'ihi seamount (Hawaii), hydrogenetic crust and nodules from the Bauer Basin (Pacific Ocean). Nickel isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using a double-spike (61Ni and 62Ni) correction method. The combination of Ni isotopes and rare earth element (REE) geochemistry show that Ni isotope fractionation in Fe-Mn deposits is essentially controlled by formation processes of the deposits (such as the rate of formation, the initial Mn-phase and sorption processes) which are also related to the depositional environment. Consistent with previous studies, pure hydrogenetic crusts are characterized by isotopically heavy Ni isotope signatures (δ60/58Ni values range from ‰ 0.9 and 2.5‰) and well-developed positive Ce anomalies. In contrast, mixed hydrothermal‑hydrogenetic crust and nodules from the Bauer Basin (East Pacific) display negative Ce anomaly and lighter δ60/58Ni values (0.3‰ to 0.4‰), which are interpreted as the result of far-field hydrothermal inputs of Fe-Mn precipitates from the East Pacific Rise. Nickel in hydrothermal deposits from the Lau Basin (0.5 and 1.1‰) and Lo'ihi seamount (−0.8 to −1.5‰) is isotopically lighter than in hydrogenetic Fe-Mn crusts. Light δ60/58Ni values in Lo'ihi deposits is due to the removal of Ni during Ni adsorption from seawater and from the hydrothermal fluid (between 0 and 1.4‰) on Fe-oxides followed by isotope fractionation between the fluid and the mineral phase. Results suggest that Ni isotopes in hydrothermal Fe-rich deposits are strongly fractionated relative to the seawater/fluid source due to partial removal of Ni on Fe-phases. Hydrothermal Mn-oxides deposits from the Lau Basin acquired their Ni isotope signature through Ni adsorption and continuous exchange of Ni with seawater. We propose that the systematic difference in Ni isotope signatures between hydrogeneous and hydrothermal Fe-Mn deposits is related to the mechanisms of Ni uptake into oxide minerals (e.g., birnessite vs. todorokite; Fe-oxides vs. Mn-oxides) which depend on the rate of formation and the source of Mn and Fe to marine ferromanganese deposits (i.e., depositional environment) rather than Ni sources