Diagenetic alteration of natural Fe–Ti oxides identified by energy dispersive spectroscopy and low-temperature magnetic remanence and hysteresis measurements

Abstract

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300–5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite–ulvöspinel and ilmenite–hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe–Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field ( B C). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6–13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in B C for the suboxic samples contrasts a 20–25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in B C at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe K α and Ti K α lines of the Fe–Ti oxides may be used to differentiate between members of the magnetite–ulvöspinel and ilmenite–hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (∼19%), titanomagnetite (∼62%), hemoilmenite (∼17%) and ∼2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (∼66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ∼5% titanomagnetite was detected. The remaining ∼28% comprises Ti-rich particles such as pseudobrookite or rutile.