Transition metal isotope fractionation in marine hydrothermal deposits of the Mohns Ridge, North Atlantic Ocean
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Seafloor hydrothermal vent systems form along mid-ocean ridges in all of the Earth’s oceans. They have a major impact on the chemical exchange between the lithosphere and the hydrosphere, as vast volumes of seawater cycle through these systems, thereby interacting with young, oceanic crust. Furthermore, seafloor hydrothermal vent systems provide an excellent environment for organisms to thrive, resulting in diverse and unique vent faunas. Due to their favourable ecological conditions and their existence throughout Earth’s history, they are regarded as a potential cradle where life on Earth could have emerged. Transition metals, such as iron, copper and zinc, are essential nutrients for all organisms on Earth and, thus, metabolic processes have direct influence on cycling of these elements in the environment. The development of high-resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in the mid-1990s enabled the use of transition metal stable isotope variations in nature as geochemical tracers. Studies on Fe, Cu and Zn isotope variations have revealed that metabolic reactions are capable of fractionating stable isotopes of these transition metals. Consequently, researchers have tried to find distinct isotopic fingerprints that allow identifying remnants of biological activity in geological samples, since DNA or microfossil structures are often missing in especially ancient samples due to later geological overprint. Unambiguous biological transition metal isotope signatures, however, have not been discovered so far. The objective of this thesis is to better understand fractionation of Fe, Cu and Zn isotopes in seafloor hydrothermal vent systems in general, and whether isotope variations of these transition metals may help to unravel (biological) formation processes of ancient hydrothermal deposits in particular. For this purpose, analytical methods were developed to determine Cu and Zn isotope variations in Fe-rich hydrothermal samples. Copper and zinc were purified from the sample matrices using a two-step ion-exchange chromatographic procedure. It was shown that no fractionation of Cu isotopes occurred during chromatographic separation of copper by applying a standard addition approach with an enriched 65Cu spike. Furthermore, a new 64Zn-67Zn double spike was calibrated in order to correct for instrumental mass bias during Zn isotope ratio determinations by MC-ICP-MS. Cu and Zn isotope measurements of international reference materials and inter-laboratory data comparison between the isotope laboratories at the University of Bergen and the Imperial College, London, confirmed the accuracy and applicability of the established analytical methods. The samples investigated in this thesis derived from the Jan Mayen and the Loki’s Castle vent fields, situated along the Mohns Ridge, North Atlantic. Low temperature hydrothermal venting at the Jan Mayen vent fields leads to the formation of extensive microbial mats that mediate the precipitation of layered, siliceous Fe oxyhydroxide deposits. These deposits exhibit substantial amounts of encrusted filaments of the Fe-oxidising bacterium Mariprofundus ferrooxidans. Fe isotopic compositions of the siliceous Fe oxyhydroxides span a range from -2.09 to -0.66 ‰ in δ56Fe, which is most likely the result of partial oxidation of hydrothermal Fe(II)aq by low concentrations of free oxygen in fluid-filled cavities within the deposits and/or by microbial iron oxidation. The Jan Mayen samples are enriched in the heavy Zn isotopes relative to the low temperature hydrothermal fluids, most likely caused by isotope fractionation during adsorption of Zn aquo complexes onto the surfaces of the siliceous Fe oxyhydroxides. Cu isotopes in the Jan Mayen samples, on the other hand, are fractionated towards lower δ65Cu values relative to igneous rocks. Here, Cu isotope fractionation might be caused by partitioning of copper into different organic and inorganic complexes and subsequent preferential, pH-dependent adsorption of Cu aquo complexes onto siliceous Fe oxyhydroxides and/or by assimilation and adsorption of isotopically light copper by microorganisms. Isotope variations in the modern Jan Mayen siliceous Fe oxyhydroxide deposits were compared to those in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which are interpreted to have formed from white smoker hydrothermal fallout deposits. Fe isotope variations in the Løkken jaspers, ranging from -0.38 to +0.89 ‰ in δ56Fe, point to partial oxidation of Fe(II)aq in the hydrothermal plume. The isotopic compositions of copper and zinc in the jaspers are comparable to those of the modern siliceous Fe oxyhydroxide deposits from the Jan Mayen vent fields, and isotope fractionation might have been caused by similar (bio)chemical reactions despite different formation pathways of the two deposits. However, interpreting reactions causing the observed Cu and Zn isotope fractionations in these hydrothermal systems remains speculative. Besides low temperature deposits, hydrothermal sulphides which formed in high temperature white smoker chimneys at the Jan Mayen vent fields and in black smoker chimneys at the Loki’s Castle vent field were investigated. In these environments, fractionation of transition metal isotopes is mostly driven by inorganic chemical reactions. Variations of Fe, Cu and Zn isotopes were used to trace reaction pathways of sulphide formation. Isotopically light iron is incorporated into iron mono- and disulphides, such as pyrrhotite and marcasite, respectively, reflecting kinetic Fe isotope fractionation during sulphide precipitation. Kinetic isotope effects are most likely also responsible for low δ56Fe and δ66Zn values in sphalerite solid solution, compared to δ56Fe and δ66Zn values of high temperature hydrothermal fluids. A correlation between FeS concentration and Zn isotopic composition in sphalerite was found, which might indicate an impact of zinc substitution for iron on Zn isotope fractionation. Equilibrium isotope fractionation of copper and iron between hydrothermal fluids and sulphides was identified during formation of isocubanite and chalcopyrite. Here, isotope fractionation is most likely driven by changes in the oxidation states of iron and copper. Overall, the results are in agreement with experimental studies published in literature. The findings of this thesis show that transition metal isotope variations can be successfully used to trace chemical reactions in hydrothermal vent systems. However, the results also confirm that investigations solely based on Fe isotope variations are not suitable to distinguish between microbial and inorganic oxidation of reduced Fe(II)aq, which is an important reaction in oceanic and terrestrial low temperature environments. Combined studies of isotope fractionation of different transition metals, as presented in this thesis, are certainly a better approach to unravel formation reactions of minerals and hydrothermal deposits. However, the results of this thesis also emphasise the need of further research on metal isotope fractionation both in nature and laboratory experiments to enhance our knowledge of transition metal isotope fractionation and, thus, to allow accurate interpretations of measured isotope variations in nature.
Paper I: Calibration of the new Certified Isotopic Reference Materials ERM-AE633 and ERM-AE647 for Copper and IRMM-3702 for Zinc Isotope Determinations (in press). Full-text not available due to publisher restrictions. The published version is available at: http://dx.doi.org/10.1111/j.1751-908X.2011.00153.xPaper II: Comparison of Transition Metal Isotope Variations of Modern and Ordovician Siliceous Fe Oxyhydroxide Deposits (in preparation). Full-text not available.Paper III: Transition Metal Isotope Fractionation during Formation of Hydrothermal Sulphides (in preparation). Full-text not available.