Iron mineral structure, reactivity, and isotopic composition in a South Pacific Gyre ferromanganese nodule over 4 Ma

摘要

Deep-sea ferromanganese nodules accumulate trace elements from seawater and underlying sediment porewaters during the growth of concentric mineral layers over millions of years. These trace elements have the potential to record past ocean geochemical conditions. The goal of this study was to determine whether Fe mineral alteration occurs and how the speciation of trace elements responds to alteration over similar to 3.7 Ma of marine ferromanganese nodule (MFN) formation, a timeline constrained by estimates from Be-9/Be-10 concentrations in the nodule material. We determined Fe-bearing phases and Fe isotope composition in a South Pacific Gyre (SPG) nodule. Specifically, the distribution patterns and speciation of trace element uptake by these Fe phases were investigated. The time interval covered by the growth of our sample of the nodule was derived from Be-9/Be-10 accelerator mass spectrometry (AMS). The composition and distribution of major and trace elements were mapped at various spatial scales, using micro-X-ray fluorescence (mu XRF), electron microprobe analysis (EMPA), and inductively coupled plasma mass spectrometry (ICP-MS). Fe phases were characterized by micro-extended X-ray absorption fine structure (mu EXAFS) spectroscopy and micro-X-ray diffraction (mu XRD). Speciation of Ti and V, associated with Fe, was measured using micro-X-ray absorption near edge structure (mu XANES) spectroscopy. Iron isotope composition (delta Fe-56/54) in subsamples of 1-3 mm increments along the radius of the nodule was determined with multiple-collector ICP-MS (MC-ICP-MS). The SPG nodule formed through primarily hydrogeneous inputs at a rate of 4.0 +/- 0.4 mm/Ma. The nodule exhibited a high diversity of Fe mineral phases: feroxyhite (delta-FeOOH), goethite (alpha-FeOOH), lepidocrocite (gamma-FeOOH), and poorly ordered ferrihydrite-like phases. These findings provide evidence that Fe oxyhydroxides within the nodule undergo alteration to more stable phases over millions of years. Trace Ti and V were spatially correlated with Fe and found to be adsorbed to Fe-bearing minerals. Ti/Fe and V/Fe ratios, and Ti and V speciation, did not vary along the nodule radius. The delta Fe-56/54 values, when averaged over sample increments representing 0.25-0.75 Ma, were homogeneous within uncertainty along the nodule radius, at -0.12 +/- 0.07 parts per thousand (2sd, n = 10). Our results indicate that the Fe isotope composition of the nodule remained constant during nodule growth and that mineral alteration did not affect the primary Fe isotope composition of the nodule. Furthermore, the average delta Fe-56/54 value of -0.12 parts per thousand we find is consistent with Fe sourced from continental eolian particles (dust). Despite mineral alteration, the trace element partitioning of Ti and V, and Fe isotope composition, do not appear to change within the sensitivity of our measurements. These findings suggest that Fe oxyhydroxides within hydrogenetic ferromanganese nodules are out of geochemical contact with seawater once they are covered by subsequent concentric mineral layers. Even though Febearing minerals are altered, trace element ratios, speciation and Fe isotope composition are preserved within the nodule. Published by Elsevier Ltd.