ALBERT

Description: Silicon and iron isotope compositions of different physically separated components of enstatite chondrites (EC) were determined in this study to understand the role of nebular and planetary scale events in fractionating Si and Fe isotopes of the terrestrial planet‐forming region. We found that the metal–sulfide nodules of EC are strongly enriched in light Si isotopes (δ〈sup〉30〈/sup〉Si ≥ −5.61 ± 0.12‰, 2SD), whereas the δ〈sup〉30〈/sup〉Si values of angular metal grains, magnetic, slightly magnetic, and non‐magnetic fractions become progressively heavier, correlating with their Mg# (Mg/(Mg+Fe)). White mineral phases, composed primarily of SiO〈sub〉2〈/sub〉 polymorphs, display the heaviest δ〈sup〉30〈/sup〉Si of up to +0.23 ± 0.10‰. The data indicate a key role of metal–silicate partitioning on the Si isotope composition of EC. The overall lighter δ〈sup〉30〈/sup〉Si of bulk EC compared to other planetary materials can be explained by the enrichment of light Si isotopes in EC metals along with the loss of isotopically heavier forsterite‐rich silicates from the EC‐forming region. In contrast to the large Si isotope heterogeneity, the average Fe isotope composition (δ〈sup〉56〈/sup〉Fe) of EC components was found to vary from −0.30 ± 0.08‰ to +0.20 ± 0.04‰. A positive correlation between δ〈sup〉56〈/sup〉Fe and Ni/S in the components suggests that the metals are enriched in heavy Fe isotopes whereas sulfides are the principal hosts of light Fe isotopes in the non‐magnetic fractions of EC. Our combined Si and Fe isotope data in different EC components reflect an inverse correlation between δ〈sup〉30〈/sup〉Si and δ〈sup〉56〈/sup〉Fe, which illustrates that partitioning of Si and Fe among metal, silicate, and sulfidic phases has significantly fractionated Si and Fe isotopes under reduced conditions. Such isotope partitioning must have occurred before the diverse components were mixed to form the EC parent body. Evaluation of diffusion coefficients of Si and Fe in the metal and non‐metallic phases suggests that the Si isotope compositions of the silicate fractions of EC largely preserve information of their nebular processing. On the other hand, the Fe isotopes might have undergone partial or complete re‐equilibration during parent body metamorphism. The relatively uniform δ〈sup〉56〈/sup〉Fe among different types of bulk chondrites and the Earth, despite Fe isotope differences among their components, demonstrates that the chondrite parent bodies were not formed by random mixing of chondritic components from different locations in the disk. Instead, the chondrite components mostly originated in the same nebular reservoir and Si and Fe isotopes were fractionated either due to gas–solid interactions and associated changes in physicochemical environment of the nebular reservoir and/or during parent body processing. The heavier Si isotope composition of the bulk silicate Earth may require accretion of chondritic and/or isotopically heavier EC silicates along with cumulation of refractory forsterite‐rich heavier silicates lost from the EC‐forming region to form the silicate reservoir of the Earth.〈/p〉

Description: DFG, German Research Foundation

Description: In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (〉 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of 〈 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Description: When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these “novel” stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants.The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (τ)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic.

Description: Ferruginous lacustrine systems, such as Lake Towuti, Indonesia, are characterized by a specific type of phosphorus cycling in which hydrous ferric iron (oxyhydr)oxides trap and precipitate phosphorus to the sediment, which reduces its bioavailability in the water column and thereby restricts primary production. The oceans were also ferruginous during the Archean, thus understanding the dynamics of phosphorus in modern-day ferruginous analogues may shed light on the marine biogeochemical cycling that dominated much of Earth's history. Here we report the presence of large crystals (〉5 mm) and nodules (〉5 cm) of vivianite – a ferrous iron phosphate – in sediment cores from Lake Towuti and address the processes of vivianite formation, phosphorus retention by iron and the related mineral transformations during early diagenesis in ferruginous sediments. Core scan imaging, together with analyses of bulk sediment and pore water geochemistry, document a 30 m long interval consisting of sideritic and non-sideritic clayey beds and diatomaceous oozes containing vivianites. High-resolution imaging of vivianite revealed continuous growth of crystals from tabular to rosette habits that eventually form large (up to 7 cm) vivianite nodules in the sediment. Mineral inclusions like millerite and siderite reflect diagenetic mineral formation antecedent to the one of vivianite that is related to microbial reduction of iron and sulfate. Together with the pore water profiles, these data suggest that the precipitation of millerite, siderite and vivianite in soft ferruginous sediments stems from the progressive consumption of dissolved terminal electron acceptors and the typical evolution of pore water geochemistry during diagenesis. Based on solute concentrations and modeled mineral saturation indices, we inferred vivianite formation to initiate around 20 m depth in the sediment. Negative δ56Fe values of vivianite indicated incorporation of kinetically fractionated light Fe2+ into the crystals, likely derived from active reduction and dissolution of ferric oxides and transient ferrous phases during early diagenesis. The size and growth history of the nodules indicate that, after formation, continued growth of vivianite crystals constitutes a sink for P during burial, resulting in long-term P sequestration in ferruginous sediment.