Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits

Formation pathways of ancient siliceous iron formations and related Fe isotopic fractionation are still not completely understood. Investigating these processes, however, is difficult as good modern analogues to ancient iron formations are scarce. Modern siliceous Fe oxyhydroxide deposits are found at marine hydrothermal vent sites, where they precipitate from diffuse, low temperature fluids along faults and fissures on the seafloor. These deposits exhibit textural and chemical features that are similar to some Phanerozoic iron formations, raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes. In this study, we present the first data on modern Fe oxyhydroxide deposits from the Jan Mayen hydrothermal vent fields, Norwegian-Greenland Sea. The samples we investigated exhibited very low δ56Fe values between -2.09‰ and -0.66‰. Due to various degrees of partial oxidation, the Fe oxyhydroxides are with one exception either indistinguishable from low-temperature hydrothermal fluids from which they precipitated (-1.84‰ and -1.53‰ in δ56Fe) or are enriched in the heavy Fe isotopes. In addition, we investigated Fe isotope variations in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume, in order to compare different formation pathways of Fe oxyhydroxide deposits. Iron isotopes in the jasper samples have higher δ56Fe values (-0.38‰ to +0.89‰) relative to modern, high-temperature hydrothermal vent fluids (ca. -0.40‰ on average), supporting the fallout model. However, formation of the Ordovician jaspers by diffuse venting cannot be excluded, due to lithological differences of the subsurface of the two investigated vent systems. Our study shows that reliable interpretation of Fe isotope variations in modern and ancient marine Fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context. Furthermore, we demonstrate that very negative δ56Fe values in such samples might not be the result of microbial dissimilatory iron reduction, but could be caused instead by inorganic reactions.

Pelagic molybdenum concentration anomalies and the impact of sediment resuspension on the molybdenum budget in two tidal systems of the North Sea

The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-Rømø and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First δ98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of ε = −0.3‰ has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column.

Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps)

Here, we present sedimentological, trace metal, and molecular evidence for tracking bottom water redox-state conditions during the past 12,500 years in nowadays sulfidic and meromictic Lake Cadagno (Switzerland). A 10.5 m long sediment core from the lake covering the Holocene period was investigated for concentration variations of the trace metals Mn and Mo (XRF core scanning and ICP-MS measurements), and for the presence of anoxygenic phototrophic sulfur bacteria (carotenoid pigment analysis and 16S rDNA real time PCR). Our trace metal analysis documents an oxic-intermediate-sulfidic redox-transition period beginning shortly after the lake formation similar to 12.5 kyr ago. The oxic period is characterized by low sedimentary Mn and Mo concentrations, as well as by the absence of any remnants of anoxygenic phototrophic sulfur bacteria. Enhanced accumulation/preservation of Mn (up to 5.6 wt%) in the sediments indicates an intermediate, Mn-enriched oxygenation state with fluctuating redox conditions during a similar to 2300-year long transition interval between similar to 12.1 and 9.8 kyr BP. We propose that the high Mn concentrations are the result of enhanced Mn2+ leaching from the sediments during reducing conditions and subsequent rapid precipitation of Mn-(oxyhydr) oxide minerals during episodic and short-term water-column mixing events mainly due to flood-induced underflows. At 9800 +/- 130 cal yr BP, a rapid transition to fully sulfidic conditions is indicated by the marked enrichment of Mo in the sediments (up to 490 ppm), accompanied by an abrupt drop in Mn concentrations and the increase of molecular biomarkers that indicate the presence of anoxygenic photosynthetic bacteria in the water column. Persistently high Mo concentrations >80 ppm provide evidence that sulfidic conditions prevailed thereafter until modern times, without any lasting hypolimnetic ventilation and reoxygenation. Hence, Lake Cadagno with its persistently stable chemocline offers a framework to study in great temporal detail over similar to 12 kyr the development of phototrophic sulfur bacteria communities and redox processes in a sulfidic environment, possibly depicting analogous conditions in an ancient ocean. Our study underscores the value of combining sedimentological, geochemical, and microbiological approaches to characterize paleo-environmental and -redox conditions in lacustrine and marine settings.

High precision determination of the terrestrial 40K abundance

Recent improvements in the precision of mass spectrometric measurements have reduced the uncertainty of K-Ar and 39Ar-40Ar ages measured on geological materials. Now the major sources of uncertainty are the uncertainties on the 40K decay constant and the absolute abundance of 40K. In order to improve on this situation we determined the abundance of the 40K isotope in terrestrial standards. A ThermoFischer Triton+ thermal ionization mass spectrometer was used for K isotope ratio measurements of the NIST K standard reference materials SRM 918b and SRM 985. Ion beams were measured in Faraday cups with amplifiers equipped with 1E10, 1E11 and 1E12 Ω resistors. Three measurement protocols were used: (A) dynamic measurement with in-run fractionation correction by normalization to the IUPAC recommended isotope ratio 41K/39K = 0.0721677; (B) total evaporation; (C) a modified total evaporation with interblock baseline measurements. Different measurement protocols were combined with different loading procedures. The best results were obtained by loading samples on single tantalum filaments with 0.1M H3PO4. The total ion yields (ionization + transmission) were tested for the evaporation procedures (B) and (C) and ranged up to 48 %. The resulting best estimate for the 40K/39K ratio is 0.000 125 116 ± 57 (2σ), corresponding to 40K/K = (1.1668 ± 8; 2σ) x 10-4.

K-feldspar hygrochronology

The significance of the multi-isotopic record preserved in K-feldspars is assessed on samples from the Aar metagranite, Central Alps, Switzerland having very tight independent geological constraints. Stepwise leaching reveals that two diachronically grown K-feldspar generations coexist: Kfs-1 (≥ 35 Ma old, Ca-poor, Rb-Cl-rich, with low 87Sr/86Sr and high 206Pb/204Pb) and Kfs-2 (≤ 10 Ma old, antithetic isotopic signatures deriving from external fluids). Microtextures imaged by cathodoluminescence, backscattered electrons, and electron probe microanalysis are patchy and chemically heterogeneous, with pronounced enrichments in Ba in the retrogressed regions. This confirms the simultaneous presence of fluid-dominated retrogression and recrystallization and isotopic inheritance. The staircase-shaped 40Ar/39Ar age spectrum correlates with the Ca/K and Cl/K signatures. This reflects a mixture of heterochemical K-feldspar generations, and not an intracrystalline Ar gradient caused by diffusion. The shape of the age spectrum and the in vacuo release kinetics proceed from entirely different physical and geological phenomena. What K-feldspars can be effectively used for is to constrain the timing of the fluids that interacted with them by multi-isotopic analyses, rather than to model a “cooling history” from 39Ar release alone. The identification of multiple mineral generations by imaging combined with multi-isotopic analysis enables the accurate dating of the events of a multistage evolution after the initial crystallization of the rock in which the minerals occur.

REE and Hf distribution among mineral phases in the CV-CK clan : a way to explain present-day Hf isotopic variations in chondrites

Chondrites are among the most primitive objects in the Solar System and constitute the main building blocks of telluric planets. Among the radiochronometers currently used for dating geological events, Sm–Nd and Lu–Hf are both composed of refractory, lithophile element. They are thought to behave similarly as the parent elements (Sm and Lu) are generally less incompatible than the daughter elements (Nd and Hf) during geological processes. As such, their respective average isotopic compositions for the solar system should be well defined by the average of chondrites, called Chondritic Uniform Reservoir (CHUR). However, while the Sm–Nd isotopic system shows an actual spread of less than 4% in the average chondritic record, the Lu–Hf system shows a larger variation range of 28% [Bouvier A., Vervoort J. D. and Patchett P. J. (2008) The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth Planet. Sci. Lett.273, 48–57]. To better understand the contrast between Sm–Nd and Lu–Hf systems, the REE and Hf distribution among mineral phases during metamorphism of Karoonda (CK) and Vigarano-type (CV) carbonaceous chondrites has been examined. Mineral modes were determined from elemental mapping on a set of five CK chondrites (from types 3–6) and one CV3 chondrite. Trace-element patterns are obtained for the first time in all the chondrite-forming minerals of a given class (CK chondrites) as well as one CV3 sample. This study reveals that REE are distributed among both phosphates and silicates. Only 30–50% of Sm and Nd are stored in phosphates (at least in chondrites types 3–5); as such, they are not mobilized during early stages of metamorphism. The remaining fraction of Sm and Nd is distributed among the same mineral phases; these elements are therefore not decoupled during metamorphism. Of the whole-rock total of Lu, the fraction held in phosphate decreases significantly as the degree of metamorphism increases (30% for types 3 and 4, less than 5% in type 6). In contrast to Lu, Hf is mainly hosted by silicates with little contribution from phosphates throughout the CK metamorphic sequence. A significant part of Sm and Nd are stored in phosphates in types 3–5, and these elements behave similarly during CK chondrite metamorphism. That explains the robustness of the Sm/Nd ratios in chondrites through metamorphism, and the slight discrepancies observed in the present-day isotopic Nd values in chondrites. On the contrary, Lu and Hf are borne by several different minerals and consequently they are redistributed during metamorphism–induced recrystallization. The Lu/Hf ratios are therefore significantly disturbed during chondrites metamorphism, leading to the high discrepancies observed in present-day Hf isotopic values in chondrites.