Fe Isotope Fractionation Research Articles

Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa)

Iron mobilization during continental weathering was pervasive before the Great Oxidation Event (GOE) that started at around 2.43 billion years (Ga) ago, due to the soluble nature of reduced iron. However, various geochemical proxies indicate transient oxygenation during deposition of the Mesoarchean (∼2.95 Ga) Pongola Supergroup, South Africa, which suggests that continental weathering could have also occurred under transiently oxic conditions before the GOE. We analyzed trace elemental and Fe, Ti, and Cr isotopic compositions of the ca. 2.95 Ga Denny Dalton paleosol in the Pongola Supergroup to better understand continental weathering and redox conditions in the ancient critical zone, and the nature of geochemical fluxes from the continents to the oceans and marine sediments. Iron isotope systematics are consistent with a model where Fe was released during intense leaching from the paleosol to concentrate in Fe-rich groundwaters in the deeper part of the soil horizon. We show for the first time that Fe isotopic fractionation during Mesoarchean continental weathering was limited, and Fe enrichments and depletions are coupled with those of divalent transition metals, Co, Ni, and particularly Zn. This suggests that Fe redox cycling was not involved in paleosol formation, and Fe2+ was mobilized under anoxic conditions. Chromium isotopes are also unfractionated relative to the parent igneous rock in this paleosol, which precludes removal of isotopically heavy Cr6+ and thus supports anoxic continental weathering. We show that previously reported Cr isotopic fractionation in another Denny Dalton paleosol profile does not follow a Cr6+ leaching trend, but instead scales with Cr enrichment and may reveal Cr enrichment from post-burial fluids. Thus, there is no clear evidence for an oxidative continental weathering during deposition of the Pongola Supergroup. Titanium isotopes are not significantly fractionated in the paleosol, suggesting that continental weathering and erosion in the Archean did not fractionate Ti isotopes. Similarly, Ni/Co and Th/Sc ratios are reasonably conserved, which validates their use as a robust proxy for upper continental crust composition in shales, whereas La/Sc, Cr/Zn and Cr/U ratios are highly variable relative to the provenance composition, which suggests that caution should be used when applying these ratios in shale studies of the ancient upper continental crust composition.

Large Fe isotope fractionations in sulfide ores and ferruginous sedimentary rocks from the Kuroko volcanogenic massive sulfide deposits in the Hokuroku district, northeast Japan

Anoxic seawater may have played an important role in the preservation of volcanogenic massive sulfide (VMS) deposits in the Hokuroku district, northeast Japan, which is the type locality for Kuroko-type VMS deposits. In this study, we investigated the Fe isotopic compositions of sulfide ores and overlying ferruginous sedimentary rocks in these deposits. These data, coupled with petrographic and geochemical data, enable us to investigate the key formation processes and conditions during the formation of large Kuroko-type VMS deposits. Large Fe isotope variations of ca. 4‰ (δ56Fe) associated with negative or positive Ce anomalies characterize the ferruginous sedimentary rocks formed in the Kuroko VMS deposits and post-Kuroko hydrothermal activity. This suggests that iron (hydr)oxides were precipitated by the partial oxidation of dissolved Fe2+ derived from hydrothermal fluids in anoxic or suboxic pools in the Hokuroku Basin. Positive Eu anomalies in the ferruginous cherts closely associated with the Kuroko VMS deposits indicate formation from high-temperature hydrothermal fluids. Zinc-rich black ores in both the Matsumine and Fukazawa deposits have lower δ56Fe values, due to rapid precipitation of pyrite triggered by the mixing of hydrothermal fluids with seawater. Positive shifts in δ56Fe values in the ferruginous cherts from the Ezuri and Fukazawa deposits may be explained by simultaneous precipitation of ferruginous sedimentary rocks with black ores, which modified the δ56Fe values of the hydrothermal fluids to positive values. However, Cu-rich yellow ores show no significant Fe isotope fractionation as compared with the dissolved Fe in the hydrothermal fluids, and were likely formed by slow growth of pyrite that replaced the black ores. The difference in the abundance of sulfide ores between the Matsumine and Fukazawa deposits may reflect the duration of hydrothermal circulation.

Iron force constants of bridgmanite at high pressure: Implications for iron isotope fractionation in the deep mantle

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.

Evolution of Retisol impacted by artificial drainage: What can we learn from stable Fe isotope ratios?

Iron oxides are one of the most reactive mineral phases in soils. As a consequence, their transformations can considerably affect the dynamics of the adsorbed elements and of the associated soil ecosystem services. Understanding the dynamics of Fe oxides in soils is therefore a key issue for the evolution of soil and associated ecosystem services. A potentially powerful tool to study the transformations of Fe-oxides in soil is stable Fe isotopes. However, there are still important gaps in our knowledge of Fe isotope fractionations.In order to examine the Fe isotope fractionations related to each process occurring in soils, we focused on a drainage-influenced sequence of Retisols, a soil type characterized by clay translocation and subsequent degradation by redox processes inducing a strong spatial Fe segregation in contrasted soil volumes. We combined the isotopic approach at the scale of a bulk horizon and at the scale of the different soil volumes, with mineralogical analyses and mass balance calculations in order to investigate the consequences of the drainage on Fe isotope fractionation. We showed that while there were no Fe isotope fractionations at the profile scale, Fe isotopic signatures varied significantly among soil volumes (δ56Fe values from −0.49 ± 0.05 to 0.29 ± 0.06‰). These variations suggest that redox processes are the main mechanisms responsible for the Fe redistribution among the volumes, and particularly that Fe accumulation during Mn oxide precipitation makes a significant contribution to Fe isotopic fractionation, during these Retisol differentiation. In contrast, drainage-induced eluviation does not result in further Fe isotope fractionations in soil volumes in these Retisols. The isotopic signatures of the different mineral phases present in the volumes were calculated using the mass balance approach and suggest that the iron oxides (goethite, ferrihydrite) have δ56Fe values close to 0‰, while the clay minerals are enriched in heavy Fe isotopes and the Mn oxides in light Fe isotopes. This study provides insight into the dynamics of Fe minerals in hydromorphic soils and offers a new perspective on stable Fe isotope fractionation in soils.

A multi-isotope study (Fe, Ge, O) of hydrothermal alteration in the Limousin ophiolite (French Massif Central)

Geochemical studies using non-traditional stable isotopes can help tracing processes of hydrothermal alteration (or hydrothermal metamorphism) of ancient oceanic lithosphere. In this study, we have measured non-traditional Ge and Fe isotopes and traditional O isotopes in a series of ultrabasic and basic rocks from the Limousin ophiolite (French Massif Central) to decipher the different signatures of hydrothermal vs. magmatic processes. Serpentinites are strongly oxidised rocks (Fe3+/ΣFe: 0.58–0.71) and display δ18O values (+5.0 to +6.1‰) typical of hydrothermally altered ultrabasic rocks. They display δ56Fe (+0.15 to +0.18‰) and δ74Ge values (+0.48 to +0.93‰) similar to heavier than ultrabasic rocks. The negative correlation between δ18O and δ74Ge and between δ18O and δ56Fe, suggests that Ge and Fe isotopes have fractionated during hydrothermal alteration. The δ74Ge shows a slight positive correlation with the δ56Fe, indicating concomittant Ge and Fe isotope fractionation towards heavier values during hydrothermal alteration. However, δ56Fe values display a larger deviation from ultrabasic rocks than δ74Ge and δ18O values, suggesting that oxidising conditions have enhanced Fe isotope fractionation to a larger extent than Ge isotopes. Amphibolites display Fe3+/ΣFe ratios (0.11–0.14) and δ56Fe values (+0.03 to +0.17‰) typical of mid-ocean ridge basalts (MORB). δ18O values are typical of high-T hydrothermally altered MORB (+6.2 to +6.6‰). δ74Ge values show a small range (+0.72 to +0.77‰) and are heavier than most basalts and gabbros. The δ18O slightly decreases with the increase of the δ56Fe, indicating that Fe isotopes may have fractionated towards lighter values during hydrothermal alteration. However, the lack of correlation between δ74Ge and δ56Fe values indicates that Ge isotope fractionation has prevailed over Fe isotope fractionation during hydrothermal alteration of basic rocks in the absence of oxidising conditions.

Iron Isotope Compositions of Podiform Chromitites from Dazhuqu and Luobusha Ophiolites, Southern Tibet

AbstractPodiform chromitites crop out in ophiolitic harzburgites as pod‐like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt‐rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes (e.g., Arai, 2013; Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent (e.g. Chen et al., 2014; Weyer and Ionov, 2007; Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, δ56Fe (relative to the standard, IRMM‐014) values range from −0.02‰ to 0.11‰ in olivines and from 0.03‰ to 0.08‰ in chromites. Chromites in Dazhuqu chromitites show δ56Fe values varying from −0.03‰ to 0.02‰. In nodular and densely disseminated chromitites from Luobusha, olivines have δ56Fe values of olivines and chromites are 0.09–0.35‰ and −0.15–0.08 ‰, respectively. Chromites from Luobusha massive chromitites have δ56Fe values of 0.07–0.12 ‰.Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions Δ56FeOl‐Chr ≈–0.08‰ at 1300 °C according to the ionic model (e.g., Macris et al., 2015; Sossi and O'Neill, 2017). However, most of our samples, except for two samples, have Δ56FeOl‐Chr values that are greater than zero, indicating a disequilibrium inter‐mineral Fe isotopic fractionation. There is a positive correlation between Fo and δ56Fe (or Δ56FeOl‐Chr) of olivines but no positive correlation between Mg# and δ56Fe (or Δ56FeOl‐Chr) of chromites. This phenomenon suggests that the Fe isotopic dis‐equilibration may be caused by migrating melts in dunitic channels rather than by the sub‐solidus Fe‐Mg exchange (Xiao et al., 2016; Zhang et al., 2019). Additionally, the wide δ56Fe range of chromites is similar to those of the subduction‐related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone.

Stable Fe isotope fractionation during dissimilatory Fe(III) reduction by a thermoacidophile in acidic hydrothermal environments

Dissimilatory iron reduction (DIR) plays an essential role in biogeochemical Fe cycling in anoxic environments. At near-neutral pH, in both biotic and abiotic systems, aqueous Fe(II) (Fe(II)aq) interacts with reactive ferric (hydr)oxides via electron transfer and atom exchange that is catalyzed by large amounts of sorbed Fe(II). This may result in substantial Fe isotope exchange, which, at equilibrium, produces up to a ∼4‰ 56Fe/54Fe fractionation between coexisting oxide/hydroxide and Fe(II)aq, depending on mineralogy. The role of biology in such systems has been interpreted to lie in the production of Fe(II) rather than a specific “vital” effect, such as enzymatic and kinetic processes. Under acidic abiotic conditions, however, the lack of sorbed Fe(II) generates little Fe isotope exchange, and, by extension, it has been expected that little exchange would occur during DIR at low pH if sorbed Fe(II) is the key component for catalyzing isotopic exchange.In this study, we explored the extent and mechanism of Fe isotope exchange between Fe(II)aq and ferric hydroxides (ferrihydrite and goethite), including determination of the 56Fe/54Fe fractionations during DIR by Acidianus strain DS80 at pH ∼ 3.0 and 80 °C, over 19 days of incubation. Significant Fe(III) reduction occurred for both minerals along with large changes in Fe isotope compositions for Fe(II)aq, indicating Fe isotope exchange. Solid-phase extractions using HCl confirmed a lack of sorbed Fe(II), which suggests that a mechanism other than sorption is required to catalyze Fe isotope exchange during DIR at low pH. Reactive Fe(III) (Fe(III)reac) extracted from the mineral surface allowed for the calculation of the Fe pools that underwent isotopic exchange. A total of ∼20% of goethite and ∼60% of ferrihydrite underwent isotopic exchange over 19 days. For goethite from biotic experiments, we calculate a Fe(III)reac-Fe(II)aq fractionation factor of 1.57 ± 0.52‰, which is larger than the abiotic equilibrium fractionation factor (∼0.73‰ at 80 °C). This result is consistent with previous work on DIR of goethite at neutral pH, where a fractionation factor larger than equilibrium was interpreted to reflect an isotopically distinct “distorted surface layer” of goethite produced during exchange with Fe(II)aq. In contrast to goethite, the difference between the Fe(III)reac-Fe(II)aq fractionation factor for ferrihydrite from our biotic reactors (2.91 ± 0.40‰) and the abiotic equilibrium fractionation factor (∼2.28‰ at 80 °C, under silica-free conditions) is smaller.Ultimately, the contrast in the extent of Fe isotope exchange between biotic and abiotic experiments emphasizes the importance of biology in promoting Fe isotope exchange in acidic systems. We speculate that the unique role of biology at low pH in catalyzing Fe isotope exchange, not seen in equivalent abiotic systems, must lie in the transport of electrons to the ferric hydroxide surface that produces Fe(II) atoms in situ. This suggests that isotopic exchange occurs on an atom-by-atom basis as Fe(III) is reduced to Fe(II), followed by the release of Fe(II) into solution. This study demonstrates that significant variations in Fe isotope compositions may be uniquely produced in acidic environments where microbial Fe cycling occurs via DIR, compared to minor isotopic variations observed previously in acidic abiotic systems.

Magnesium and iron isotopic evidence of inter-mineral diffusion in ultramafic cumulates of the Peridotite Zone, Stillwater Complex

To better understand the role and extent of inter-mineral diffusion during subsolidus processes, we investigated the iron and magnesium isotopic compositions of olivine, orthopyroxene, and chromite separates from the Peridotite Zone of the Stillwater Complex. Olivine and orthopyroxene show limited Mg isotopic variations, with δ26Mg values ranging from −0.40 to −0.26‰ and from −0.29 to −0.22‰, respectively. In contrast, chromite displays extremely large Mg isotopic variations, with δ26Mg values ranging from −0.05 to +0.84‰. The δ56Fe values of olivine and orthopyroxene range from 0.00 to +0.17‰ and −0.04 to +0.06‰, respectively, whereas those of chromite range from −0.09 to +0.13‰. Most olivine-orthopyroxene pairs in our samples show no clear Fe-Mg isotopic differences, whereas silicate and coexisting chromite pairs plot away from theoretically predicted equilibrium lines, indicating disequilibrium inter-mineral fractionation. Higher δ56Fe values in silicates than in chromite and significantly higher δ26Mg values in chromite than in silicates indicate that subsolidus re-equilibration is responsible for the observed disequilibrium Fe-Mg isotope fractionation. We develop a quantitative model based on the investigated olivine and chromite to reproduce the observed kinetic Fe-Mg isotope fractionations. Our model results indicate a systematic difference between the Fe and Mg isotope fractionation factors, β, of olivine (βFe ≈ 0.27 and βMg ≈ 0.16) and chromite (0.03 < βFe < 0.08 and 0.05 < βMg < 0.16). This study demonstrates that combined investigations of the Fe-Mg isotopic compositions of cumulus minerals provide more recognizable and more reliable information on inter-mineral diffusion among igneous minerals in magma chambers than elemental analyses alone.

Isotopic fingerprinting of dissolved iron sources in the deep western Pacific since the late Miocene

Iron (Fe) is a productivity-limiting nutrient in the ocean. However, the sources of dissolved Fe (dFe) in the deep ocean and how they respond to tectonic and climate changes are still poorly understood. In the northern hemisphere, dust flux to the low-latitude western Pacific has increased dramatically since the late Miocene associated with intense aridification of the Asian inland. Meanwhile, the terrigenous material supply to the open ocean might have also changed as a result of the reorganization of the Pacific circulation due to the gradual closure of seaways in the low latitudes. Therefore, the western Pacific is a characteristic region for understanding the sources of dFe in the deep ocean and their responses to long term climate changes. Here, we present data on isotopic evolution of dFe and dissolved Pb since ∼8 Ma based on ferromanganese crust METG-03 (16.0°N, 152.0°E, 3850 m water depth) in the western Pacific deep water. Our results show that δ56Fe of the crust remains fairly stable since the late Miocene, i.e., about −0.32±0.08‰ (2SD). We infer that δ56Fe of dFe in the deep western Pacific is relatively invariant at ∼0.45 ±0.1‰ based on the Fe isotopic fractionation between hydrogenetic crust and the seawater dissolved component. The reconstructed isotope signature is similar to the measured δ56Fe value (0.37±0.15‰) of the intermediate to deep waters in the modern low-latitude western Pacific region close to the island arcs, but is significantly higher than that of the eastern Pacific deep waters near South America which is controlled by the reductive dissolution of continental shelf sediments and the hydrothermal inputs (δ56Fe<−0.1‰). The deep-water 206Pb/204Pb ratio recorded by METG-03 displays systematic increase at about 8–4 Ma, reflecting increased input from sediment dissolution of low-latitude island arcs associated with reorganization of the western Pacific deep circulation. Notably, Fe isotopes of terrigenous materials from different sources are similar, while their dissolved Fe isotopic signatures released to the ocean are mainly controlled by the mechanism of particle dissolution. The stability of δ56Fe and systematic changes in Pb isotopes over the last ∼8 Ma thus suggest that Asian dust dissolution and hydrothermal inputs are likely only minor sources of dFe in the low-latitude deep western Pacific, while the acquisition and transport of dFe from shelf sediments by organic ligand binding in the oxic environment is the major dFe source which keeps stable on tectonic time scales since the late Miocene.

Iron isotope exchange and fractionation between jarosite and aqueous Fe(II)

Jarosite is one of the critical minerals that regulates acidity and contaminants in acid-sulfate environments and its Fe isotope composition may shed light on its formation, transformation and recrystallization over time. Interpretation of its Fe isotope composition requires understanding the equilibrium Fe isotope fractionation factor between jarosite and other Fe-bearing minerals and aqueous species. Here we explore Fe isotope exchange and fractionation between jarosite and Fe(II)aq under acidic conditions using the three-isotope method (54Fe-56Fe-57Fe). A reversal-approach to equilibrium was applied by reacting synthetic jarosite and natural natrojarosite with two 57Fe-enriched Fe(II)aq solutions that had initial 56Fe/54Fe ratios above and below the predicted equilibrium value. No change in dissolved Fe(II) concentrations were observed with time but the 57Fe/56Fe ratio of Fe(II)aq decreased towards the system mass balance, suggesting a high degree of equilibration of the fluid with the solid phase despite no net Fe(II) sorption (within error). There is a negative relationship between pH and Fe isotope exchange, with Fe isotope exchange proceeding as pH decreases. This may be explained by dissolution of hydronium jarosite and reprecipitation of natrojarosite, coupled H3O+ - Na+ exchange, or jarosite decomposition, although no Fe-oxyhydroxide phases were identified from XRD. Calculation of the amount of Fe atoms in jarosite that exchanged with Fe(II)aq indicates that jarosite recrystallization was limited to a few percent. When the initial δ56Fe value of Fe(II)aq was greater than the presumed equilibrium value its isotopic value substantially decreased with time whereas the δ56Fe values of Fe(II)aq increased with time when it had an initial value below the suspected equilibrium composition. In each case, the isotopic composition of Fe(II)aq approached similar values, providing a high degree of confidence of an attainment of equilibrium. Calculation of the Fe(II)aq–jarosite and Fe(II)aq-natrojarosite equilibrium fractionation factors at 22 °C were −2.26‰ (±0.27‰, 2σ) and −2.19‰ (±0.18‰, 2σ), respectively. This indicates that during jarosite recrystallization in the presence of Fe(II), jarosite is expected to become isotopically heavier as lighter isotopes are fractionated into Fe(II). These values differ from the estimated fractionation factors derived from NRIXS spectroscopy and molecular modeling. The differences between experiments and theory may reflect surface exchange, which was likely in our study, versus predicted bulk thermodynamic properties of the mineral.

Geochemical and Stable Fe Isotopic Analysis of Dissimilatory Microbial Iron Reduction in Chocolate Pots Hot Spring, Yellowstone National Park

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

Archean lithospheric differentiation: Insights from Fe and Zn isotopes

Abstract The Archean continental lithosphere consists of a dominantly felsic continental crust, made of tonalite-trondhjemite-granodiorite (TTG) and subordinate granitoids, and a cratonic lithospheric mantle, made of highly refractory peridotites. Whether they stemmed from the same process of differentiation from the primitive mantle, or were two distinct components that were physically juxtaposed, remains debated. Metal stable isotope ratios are sensitive to magmatic and metamorphic processes and do not evolve with time. Therefore, stable isotope ratios are complementary to radiogenic isotope ratios, and they allow direct comparisons to be made between different terrestrial components without age corrections. Isotopes of iron and zinc, metals ubiquitous in Earth’s lithosphere, can be tracers of lithospheric formation and evolution because they are affected by partial melting (Fe, Zn), redox state (Fe), and the presence of sulfides (Fe, Zn). Here, using stable Fe and Zn isotopic data from Archean samples of the lithospheric mantle and the continental crust, we show that Fe and Zn isotopes define a linear array, best explained by their coupled fractionation behavior during magmatic processes. Our data show that high degrees of partial melting (&amp;gt;30%) during the formation of the cratonic mantle and mafic protocrust, and reworking of the early crust significantly fractionate Fe and Zn isotopes. Conversely, Fe and Zn isotope ratios in the TTG are similar to those in Archean mafic rocks, suggesting an origin by fractional crystallization of basalt, and implying limited Fe and Zn isotopic fractionation, instead of partial melting of mafic crust. Moreover, the absence of Fe and Zn isotope decoupling due to redox effects, melt (fluid)–rock or sediment-rock interaction, and decarbonation indicates that subduction, at least as we understand it now, is not required to explain the Fe and Zn isotope composition of the Archean lithosphere.

Coupled Fe–S isotope composition of sulfide chimneys dominated by temperature heterogeneity in seafloor hydrothermal systems

Seafloor hydrothermal chimneys are significant metal resources and have fundamental effects on marine chemistry balance. Previous studies on growth of the chimneys focused on mineral composition and elemental distribution in chimney walls, but the isotopic composition of different elements and their correlations remain poorly understood. Here, we report lateral Fe and S isotopes from sulfides across a chimney wall in detail from the Deyin-1 hydrothermal field (DHF) on the South Mid-Atlantic Ridge. Pyrite was characterized by increases in both δ34S and δ56Fe from the exterior to the interior within the chimney wall, which is likely related to enhanced S and Fe isotope fractionation between pyrite and fluids derived from the temperature gradient. Furthermore, δ56Fe displayed a well-defined, linear, positive correlation with δ34S in pyrite from the DHF as well as in pyrite from other areas. A model of hydrothermal pyrite formation suggests that, under conditions with uniform hydrothermal fluid, different temperatures and similar pyrite–fluid exchange degrees could result in the observed linear relationships between δ34S and δ56Fe. The regular and coupled Fe–S isotope variations within the chimney wall efficiently constrain the process of hydrothermal sulfide formation.

Iron isotopic and chemical tracing of basalt alteration and hematite spherule formation in Hawaii: A prospective study for Mars

Hematite spherules from Mauna Kea volcano (Hawaii) resemble Martian hematite spherules at Meridiani Planum in morphology, mineralogy, and geochemistry, and they both formed in association with acid-sulfate alteration of basalt. To gain insights into the processes involved in the formation of these spherules, we have measured the Fe isotopic compositions of unaltered basalts and tephras, as well as a variety of weathering/alteration products including altered basalts, hematite-rich breccia and individual hematite spherules, jarosite-bearing tephras, palagonitic tephras, calcined tephras (high-T dry oxidation) and steam-vent tephras.Compared to other alteration processes that induce little Fe mobility, acid-sulfate alteration produces large Fe isotopic variations (δ56Fe values ranging from −0.15 to +0.94‰). We used rock slabs from a variably altered basaltic rock to constrain the Fe isotope fractionation factor during acid-sulfate alteration of basaltic rocks. We found that the process followed a Rayleigh distillation with an instantaneous fractionation factor of −0.24‰ between the fluid and the residue for δ56Fe, resulting in a heavy Fe isotope enrichment in the alteration residue. However, the constant Mn/Fe ratios of the slabs suggest that acid-sulfate leaching does not significantly fractionate Mn/Fe ratios in basalts, which is consistent with the mobilization of iron as Fe(II).We also measured the Fe isotopic compositions and Mn/Fe ratios (using NanoSIMS) of individual hematite spherules from a spherule-rich breccia HWMK745R. The spherules show heavy Fe isotopic enrichments and low Mn/Fe ratios compared to unaltered samples, which is similar to the fractionations encountered in iron formations. A Rayleigh fractionation model suggests that both Fe isotopic compositions and Mn/Fe ratios of the spherules are consistent with ∼80% Fe oxidation and precipitation from an acid-sulfate fluid. Therefore, direct Fe precipitation from a fluid can readily explain the hematite spherule formation, and a more complex scenario involving jarosite precipitation followed by conversion to hematite is not needed. Combined thermodynamic and kinetic constraints also suggest that precipitation of jarosite would be hindered. The study presents a roadmap for studying future returned Martian samples in the laboratory.

Iron and oxygen isotope systematics during corrosion of iron objects: a first approach

Iron objects are among the most abundant type of metal artefacts in the archaeological record and help to deepen our understanding of past societies and their technologies. However, sampling of them is often problematic due to the destructive character of most analytical methods. In this study, iron and oxygen isotope compositions of iron artefacts from marine and water under-saturated oxidising environments were analysed in a first attempt to gather artefact information from the corrosion layer without sampling the object directly. No Fe isotope fractionation between artefact and its corrosion products was recognised for both environments but cannot be excluded for marine environments. Hence, the artefact’s Fe isotope composition can be determined from the corrosion layer. This allows the characterisation of artefacts which cannot be sampled directly. Because the available data precluded a clear identification of the underlying processes, possible fractionation mechanisms resulting in this situation are presented. Furthermore, the results of this study indicate that corrosion products have the same oxygen isotope composition as their source water. As for marine corrosion, general absence of oxygen isotope fractionation could not be manifested here, because only a small sample size was available for the present study. However, a complex interplay of many parameters governs the oxygen isotope compositions of corrosion products on metallic iron. Especially oxidising environments above the water table have a strong impact on the oxygen isotope composition of the corrosion layer. The first-order controlling mechanisms, such as evaporation, are set by the local environment and cannot be reconstructed. Therefore, the oxygen isotope composition of corrosion products seems to bear no valuable potential for archaeometallurgical research.

Evaluation of Fe isotope values as a provenance tool for chert artefacts from the north‐eastern United States

Iron (Fe) isotope compositions of prehistoric stone tools and geological sources were compared to evaluate the robustness of this isotopic fingerprinting technique. Artefacts and source materials were collected from the Hatch site in central Pennsylvania, United States, where both veined chert (Bald Eagle chert) and stone tools coexist within several metres. Yellow artefacts (δ56Fe = 0.38 ± 0.1, n = 7) and source materials (δ56Fe = 0.42 ± 0.1, n = 8) isotopically matched within error. The source values also overlap yellow chert samples from three other Bald Eagle chert locations in the area. These values are different from six other chert locations in the north‐eastern United States. These data suggest that the Fe isotope composition of chert artefacts reflect distinct geological sources. To enhance the mechanical characteristics of the stone tools, the chert experienced heat treatment, which induced a phase shift of the Fe oxide mineral goethite to hematite. This phase shift changes the colour of the chert to red. The red chert artefacts and source material also have overlapping Fe isotope values, but are 0.2‰ higher than the initial yellow chert. Experimental work where cherts were heated with different clays shows that Fe exchange with site soils induces the +0.2 fractionation. These data demonstrate that predictable Fe isotope fractionation occurs during heating, resulting in isotopically distinct artefacts.

Iron isotope fractionation in hydrous basaltic magmas in deep crustal hot zones

Mafic magmatism within deep crustal hot zones plays an important role in the crustal formation and evolution. High-pressure garnet crystallization drives magma differentiation and Fe isotope fractionation at the base of the lower crust. The ∼840 Ma intrusion and the associated ∼830 Ma dikes from the Tongde region in South China consist mainly of gabbro and diorite, which record complicated magmatic processes of the deep crustal zones and provide new insights into the Fe isotope fractionation. Rocks from the Tongde intrusion have high Sr/Y ratios (61–163) and variable δ57Fe values (+0.07 to +0.21‰ relative to IRMM-014) that initially increase then decrease with progressive magmatic differentiation. All samples do not display obvious imprint of deuteric fluid exsolution, and residual garnet in the mantle source did not fractionate the Fe isotope compositions of the primary magmas significantly. The variable Fe isotope signatures in this study can be reproduced by sequential fractional crystallization of olivine/pyroxene and garnet in the deep crust followed by amphibole saturation in the upper section. The host rocks of the intrusion underwent diffusive exchange with the dikes. Removal of both isotopically light- and heavy-Fe minerals well explains the small Fe isotope fractionation in mafic and intermediate rocks. The high-Sr/Y suites of the intrusion and dikes have an average δ57Fe of +0.11 ± 0.02‰ (2SD), similar to the value of arc crust at convergent continental margins. Therefore, garnet fractionation is an important process in arc roots, which drives the melts evolving to calc-alkaline series. Fractional crystallization of hydrous basaltic magmas under high pressures in deep crustal hot zones makes a significant contribution to the formation and evolution of continental crust.

Sedimentary iron cycling in the Benguela upwelling system off Namibia

Iron (Fe) is an essential micronutrient and its availability controls primary production in large parts of the ocean. Further, the variability of oxygen minimum zones (OMZ) exerts a major control on the availability of Fe, concentrations of which are elevated off Namibia. Therefore, to understand the sources and processes involved in the cycling of Fe off Namibia, we determined major and trace element contents (Fe, Al, Si, K, P, organic carbon, Mn, Co, Zr) and the isotopic composition of Fe (δ56Fe) in seven sediment cores along a transect from the shelf to the abyssal plain at ∼5 km water depth (Meteor cruise M76-1) as well as in the local lithogenic background (Namibian river bed sediments and dust).The depletion of Mn vs. the local lithogenic background (LLB) reflects the reducing character of sediments from the shelf to the lower slope. While depletion of Mn and enrichment of Co indicates the anoxic–sulfidic environment on the shelf, the enrichment of both elements in abyssal sediments documents highly oxidizing conditions there.Iron/Al ratios are elevated (0.6 to 1.6) and therefore equal to, or higher than the LLB. The highest Fe enrichment is seen on the shelf (core 1) and the shelf break (core 2) with a clear decrease towards the abyss. The bulk δ56Fe values range from +0.30 to −0.05‰ in the marine sediments, with an average of ∼0.08‰, which is slightly lighter than the δ56Fe of the LLB (0.16 to 0.21‰). Further, combined enrichments of K and Fe suggest the presence of detrital biotite (core 1) and authigenic glauconite (core 2), both of which are perfectly correlated with Zr/Al ratios reflecting the highly dynamic conditions of the shelf environment (0-300 m water depth).The absent large sedimentary Fe isotope fractionation in the sediments of this study compared to Peru sediments is likely due to a combination of the following: (i) a highly variable OMZ leading to random re-oxidation of any sediment-sourced Fe which prevents the export of larger amounts of dissolved Fe to the interior of the ocean, and which also prevents focused enrichments of Fe, and (ii) an enhanced dust flux, which frequently resupplies terrestrial Fe dampening any Fe isotope fractionation.This study is a snapshot on sedimentary Fe cycling in one area from the vast Namibian margin. Compared to the Peruvian margin, the data suggest little sedimentary Fe release, transport and redeposition. However, previous water column data suggest substantial Fe release. Hence, for a more subtle investigation of the Fe loss and re-supply a higher sampling resolution of multiple cores across the shelf and slope will be required.

Iron isotope fractionation in soil and graminaceous crops after 100 years of liming in the long‐term agricultural experimental site at Berlin‐Dahlem, Germany

AbstractSustainable arable cropping relies on repeated liming. Yet, the associated increase in soil pH can reduce the availability of iron (Fe) to plants. We hypothesized that repeated liming, but not pedogenic processes such as lessivage (i.e., translocation of clay particles), alters the Fe cycle in Luvisol soil, thereby affecting Fe isotope composition in soils and crops. Hence, we analysed Fe concentrations and isotope compositions in soil profiles and winter rye from the long‐term agricultural experimental site in Berlin‐Dahlem, Germany, where a controlled liming trial with three field replicates per treatment has been conducted on Albic Luvisols since 1923. Heterogeneity in subsoil was observed at this site for Fe concentration but not for Fe isotope composition. Lessivage had not affected Fe isotope composition in the soil profiles. The results also showed that almost 100 years of liming lowered the concentration of the HCl‐extractable Fe that was potentially available for plant uptake in the surface soil (0–15 cm) from 1.03 (standard error (SE) 0.03) to 0.94 (SE 0.01) g kg−1. This HCl‐extractable Fe pool contained isotopically lighter Fe (δ56Fe = −0.05 to −0.29‰) than the bulk soil (δ56Fe = −0.08 to 0.08‰). However, its Fe isotope composition was not altered by the long‐term lime application. Liming resulted in relatively lower Fe concentrations in the roots of winter rye. In addition, liming led to a heavier Fe isotope composition of the whole plants compared with those grown in the non‐limed plots (δ56FeWholePlant_ + Lime = −0.12‰, SE 0.03 vs. δ56FeWholePlant_‐Lime = −0.21‰, SE 0.01). This suggests that the elevated soil pH (increased by one unit due to liming) promoted the Fe uptake strategy through complexation of Fe(III) from the rhizosphere, which favoured heavier Fe isotopes. Overall, the present study showed that liming and a related increase in pH did not affect the Fe isotope compositions of the soil, but may influence the Fe isotope composition of plants grown in the soil if they alter their Fe uptake strategy upon the change of Fe availability.Highlights Fe concentrations and stocks, but not Fe isotope compositions, were more heterogeneous in subsoil than in topsoil. Translocation of clay minerals did not result in Fe isotope fractionation in the soil profile of a Luvisol. Liming decreased Fe availability in topsoil, but did not affect its δ56Fe values. Uptake of heavier Fe isotopes by graminaceous crops was more pronounced at elevated pH.

Novel insights from Fe-isotopes into the lithological heterogeneity of Ocean Island Basalts and plume-influenced MORBs

The extent of lithological heterogeneity in the Earth's convecting mantle is highly debated. Whilst the presence of pyroxenite in the mantle source regions of Ocean Island Basalts (OIBs) has traditionally been constrained using the minor-element chemistry of olivine phenocrysts, recent studies have shown that the Ni and Mn contents of primitive olivines are influenced by the conditions of mantle melting, as well as magma chamber processes. Nevertheless, constraining the lithological properties of the mantle is important due to its influence on the P-T path followed by solid mantle material during adiabatic ascent, as well as the density of upwelling mantle plumes. We have therefore explored the use of Fe-isotopes as a novel method of tracing lithological heterogeneity in the mantle source regions beneath plume-influenced segments of the global Mid-Ocean Ridge system as well as OIBs.We present new Fe-isotope (δ56Fe) and trace-element data for 26 basaltic glasses from the plume-influenced Galápagos Spreading Centre to investigate the relative roles of pyroxenite and peridotite in the mantle source region of oceanic basalts. Our data reveals significant heterogeneity in the Fe-isotope composition of the Galápagos Spreading Centre basalts (+0.05 to +0.25‰ δ56Fe), which correlates with key major- and trace-element parameters (e.g. CaO(8)/Al2O3(8), [La/Sm]n). Application of new models developed to calculate Fe-isotope fractionation during mantle melting, alongside Monte Carlo simulations for melting of a 2-component peridotite mantle, show that this variation cannot be caused by changes in melting processes and/or oxygen fugacity of a peridotitic mantle. Instead, our new δ56Fe data is best explained by variations in the proportion of isotopically-heavy pyroxenite-derived melt that contributes to the GSC basalts, and conclusively shows that lithological heterogeneity exists in the Galápagos mantle plume. Our findings have implications for the moderately-heavy δ56Fe compositions measured in plume-influenced basalts from the Society Islands, Rochambeau Ridges of the Lau back-arc basin, and the FAMOUS segment of the Mid-Atlantic Ridge, which we suggest may also represent contribution from pyroxenite-derived melts.