Iron Isotope Composition Research Articles

Diffusion-driven Ca-Fe isotope fractionations in the upper mantle: Implications for mantle cooling and melt infiltration

This study reports Ca-Fe isotope compositions of mineral separates in a set of peridotite xenoliths including ten lherzolites, one harzburgite, and one wehrlite from Hainan Island, south China. Iron isotope compositions show a small variation (<0.15‰ of δ56/54Fe) among olivine (ol), orthopyroxene (opx), and clinopyroxene (cpx). Δ44/40Caopx-cpx (δ44/40Caopx-δ44/40Cacpx) show a large range from 0.00‰ to 1.23‰. Together with data in previous studies, Δ44/40Caopx-cpx in mantle xenoliths worldwide show a negative correlation with the apparent temperatures, which cannot be explained by equilibrium fractionation. Instead, the observed inter-mineral Ca isotope fractionation between opx and cpx (>2‰) in previous studies should be dominated by disequilibrium sub-solidus Ca redistribution between cpx and opx, which is driven by the changes of mantle temperature and chemical composition in pyroxene. The wehrlite, which was formed by reaction of peridotite with an evolved, silica-undersaturated melt, has δ44/40Ca of 3.22‰ and δ56/54Fe of 0.22‰, significantly heavier than the value of fertile mantle. Such heavy isotope signatures in the wehrlite are best explained by diffusion during the metasomatic melt percolating the refractory mantle. In summary, this study reveals that significant kinetic Ca isotope fractionations could occur in the mantle by inter-mineral Ca diffusion during re-equilibration upon cooling and Ca diffusion from melt into peridotite during melt percolation.

Heavy iron isotope composition of iron meteorites explained by core crystallization.

Similar to Earth, many large planetesimals in the Solar System experienced planetary-scale processes such as accretion, melting, and differentiation. As their cores cooled and solidified, significant chemical fractionation occurred due to solid metal-liquid metal fractionation. Iron meteorites -- core remnants of these ancient planetesimals -- record a history of this process. Recent Fe isotope analyses of iron meteorites found δ57/54Fe to be heavier than chondritic by approximately 0.1 to 0.2 ‰ for most meteorites, indicating that a common parent body process was responsible. However, the mechanism for this fractionation remains poorly understood. Here we experimentally show that the Fe isotopic composition of iron meteorites can be explained solely by core crystallization. In our experiments of core crystallization at 1300 °C, we find that solid metal becomes enriched in δ57/54Fe by 0.13 ‰ relative to liquid metal. Fractional crystallization modelling of the IIIAB iron meteorite parent body shows that observed Ir, Au and Fe isotopic compositions can be simultaneously reproduced during core crystallization. The model implies the formation of complementary S-rich components of the iron meteorite parental cores that remain unsampled by meteorite records and may be the missing reservoir of isotopically-light Fe. The lack of sulfide meteorites and previous trace element modeling predicting significant unsampled volumes of iron meteorite parent cores support our findings.

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.

Origin of the Oligocene manganese deposit at Obrochishte (Bulgaria): Insights from C, O, Fe, Sr, Nd, and Pb isotopes

The large manganese (Mn) deposit at Obrochishte (NE Bulgaria) is part of a cluster of similar Early Oligocene deposits located around present-day Black Sea. They collectively constitute the Earth’s second largest endowment of Mn, after the Kalahari Manganese Field in Africa. We have employed a battery of isotopic techniques (C, O, Fe, Sr, Nd, Pb) to help understand the genesis of this deposit. Carbon isotope data indicates that some sections of the Mn-ore layer have diagenetic MnCO3 mineralization, formed by reaction of Mn oxides with organic carbon (Corg), whereas other sections have MnCO3 precipitated directly from the seawater column. Oxygen isotopes show that the high-grade Mn mineralization had seawater as the fluid source, whereas some lower-grade sections had a mix of ground water and seawater as fluid sources. Sr and Nd isotope values of ore leachates also indicate that the Mn deposition occurred in normal Early Oligocene seawater. Nd and Pb isotope values suggest that the clastic host sediments were sourced from continental bedrock rather than younger arc volcanic rocks to the west. Iron isotope composition of the Mn ore implies deposition in a redox-stratified basin, similar to the modern Black Sea, with much of the Fe sequestered in deep, anoxic-euxinic water as sulfides. Similar to the modern Black Sea, most of the detrital Fe was transferred from shallow oxic sediments into deep, anoxic-euxinic water by an “iron shuttle” and remobilized Mn sequestered in the upper suboxic water layer. However, during the Oligocene, the “iron shuttle” operated intermittently due to the chemocline falling mostly below the shelf break, thereby limiting the efficiency of the shuttle mechanism. We propose a model for the Lower Oligocene strata in which intense weathering during the Eocene weathering phase produced a thick lateritic crust on the southern European continent. The drastic sea-level drop at the end of the Eocene initiated downcutting of streams through this weathered material, transferring Fe- and Mn-oxides to the redox-stratified Western Black Sea. Here, these oxides were partly or entirely dissolved in the suboxic (Mn-oxides partly, Fe-oxyhydroxides entirely dissolved) and anoxic-euxinic (Mn-oxides entirely dissolved, dissolved Fe2+ re-precipitated) water layers. Eventually, Fe was re-precipitated as sulfide in the deep anoxic-euxinic water, while Mn accumulated in the suboxic water layer. Transgression in the Early Oligocene brought this Mn-rich water onto the shallow shelf where it precipitated as Mn-oxide, then converted to Mn-carbonates during early diagenesis. Some Mn was also contributed by submarine groundwater discharge. Further transgression brought lower-oxygen water onto the shelf and Mn-carbonate precipitated directly from the water column. The findings from this work provide insights about the unique Oligocene geochemical event in the region that lead to the formation of the 2nd largest cluster of Mn deposits in the world.

Iron isotope fractionation at the core–mantle boundary by thermodiffusion

The D” layer at the base of the Earth’s mantle exhibits anomalous seismic properties, which are attributed to heat loss from and chemical interaction with the underlying molten Fe-rich outer core. Here we show that mass transfer due to temperature variations within the D” layer could lead to resolvable fractionation of iron isotopes. We constrain the degree of isotope fractionation by experiments on core-forming Fe alloy liquids at 2100–2300 K and 2 GPa, which demonstrate that heavy Fe isotopes preferentially migrate towards lower temperature and vice versa. We find that this isotope fractionation occurs rapidly due to the high mobility of iron, which reaches 0.013 ± 0.002‰ (2σ) per degree per amu at steady state. Numerical simulations of mantle convection capturing the evolution of a basal thermal boundary layer show that iron isotope fractionation immediately above the core–mantle boundary can reach measurable levels on geologic timescales and that plumes can entrain this fractionated material into the convecting mantle. We suggest that such a process may contribute to the heavy Fe isotope composition of the upper mantle inferred from mantle melts (basalts) and residues (peridotites) relative to chondrites. That being the case, non-traditional stable isotope systems such as Fe may constrain the interactions between the core and mantle. Iron isotopic fractionation at the core–mantle boundary due to thermal diffusion may partly explain the iron isotope composition of the upper mantle, according to high-temperature experiments and numerical simulations.

Deep-water dissolved iron cycling and reservoir size across the Ediacaran-Cambrian transition

The majority of the deep ocean was likely under ferruginous conditions during the first four billion years of Earth's history. As the atmosphere was gradually oxygenated, the sources, sinks, redox cycling, and reservoir size of dissolved iron in the deep ocean are likely to have changed dramatically. Whether deep water was thoroughly oxygenated by the time of the Ediacaran-Cambrian transition, and the relationship of this oxygenation to the Cambrian explosion, remains debated. To explore the degree of oceanic oxygenation and its effect on Cambrian explosion, we measured the iron isotopic composition (δ56Fe) of bulk rock (i.e., cherts and mudstones/shales) through the Piyuancun and Hetang formations, using samples collected from the Chunye-1 core, on the Lower Yangtze Block in western Zhejiang. The limited variation in δ56Fe values (<0.7‰) and low FeT/Al ratios (<0.77) in euxinic samples show that the deep-water Fe2+ reservoir was quite limited, and likely similar to that of the modern ocean, during the latest Ediacaran and Cambrian Stages 1–3. Iron isotope results, combined with published data from sections on the Middle and Upper Yangtze Block, record a general decline in seawater δ56Fe values from >0.55‰ during the end-Ediacaran and Cambrian Stages 1–3 to <0‰ during Cambrian Stage 4. Seawater δ56Fe values in the lower and middle Hetang Formation range between 0 and 0.2‰, suggesting that the riverine dissolved and suspended flux and/or aeolian dust was the predominant source of highly reactive iron to the deep basin. Positive deep-water δ56Fe values, above 0.55‰ during the terminal Ediacaran and Cambrian Stages 1–3, likely reflect a basin where pyritization, rather than oxidation, was the predominant sink for deep-water ferrous Fe. Thus, we infer that only the shallow water was sufficiently oxygenated to support complex metazoans and the evolutions of skeletons, and that atmospheric oxygen levels were not high enough to directly oxygenate deep water environments during the Cambrian explosion.

Correction to: A re-assessment of nickel-doping method in iron isotope analysis on rock samples using multi-collector inductively coupled plasma mass spectrometry

Element doping has been proved to be a useful method to correct for the mass bias fractionation when analyzing iron isotope compositions. We present a systematic re-assessment on how the doped nickel may affect the iron isotope analysis in this study by carrying out several experiments. We find three important factors that can affect the analytical results, including the Ni:Fe ratio in the analyte solutions, the match of the Ni:Fe ratio between the unknown sample and standard solutions, and the match of the Fe concentration between the sample and standard solutions. Thus, caution is required when adding Ni to the analyte Fe solutions before analysis. Using our method, the δ56Fe and δ57Fe values of the USGS standards W-2a, BHVO-2, BCR-2, AGV-2 and GSP-2 are consistent with the recommended literature values, and the long-term (one year) external reproducibility is better than 0.03 and 0.05‰ (2SD) for δ56Fe and δ57Fe, respectively. Therefore, the analytical method established in our laboratory is a method of choice for high quantity Fe isotope data in geological materials.

Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic Fe[sbnd]Ti deposits: A case study of the Damiao titanomagnetite ore deposit, North China Craton

Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the FeTi oxide ores hosted in the Damiao anorthosite-type FeTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The FeTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mtss) and hematite-ilmenite (Hem-Ilmss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mtss by oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ57Fe (+0.27 − +0.65‰), whereas the ilmenite separates have lower δ57Fe (−0.65 to −0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyriteI) with higher δ57Fe (δ57Fe = +0.63 − +0.95‰) is consistent with magmatic origin, and type II pyrite (pyriteII) with lower δ57Fe (δ57Fe = −0.90 to −0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyriteI probably indicate variations of oxygen fugacity, whereas those of the pyriteII may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not.

Iron isotope geochemistry and mineralogy of jarosite in sulfur-rich sediments

Jarosite is a common mineral in acidic, sulfate-rich environments where it is critical in regulating the acidity of aquatic systems and the mobility of trace elements and potential contaminants. This research aims to understand jarosite formation and recrystallization in these environments by examining the stable iron isotope geochemistry of jarosite at two coastal sites in Victoria, Australia: Fossil Beach and Southside Beach. Jarosite occurs at high abundance as beds, veins, surface coatings and nodules within oxidized zones of sulfidic sediment outcrops and as pebbles, cobbles, and boulders at the base of the outcrops within the intertidal zone, making these two beaches ideal natural laboratories. Synchrotron powder X-ray diffraction (XRD) and ICP-MS results indicate that samples are comprised predominantly of natrojarosite, often with substantial K substitution. Rietveld refinement of XRD patterns shows that most jarosite samples are a solid-solution of Na-K jarosite, differing from previous observations that (near-)end-member mixing predominantly occurs in nature. The iron isotope composition of the jarosite samples have δ56Fe values between −1.91 and +1.24‰ (relative to IRMM-014), an exceptionally large range that partially overlaps with the δ56Fe values of the sulfidic sediment precursor (−0.54 to +1.30‰). There is a negative relationship between the alkali ratio [Na/(Na + K)] and iron isotope composition, with the heavier iron isotopes preferentially partitioned into K-rich jarosite. The large range in δ56Fe values of jarosite likely results from a combination of the variable δ56Fe values of the precursor sulfides, thermodynamic differences between Na- versus K-bearing jarosite, and an open-system Rayleigh distillation during jarosite formation.

Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones. Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

Iron isotopic composition of very low-titanium basalt deduced from the iron isotopic signature in Luna 16, 20, and 24 soils

Isotopic analysis of Fe has proved to be a productive tool for the study of the formation and evolution of planetary bodies. The Fe isotopic studies on the Moon revealed that low-Ti and high-Ti basalts are positively correlated with TiO2 contents, indicating the occurrence of Fe isotopic fractionation during lunar magma ocean differentiation (Weyer et al., 2005; Liu et al., 2010; Wang et al., 2015; Sossi and Moynier, 2017). If true, very low-Ti basalts may be more representative of the lunar mantle than high-Ti and low-Ti basalts. Thus, an Fe isotopic study of very low-Ti basalts is required to investigate the Fe isotopic evolution scenario during lunar magma ocean differentiation.In this study, we measured Fe isotopic ratios of soil samples from the Soviet Luna missions, Luna 16, 20, and 24, using MC-ICP-MS. Bulk soil samples without isolation of components and separated monomineralic grains (olivine, pyroxene, and agglutinate) were dedicated for the Fe isotopic study. Among our samples, Luna 24 soil is recognized as very low-Ti basalt. Evidence of non-mass-dependent Fe isotopic fractionation was not found in bulk soils and separated monomineralic olivine, pyroxene, and agglutinate grains. Lunar regolith samples are weathered by meteorite impacts on the Moon’s surface. A mature regolith sample, L1613, showed clearly distinguished Fe isotopic compositions between the fine-grained fraction (δ57Fe = 0.54 ± 0.06‰) and the coarse-grained fraction (δ57Fe = 0.20 ± 0.06‰). Such Fe isotopic differences among different size fractions were smaller in the submature sample (L2001) and the immature sample (L24130.3). The Fe isotopic ratios of separated grains show that agglutinates have higher δ57Fe values than bulk and monomineralic grains, indicating abundant nanophase Fe in agglutinates and partial evaporation of Fe during its melting.The δ57Fe value for the coarsest-grained fraction of L24130.3 was found to be 0.08 ± 0.04‰, which is the first Fe isotopic composition reported for lunar samples with very low-Ti mare basalt composition. This value is clearly lower than previously reported δ57Fe values for high-Ti and low-Ti basalts and is nearly identical to those of Mg-suite rocks. This observation is consistent with the Fe isotopic evolution model during lunar magma ocean differentiation (Sossi and Moynier, 2017). Our results related to very low-Ti basalt showed that the bulk silicate Moon has lower Fe isotopic composition than the value estimated from averaged low-Ti or high-Ti basalts.Given the data known to date, including our results, we conclude that it is most likely that Earth and the Moon have the same Fe isotopic composition. We believe that isotopically heavy Fe of silicate Earth is balanced by isotopically light Fe of Earth’s core, as suggested by Galimov’s (2011) model for the formation of the Earth-Moon system.

A reassessment of the iron isotope composition of the Moon and its implications for the accretion and differentiation of terrestrial planets

The Fe isotope composition of planetary bodies may provide constraints on their accretion modes and/or differentiation processes, but to do so, the Fe isotope systematics of key planetary reservoirs needs to be determined. To investigate this for the Moon, we measured the Fe isotope compositions for a suite of 33 bulk lunar mare basalts and highland rocks. Combined with published data, a compendium of 73 different lunar bulk rocks reveals a statistically significant Fe isotope difference between low-Ti and high-Ti mare basalts, yielding average δ57Fe = 0.127 ± 0.012‰ (2SE; n = 27) and δ57Fe = 0.274 ± 0.020‰ (2 SE; n = 25), respectively, relative to the IRMM-14 isotopic reference material. As lunar basalts are thought to reflect the Fe isotope composition of their respective mantle sources, the estimated relative proportion of the low-Ti and high-Ti source mantle suggests that the lunar upper mantle δ57Fe value should be close to 0.142 ± 0.026‰. Whilst the composition of highland rocks (ferroan anorthosites and Mg-suite rocks) should provide a more global view of the Moon, the calculation of the mean δ57Fe value of 15 available highland rock analyses yields δ57Fe = 0.078 ± 0.124‰. Such a value is not defined precisely enough to be of critical use for comparative planetology. Ferroan anorthosites and Mg-suite rocks also give unresolvable means. It appears that Fe isotope heterogeneity among the lunar highland rocks is caused by non-representatively too small sample aliquots of coarse-grained rocks. It can also be the result of mixed lithologies for some. When the (kinetic) effect of olivine tending towards low δ57Fe and feldspar with predominantly high δ57Fe is cancelled, a more precise δ57Fe value of 0.094 ± 0.035‰ is calculated. It is indistinguishable from the mean δ57Fe of impact melts and is also similar to the upper lunar mantle estimate obtained from mare basalts. Collectively, this newly determined Fe isotope composition of the bulk Moon is indistinguishable from that of the Earth, and heavier than those reported for other planetary bodies. This planetary isotope relationship is only observed for silicon given the currently available mass-dependent stable isotope database. Because both iron and silicon reside in the Earth’s metallic core in significant quantities, this may point to the involvement of metallic cores of the Earth and Moon in the interplanetary Fe and Si isotope fractionation. Rather than via high-pressure metal–silicate fractionation at the core–mantle boundary, this would more likely be achieved by partial vaporization of the liquid outer metallic core in the aftermath of a Moon-forming giant impact.

Iron and magnesium isotopic compositions of subduction-zone fluids and implications for arc volcanism

High-pressure (HP) metamorphic veins in eclogites provide insights into the composition and evolution of fluids in subduction zones. We here present the first Fe-Mg isotope data for three types of HP veins, eclogites and their mineral separates from the Dabie Orogen to constrain the Fe-Mg isotopic compositions of subduction-zone fluids and the Fe-Mg isotope behaviors during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich vein-forming fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the formation of the Omp-Ep veins. The early Omp-Ep veins have much heavier Fe-Mg isotopic compositions compared to the host eclogites, indicating Fe-Mg isotope fractionation during fluid-rock interaction due to preferential dissolution of isotopically heavy Omp and Ep from the eclogites into the vein-forming fluids. The δ56Fe and δ26Mg values of the Omp-Ep, Ep-Qtz, and Ky-Ep-Qtz veins gradually decrease and positively correlate with the Eu/Eu* values of whole rock and epidote. This indicates that Fe-Mg isotope fractionation during fluid evolution results from continuous crystallization of isotopically heavy Omp and Ep from the vein-forming fluids. Our results thus demonstrate that Fe-Mg isotopes can significantly fractionate during the dissolution-precipitation processes of minerals in subduction zones.The high δ56Fe (0.04–0.21‰) and δ26Mg (−0.08 to 0.15‰) values of fluid-related HP veins within mafic eclogites indicate that fluids derived from subducted altered oceanic crust (AOC) probably have Fe-Mg isotopic compositions similar to or higher than those of mid-ocean ridge basalts (MORBs). Thus, contribution from AOC-derived fluids is unlikely to explain the light Fe and heavy Mg isotopic compositions of arc lavas. We propose that the light Fe and heavy Mg isotopic compositions of arc lavas may result from a combination of prior melt depletion and addition of serpentinite-derived 54Fe-26Mg-rich fluids into the overlying mantle wedge.

Reconciling petrological and isotopic mixing mechanisms in the Pitcairn mantle plume using stable Fe isotopes

Ocean-island basalts (OIB) are the products of intra-plate mantle melting above mantle plumes. These hot, buoyant upwellings rise from the deep mantle, possibly the core-mantle boundary, to the Earth's surface. Radiogenic isotope signatures of trace elements in OIB are consistent with a variety of crustally-derived recycled components in their source that are suggested to reside in the plume as eclogite or their pyroxenitic melt-reaction products. The global OIB compositional spectrum exhibits a continuum of radiogenic isotope characteristics (Sr-Nd-Pb-Hf) that indicate mixing of these plume components. However, the petrological mechanisms underpinning this mixing remain elusive. Here, we report stable Fe isotope data for plume-derived lavas of Pitcairn Island from the southwest Pacific. Iron isotope compositions, corrected for olivine fractionation, show a strong co-variation with radiogenic Nd-Sr-Pb isotopes. At Pitcairn, two isotopically distinct components have been identified, one being an enriched-mantle 1 (EM-1) component and the other being a more primitive component, which plots in multiple radiogenic isotope space in the so-called focal zone (FOZO), tentatively identified here as the plume matrix. We suggest a two-stage scenario in which Fe isotopes are first fractionated towards heavier values during formation of a reaction-zone pyroxenite. Partial silicate melt derived from a recycled crustal eclogite, assumed here to be of a subducted sediment origin, reacts with ambient peridotite to form an isotopically heavier Fe isotope pyroxenite with memory of its precursor in radiogenic isotopes. Subsequently, both, isolated and enriched reaction zone pyroxenites and ambient plume matrix reach their solidus and mixing of their partial melts to variable proportions create the spread in radiogenic and stable isotope systematics. This mixing of components is coherent with the widely observed disparity in trajectories between plume matrix and enriched components in long-lived (Sr-Nd-Pb-Hf) and extinct (W) radiogenic and stable (Fe-Mg) isotope systematics and marks a fundamental process for chemical diversity in OIB. It supports a model of isolated plume components in a primitive mantle matrix. Enriched components in the plume that contribute to this mixing appear to be solely pyroxenitic with no or very little direct contribution from the original eclogite. Timing and loci of pyroxenite formation remain to be elucidated.

Iron isotopic composition of supra-subduction zone ophiolitic peridotite from northern Tibet

Abstract We present high-precision iron isotope data for harzburgites and constituting minerals from the Yushigou supra-subduction zone ophiolite in the North Qilian orogen of northern Tibet to provide insights into iron isotope behavior in subduction zones. The Yushigou harzburgites represent typical melting residues of silica-enriched forearc mantle wedge with significant melt extraction and minimal melt percolation. The δ56Fe ranges from −0.096‰ to 0.110‰ in olivine, from −0.111‰ to 0.105‰ in orthopyroxene and from −0.141‰ to 0.054‰ in spinel. Coexisting minerals fall on a line with a slope of 1 in the δ-δ plot, indicating iron isotope equilibrium between mineral pairs. Iron isotopic fractionation between olivine and orthopyroxene is minor, which is consistent with theoretical predictions and previous observations. Good iron isotopic equilibrium between olivine and orthopyroxene is a further indicator that silica enrichment in arc peridotites is inherited from their mantle source. In contrast, significantly negative iron isotopic fractionation between spinel and olivine contradicts with most previous constraints, which may be ascribed to Cr substitution in spinel. Bulk rock harzburgites have δ56Fe ranging from −0.097‰ to 0.076‰, the average of which (0.002 ± 0.020‰) is similar to average δ56Fe determined for abyssal peridotites. Based on this inference, the lower δ56Fe of arc magmas than MORB should be due to smaller discrepancy in Fe3+/ΣFe between arc magmas and arc peridotites when compared with MORB and abyssal peridotites. Our modelling demonstrates that this can be achieved if arc magmas are produced by redox-buffered melting of silica-enriched mantle wedge while MORB are produced by redox-unbuffered melting of the depleted mantle.

Iron Isotopic Compositions of Forearc Mantle Peridotites Constrained by Ophiolites from the North Qilian Orogen, Northern Tibet

AbstractPrevious studies on iron isotope compositions of subduction zone magmas have revealed significant and complex variations that have great bearings on petrogenetic processes in the mantle wedge, e.g., partial melting, fluid metasomatism and redox state. However, interpretations for the fractionations are highly debatable and lack direct constraints from mantle wedge peridotites. This study presents iron isotope compositions for whole rocks and mineral separates in fresh forearc peridotites from the Yushigou ophiolite, North Qilian orogen in northern Tibet. Major and trace element compositions of whole rock and mineral indicate that the peridotites are highly depleted forearc peridotites overprinted by melt metasomatism, in contrast to the long‐holding opinion that the peridotites are derived from mid‐oceanic ridges. The minerals fall on a line with a slope of ∼1 on the plot of δ56Fe vs. δ56Fe, indicating isotope equilibrium between minerals. δ56Fe fractionation between olivine and orthopyroxene is within the range of 0∼0.05, while fractionation between olivine and spinel is about 0.05∼0.10. The fractionation trend between olivine and spinel is opposite to previous theoretical and experimental constraints, which may be due to substantial Cr substitution into the spinel. This indicates that negative correlations between spinel Cr#, fO2 and spinel δ56Fe in previous studies are probably a reflection of gradual Cr enrichment in spinel during melt extraction, and spinel δ56Fe values are not a proxy for oxygen fugacity. Whole rock δ56Fe values are well correlated with mineral δ56Fe values, varying from overlapping with depleted mantle to slightly lower than depleted mantle. Therefore, variations in iron isotope compositions of subduction zone magmas are probably due to combined effect of source heterogeneity and partial melting fractionation.

Iron isotope transformations in the meromictic Lake Cadagno

Abstract The iron isotope composition of sedimentary deposits is a key tool for tracking changes in the biogeochemical and redox conditions of modern and geologically ancient aquatic systems. The use of iron isotopes to reconstruct oxic, anoxic and redox conditions is based on iron isotope fractionation associated with the iron(II)–iron(III) redox reaction and the formation of various iron oxy, hydroxy and sulfur species. However, the degree of iron isotope fractionation varies within the sedimentary record, and the processes leading to isotope fractionation within aquatic systems also vary. Here we investigate iron isotope fractionation within the water column of Lake Cadagno, Switzerland. Lake Cadagno is a 21 m deep alpine meromictic lake that is permanently stratified. It has two chemically distinct water layers: an oxic mixolimnion separated by a narrow chemocline from an anoxic monimolimnion. The chemocline is located across a narrow band between 10 and 13.5 m and is defined by steep chemical gradients in dissolved oxygen, redox potential, nutrients (nitrite + nitrate, ammonia), dissolved and particulate trace metals (iron and manganese) and sulfur species. Iron isotope determination (56Fe/54Fe ratio; expressed as δ56Fe) of both dissolved (δ56Fedissolved) and particulate (δ56Feparticulate) iron reveals a sharp transition within the chemocline with a heavy δ56Fedissolved value (+0.75‰) at 11.5 m and light δ56Fedissolved value (−0.61‰) below at 12 m. The large shift in δ56Fedissolved occurs where anoxygenic phototrophic bacteria become abundant. Modelling of the dissolved iron isotope fractionation within the chemocline produced fractionation factors of −0.60‰ (kinetic model) and −1.52‰ (equilibrium model) assuming a two-step process involving iron oxidation followed by precipitation. Changes in the isotope composition of particulate iron with depth are subtler to that of dissolved iron, with slightly lighter values (−0.18‰) above the chemocline and slightly heavier values (+0.13‰) below. The production of reduced iron(II) within and below the chemocline is likely coupled to dissimilatory iron(III) reduction and dissolution reactions associated with sinking iron(III) oxy and hydroxy species. These processes lead to a peak in dissolved iron concentrations at 12.5 m. The decline in dissolved iron concentration below this depth and its change in isotope composition is consistent with the formation of iron sulphide species, e.g. mackinawite, under mildly euxinic conditions. Overall our results indicate that variations in δ56Fedissolved likely involve a combination of biotic and abiotic processes with biological mixing enhancing the rate at which iron(II) is transferred across the chemocline. The observed isotope transformation of dissolved iron across the chemocline of Lake Cadagno may make it a reasonable analogue to past shallow-water systems with mild euxinia.

A novel framework for interpreting pyrite-based Fe isotope records of the past

Abstract The variation in the iron isotopic composition (δ56Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II)aq) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H2S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe3S4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fepyrite - δ56FeFeSx, where FeSx includes FeS, Fe(II)aq, and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the

Iron Isotopic Composition of Suspended Particulate Matter in Hongfeng Lake

The geochemical study of iron isotopes is of great significance to comprehensively understand the surface material circulation process and its environmental effects in surface and subsurface environments. Eutrophic lakes are an important part of the surface and subsurface environment; however, knowledge of the geochemical behaviour and fractionation mechanism of iron isotopes in the biogeochemical cycling of eutrophic lakes is still scarce. In this study, a eutrophic lake with seasonal anaerobic characteristics (Hongfeng Lake) was selected as the study object to systematically analyse the iron isotope composition of suspended particles in lake water and the main tributaries in different seasons. The results show that the value of δ56Fe in Hongfeng Lake is between −0.85‰ and +0.14‰, and the value of δ56Fe has a high linear correlation with Fe/Al, indicating that the continental source material carried by the main inflow tributaries of the lake has an important influence on the source of iron in the lake. And Hongfeng Lake is moderately eutrophic lakes. Algal bloom and the content of chlorophyll a (Chl-a) are high, combined with the high correlation between Chl-a and the value of δ56Fe, which indicates that the growth of algae has an important influence on the change in the iron isotope composition of suspended particulate matter (SPM) in lake water and that the adsorption and growth absorption of Fe by algae are the main reason for the change in the value of δ56Fe; therefore, Fe isotope can be used to trace the lake’s biological action. For the lake and its inflow tributaries, δ56Fe values are higher in summer than in winter. The variation in the δ56Fe value of SPM with lake depth is more distinct in summer than in winter. In addition, there is a distinct thermocline in summer, which leads to hydrochemical stratification. Moreover, according to a linear correlation analysis, the content of dissolved organic matter (DOC) in Hongfeng Lake’s upper and lower water bodies, respectively, has a high correlation with the value of δ56Fe. Specifically, the correlation is positive in the upper water but negative in the lower water, which indicates that the difference in algae metabolism patterns between the upper and lower water bodies of Hongfeng Lake plays an important role in the iron isotope composition of SPM. The composition of the iron isotope in SPM is altered by organic adsorption and growth absorption of algae in the upper water. With an increase in depth, degradation becomes the main process. In addition, the value of δ56Fe is low and that of Fe/Al is high in the water bottom, which indicates that a “ferrous-wheel” cycle forms at the bottom of the water.