Iron Isotope Composition Research Articles

Zinc and iron isotopic compositions of Cenozoic basalts in Inner Mongolia: New insights into deep carbon recycling related to Paleo-Asian slab subduction

The origin of Cenozoic basalts in Inner Mongolia has been debated in recent years. Although the heavier-than-mantle zinc isotopic compositions indicate recycled carbonate in their sources, the recycled carbonate has been attributed to two different subducting oceanic slabs (Paleo-Asian vs. Paleo-Pacific). To further address this issue, Cenozoic basalts located in the west of the North-South Gravity Lineament (NSGL) in Inner Mongolia are investigated, and the sampling profile is approximately parallel to the NSGL (i.e., from southwest to northeast; SW-NE). These basalts have notably heavier zinc (δ66Zn = 0.47–0.52 ‰) and iron (δ56Fe = 0.18–0.24 ‰) isotopic compositions than those of the mantle, which cannot be interpreted by magmatic processes (differentiation and partial melting). Instead, the involvement of recycled carbonates in sources is a viable explanation for the high δ66Zn characteristics, supported by high P2O5 contents and low Hf/Hf*. In addition, samples with high δ56Fe also display high Fe/Mn (69–81) related to Cenozoic basalts from the east of the NSGL, indicating the existence of a pyroxenite component in sources in addition to carbonated peridotites. Combining with literature data, we found that basalts distributed in the SW-NE direction in Inner Mongolia have similarly heavy Zn and Fe isotopic compositions. This provides further constraints that mantle carbonate metasomatism in the west of the NSGL was most likely to have been caused by southeastward subduction of the Paleo-Asian oceanic slab, instead of northward subduction of the Paleo-Asian oceanic slab or subduction of the Paleo-Pacific oceanic slab.

Heavy iron isotopes reveal mantle oxidation in addition to pyroxenite sources for intraplate basalts

The heavier iron isotopic composition of some oceanic island basalts (OIB) relative to primary mid-ocean ridge basalts (MORB) has commonly been interpreted to reflect the heavy bulk isotopic compositions of their sources (e.g., of a pyroxenite-dominated mantle region). Alternatively, comparable Fe isotopic signatures observed in continental intraplate basalts (CIB) may record enhanced isotope fractionation during partial melting of an oxidized source, offering valuable insights into mantle oxidation induced by subducted carbonate. To distinguish between these two independent contributions, here we present high-precision Fe isotopic data (δ57Fe) and Fe3+/∑Fe ratios for a suite of Cenozoic intraplate basalts from Inner Mongolia, East Asia. These basalts are distributed along a northwest-southeast (NW–SE) profile and associated with earlier subduction of the Paleo-Asian oceanic slab. Variations in Fe/Mn ratios and zinc isotopic compositions (δ66Zn) of these basalts reveal that altered oceanic crust has always contributed to their mantle sources but the amounts of recycled carbonate gradually vary. Together with existing data, we demonstrate that all of these basalts generally exhibit heavier δ57Fe (0.07 ‰ to 0.31 ‰), higher Fe3+/∑Femelt (0.08 to 0.39) and Fe/Mn (> 65) relative to primary MORB (δ57Fe ∼0.10 ‰; Fe3+/∑Femelt ∼0.10; Fe/Mn ∼60), pointing to a common origin of heavy Fe isotopes in intraplate basalts that may reflect those of pyroxenite sources (e.g., recycled crust). However, these basalts containing larger amounts of recycled carbonate in their sources (i.e., higher δ66Zn) have heavier δ57Fe at similar Fe/Mn ratios and proportions of pyroxenite melt. Such Fe isotopic signatures exceed the exclusive role of pyroxenite and are best attributed to partial melting at oxidizing conditions (i.e., high Fe3+/∑Fesource) with an inferred enlarged fractionation factor. This oxidized mantle source formed through the reduction of recycled carbonate to graphite/diamond at depths greater than 150 km. A global dataset of Fe isotopes in intraplate basalts is compiled in order to examine this interpretation further. Compared with OIB typically from a pyroxenite/eclogite-dominant source, CIB with similarly heavy δ57Fe and formed by similar melting degrees are commonly more silica-undersaturated, indicating a widespread carbonated mantle source. Thus, these observations highlight that heavy Fe isotopes can record carbonate-induced mantle oxidation in addition to source lithological heterogeneity for intraplate basalts.

Fe, Zn, and Mg stable isotope systematics of acapulcoite lodranite clan meteorites

AbstractThe processes of planetary accretion and differentiation, whereby an unsorted mass of primitive solar system material evolves into a body composed of a silicate mantle and metallic core, remain poorly understood. Mass‐dependent variations of the isotope ratios of non‐traditional stable isotope systems in meteorites are known to record events in the nebula and planetary evolution processes. Partial melting and melt separation, evaporation and condensation, diffusion, and thermal equilibration between minerals at the parent body (PB) scale can be recorded in the isotopic signatures of meteorites. In this context, the acapulcoite–lodranite meteorite clan (ALC), which represents the products of thermal metamorphism and low‐degree partial melting of a primitive asteroid, is an attractive target to study the processes of early planetary differentiation. Here, we present a comprehensive data set of mass‐dependent Fe, Zn, and Mg isotope ratio variations in bulk ALC species, their separated silicate and metal phases, and in handpicked mineral fractions. These non‐traditional stable isotope ratios are governed by mass‐dependent isotope fractionation and provide a state‐of‐the‐art perspective on the evolution of the ALC PB, which is complementary to interpretations based on the petrology, trace element composition, and isotope geochemistry of the ALC. None of the isotopic signatures of ALC species show convincing co‐variation with the oxygen isotope ratios, which are considered to record nebular processes occurring prior to the PB formation. Iron isotopic compositions of ALC metal and silicate phases broadly fall on the isotherms within the temperature ranges predicted by pyroxene thermometry. The isotope ratios of Mg in ALC meteorites and their silicate minerals are within the range of chondritic meteorites, with only accessory spinel group minerals having significantly different compositions. Overall, the Mg and Fe isotopic signatures of the ALC species analyzed are in line with their formation as products of high‐degree thermal metamorphism and low‐degree partial melting of primitive precursors. The δ66/64Zn values of the ALC meteorites demonstrate a range of ~3.5‰ and the Zn is overall isotopically heavier than in chondrites. The superchondritic Zn isotopic signatures have possibly resulted from evaporative Zn losses, as observed for other meteorite parent bodies. This is unlikely to be the result of PB differentiation processes, as the Zn isotope ratio data show no covariation with the proxies of partial melting, such as the mass fractions of the platinum group and rare earth elements.

Iron species and sulfur isotopic compositions of authigenic pyrite in deep-sea sediments at southern Hydrate Ridge, Cascadia margin (ODP Leg 204): implications for non-steady-state depositional and diagenetic processes

Two accretionary sediment sequences from Sites 1245 and 1252 recovered during Ocean Drilling Program (ODP) Leg 204 at southern Hydrate Ridge were investigated to explore the response of geochemical partitioning of iron and sulfur isotopic composition of authigenic pyrite to non-steady-state depositional and diagenetic scenarios. Five iron species were characterized by a modified sequential extraction procedure that covers almost all iron-bearing minerals in sediment cores, including: (1) iron-bearing carbonates, mainly siderite; (2) ferric (hydr)oxides, probably ferrihydrite and/or lepidocrocite; (3) magnetite; (4) iron-bearing silicates; and (5) pyrite. Highly reactive iron has been accumulated for a long-term steady-state history and its pyritization, to varying degrees, is limited by availability of dissolved sulfide. This causes pyrite and siderite occurred in the same sedimentary layer and shows an inverse relationship between their concentrations. From this, their proportions to highly reactive iron can be chosen for evaluating the degree of sulfidization. A significant change in sulfur isotopic composition of pyrite (-42.4 to +16.8‰ VCDT) indicates that the steady-state conditions are dramatically limited, where the δ34S values higher than -20‰ may result from an upward shift of SMT zone close to the seafloor or a sudden, massive depositional event. To explain the downcore sulfidization effects and pyrite δ34S values, we developed two categories of conceptual scenarios based on variations in sedimentation rate and methane flux. The geochemical features similar to those derived from each scenario were searched in the sediment columns and the non-steady-state events behind the scenarios were proved to be consistent with the real observations. Thus, iron species and pyrite δ34S values can be regarded as a proxy to differentiate different non-steady-state depositional and diagenetic controls on the sedimentary record.

Impact of deforestation on the hydrogen, oxygen and iron isotope compositions of Amazonian streams

We examined the consequences of deforestation and pasture establishment on H, O and Fe isotope cycling in the hydrosphere, with focus on small watersheds from the central and eastern part of Brazilian Amazonia. The δ18O- δD relationships in stream water samples show that the evaporation effect is greater in pasture than in forest, thereby highlighting the impact of deforestation on atmospheric water circulation and rainfall.A clear effect of deforestation was also found on the dissolved load of these streams. Water temperature, pH, conductivity, major cations and dissolved carbon organic content are lower in forest streams compared to those from deforested areas. Dissolved (i.e., filtrate <0.45 μm) iron concentrations are typically higher in these tropical forest streams. They also show δ57Fe signatures commonly higher than that of the continental crust (i.e., > 0.1‰). In these black waters enriched in organic matter, the heavier iron isotope composition in the forested areas is related to the oxidized Fe strongly bound to colloidal organic matter. On the other hand, dissolved iron from streams draining older deforested areas shows isotopic compositions usually lighter than the continental crust (δ57Fe < 0.1‰). The negative δ57Fe signatures of these areas are likely caused by deforestation involving erosion and rejuvenation of soils in valleys, which leads to the occurrence of a partial iron reduction in the soils. These modifications increased the proportion of isotopically light, dissolved and more reduced Fe in the streams.Those effects are less obvious on more recent pastures since they display more chemical and isotopic scatter along the sampling months. In a stream flowing through an area undergoing slash and burned deforestation within the year, dissolved iron concentrations and isotopic compositions are much more scattered, with δ57Fe values varying by >10 ‰ as a result of fire or heavy rain events. This likely depicts the disruption of the iron cycling in the water-soil-plant system with strong changes in the soil mineralogy, organic matter characteristics and biomass activity resulting from the forest fire. Our results suggest that more than a decade is needed to reach a new dynamic steady state of the Fe biogeochemical cycle in pastures.Hence, the contrasted Fe isotopic compositions between the streams draining pristine tropical forests and those from deforested areas indicate that, like H and O isotopes, Fe isotopes are a sensitive indicator of the transformations affecting the biogeochemical cycling of tropical environment in response to deforestation. However, in contrast to hydrogen and oxygen isotopes that reveal the exchanges between surface waters and atmosphere, iron isotopes rather highlight the physical-chemical reactions occurring between surface waters and the soils, but with the biomass mediation for all three isotopic tracers.

Iron Composition of a Typical Loess-Paleosol Sequence in Northeast China

Iron isotope compositions, along with the partial extraction of iron in its various forms, can be utilized to investigate the complex interplay of iron migration and transformation with respect to iron isotope patterns. This study investigated the iron composition of a typical loess-paleosol sequence in Northeast China and aimed to understand the influence of iron migration and transformation of the typical loess-paleosol sequence on iron isotopes and environmental and climatic changes that occurred in the region over time by analyzing the distribution and characteristics of iron compositions in sedimentary layers. Samples were collected from Chaoyang in Northeast China, and the iron isotopic composition was analyzed using the multi-receiver inductively coupled plasma mass spectrometer (MC-ICP-MS). The findings revealed depth-dependent variations in the content of different iron forms, reflecting paleoclimatic shifts primarily through pedogenic transformation processes. Notably, iron migration within the section was observed to be limited. The variations in the reddening index and magnetic susceptibility of the loess-paleosol were primarily influenced by the presence of free iron (Fed), exhibiting a range of colors from yellow to red-yellow and red. The δ56Fe values for loess and paleosols ranged from 0.097 ± 0.035‰ to 0.167 ± 0.010‰, with an average of 0.133 ± 0.024‰ and a coefficient of variation (CV) of 15.66% at the stratum scale. These values indicated a systematic enrichment of heavy iron isotopes and a significant negative correlation with the slightly fluctuating total iron content. Specifically, our analysis highlighted distinct differences in δ56Fe values between paleosol (0.126 ± 0.024‰) and loess (0.146 ± 0.021‰). The δ56Fe in Fed was negative, averaging −0.101 ± 0.022‰, while the δ56Fe in silicate-bound iron was positive, averaging 0.156 ± 0.032‰. Intense pedogenesis, driven by warm and wet climates, facilitated iron transformations and migrations, resulting in the accumulation of light iron isotopes in the paleosols. These transformations and migrations were predominantly observed in microdomains characterized by iron depletions and concentrations, as reflected in the profile morphologies. However, the limited iron transformations and migrations did not result in significant Fe redistribution within the soil section, as evidenced by the limited variations in δ56Fe with soil depth at the stratum scale. Sampling from the stratum or pedogenic horizon could potentially create the illusion of the minimal fractionation of iron isotopes within the sequence. Therefore, a detailed examination of the iron isotope composition in the micro-domains of the loess-paleosol sequence is crucial to elucidate the fractionation processes and mechanisms of iron isotopes during the formation of these sequences.

Precise measurement of Fe isotopes in marine and biological samples by pseudo-high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS).

This paper introduces an enhanced technique for analyzing iron isotopes in complex marine and biological samples. A dedicated iron purification method for biological marine matrices, utilizing three ion exchange columns, is validated. The MC-ICPMS in pseudo-high-resolution mode determines precise iron isotopic ratios, with sensitivity improved through the DSN-100 desolvating nebulizer system and Apex-IR. Only 2µg of iron on DSN versus 1µg on Apex is needed for six replicates (30-60 times improvement) while 10 to 20µg is required for a single measurement on awet system considering the resolution power (Rp) is maintained at 11,000-13,000. The Ni-doping method with a Fe/Ni ratio of 1 yields more accurate isotopic ratios than standard-sample bracketing alone. Measurement reproducibility of triplicate samples from marine biological experiments on MC-ICPMS is ± 0.03‰ (2SD) for δ56Fe and ± 0.07‰ for δ57Fe (2SD). This study introduces a novel iron purification process specifically designed for marine and biological samples, enhancing sensitivity and enabling more reliable measurements with smaller sample sizes and reduced uncertainties. It proposes iron isotopic compositions for biological reference materials, offering a valuable reference dataset in diverse scientific disciplines.

Genesis of the Beizhan Iron Deposit in Western Tianshan, China: Insights from Trace Element and Fe-O Isotope Compositions of Magnetite

The Beizhan iron deposit (468 Mt at an average grade of 41% Fe) is the largest iron deposit in the Awulale iron metallogenic belt of Western Tianshan, northwest China. The high-grade magnetite ores are hosted in the Carboniferous volcanic rocks with extensive development of skarn alteration assemblages. While considerable progress has been made in understanding the characteristics of Beizhan and its genetic association with volcanic rocks, the genetic models for ore formation are poorly constrained and remain controversial. This study combines detailed petrographic investigations with in situ LA-ICP-MS analyses of trace elements and Fe-O isotope compositions of magnetite to elucidate the origin of magnetite and the conditions of ore formation. The trace element concentrations in magnetite unveil intricate origins for various ore types, implying the precipitation of magnetite from both magmatic and hydrothermal fluids. The application of the Mg-in magnetite thermometer (TMg-mag) reveals a notable temperature divergence across different magnetite varieties, spanning from relatively higher temperatures in magmatic brecciated magnetite (averaging ~641 and 612 °C) to comparatively lower temperatures in hydrothermal platy magnetite (averaging ~552 °C). The iron isotopic composition in massive and brecciated magnetite grains, characterized by lighter δ56Fe values (ranging from −0.078 to +0.005‰ and −0.178 to −0.015‰, respectively), suggest a magmatic or high-temperature hydrothermal origin. Conversely, the heavier δ56Fe values observed in platy magnetite (+0.177 to +0.200‰) are attributed to the influence of pyrrhotite, signifying late precipitation from low-temperature hydrothermal fluids. Additionally, the δ18O values of magnetite, ranging from +0.6 to +4.6‰, provide additional evidence supporting a magmatic–hydrothermal origin for the Beizhan iron deposit. Overall, the identified genetic associations among the three magnetite types at Beizhan provide valuable insights into the evolution of ore-forming conditions and the genesis of the deposit. These findings strongly support the conclusion that the Beizhan iron deposit underwent a process of magmatic–hydrothermal mineralization.

Iron isotopic compositions of HIMU Ocean island basalts: Implications for the mantle source lithology

Basalts from the islands of Mangaia, Tubuai, Rurutu (old stage), and Raivavae (Rairua stage) in the South Pacific, and from St. Helena Island in the South Atlantic, are characterized by extremely radiogenic Pb isotopic compositions (e.g., 206Pb/204Pb > 20.5). These ocean island basalts (OIBs) define the famous HIMU (high μ, where μ = 238U/204Pb) component in the mantle ‘zoo’, which has traditionally been thought to represent ancient recycled oceanic crust with highly fractionated U/Pb and Th/Pb ratios. However, recent high-precision analyses of minor and trace elements in olivine phenocrysts from HIMU OIBs suggest a peridotitic mantle source rather than pyroxenitic/eclogitic remnants of subducted oceanic crust. To better constrain the lithology of the HIMU source is crucial for further understanding the nature of the HIMU component. Here we utilize stable iron (Fe) isotopes, a novel tool for identifying the mantle source lithology of basalts, to further investigate the lithology of the HIMU source by examining classic HIMU OIBs from the Cook-Austral volcanic chain in the South Pacific. The results show that these OIBs have δ57Fe values varying from 0.09‰ to 0.18‰, which are similar to those of normal mid-ocean ridge basalts (N-MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). Quantitative calculations indicate that the fractional-crystallization-corrected δ57Fecorr values (0.06–0.15‰) of these basalts can well be explained by partial melting of garnet peridotite with δ57Fe values ranging from 0.05‰ to 0.09‰. Such low δ57Fecorr values, however, are difficult to be reproduced by partial melting of eclogite with a MORB-like δ57Fe value (δ57Fe = 0.15‰). Our new Fe isotope data, combined with previously reported heavy whole-rock zinc isotopes (δ66Zn = 0.38 ± 0.03‰) as well as high Mn/Fe (100Mn/Fe = 1.5–1.7) and Ca/Al (mostly >15) ratios of olivine phenocrysts in the HIMU OIBs, further confirm that the lithology of the HIMU mantle source is carbonated peridotite.

Fractionation of iron and titanium isotopes by ilmenite and the isotopic compositions of lunar magma ocean cumulates

Basaltic volcanism on the Moon produced low- and high-Ti mare basalt suites that are also distinct with respect to their iron, titanium, and magnesium isotopic compositions. Here, the equilibrium fractionation of Fe and Ti isotopes between ilmenite and melt was experimentally investigated in order to evaluate the role of ilmenite in generating the isotopic compositional variability among the lunar mare basalts. Ilmenite crystallization experiments were conducted using two bulk compositions: an ilmenite-saturated basaltic andesite and an ilmenite-saturated Apollo 14 black glass, and the Fe and Ti isotopic compositions of the experimental ilmenites and glass (quenched melt) were analyzed using solution MC-ICPMS after hand-picking. Additionally, Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements on synthetic ilmenite were conducted and compared to previous NRIXS measurements on synthetic lunar glasses in order to derive temperature-dependent equilibrium ilmenite-melt Fe isotopic fractionations. Experimentally determined ilmenite-melt fractionations were then incorporated into a lunar magma ocean crystallization model that tracks the major element and isotopic compositional evolution of lunar magma ocean cumulates and residual liquid. There is good agreement between the Fe equilibrium isotopic fractionation measured by NRIXS and the laboratory equilibration experiments, and we find that the isotopic fractionation is sensitive to ilmenite compositional differences (0 vs. 10% Fe3+). Further, the light Ti isotopic composition of ilmenite relative to the melt (Δ49Ti=ilmenite-melt−0.09±0.03‰ at 1100∘C) is consistent with the higher coordination of Ti in ilmenite relative to melts and results of previous studies. The modeled Ti isotopic compositions for lunar magma ocean cumulates display Ti isotopic variability sufficient to explain the low- and high-Ti mare basalt sources. However, the difference in Fe isotopic composition between the low- and high-Ti mare basalts cannot be attributed solely to ilmenite fractionation. Instead, Fe isotopic fractionation by additional products of lunar magma ocean crystallization, such as clinopyroxene, is required to generate the inferred Fe and Mg isotopic variability in the lunar mantle. Alternatively, the Fe and Mg isotopic compositions of the lunar mare basalts may indicate Fe-Mg interdiffusion has occurred in the Ti-rich component of the mare basalt source regions via reaction between ilmenite cumulates and the olivine- and pyroxene-rich lunar mantle.

Iron and Molybdenum Isotope Application for Tracing Industrial Contamination in a Highly Polluted River

Rivers adjacent to industrial zones usually suffer from severe pollution issues. Industrial wastewater that has undergone sewage treatment processes may be legally discharged into rivers under water quality permits. Previous studies have frequently employed isotopic tracers to identify potential contaminants for pollution control. Conventional radiogenic isotopes utilized in tracing studies cannot discern whether the source is untreated (primary) industrial wastewater, which can have serious impact to the environment. By analyzing the iron (Fe) and molybdenum (Mo) isotopic compositions in original industrial wastewater and treated effluent, this study aims to investigate whether the heavily polluted Agongdian River is contaminated by the untreated wastewater. Based on the results from this study, the original industrial wastewater exhibits higher concentrations of metallic elements and heavier Fe and lighter Mo isotopic compositions, compared to the treated effluent. Consequently, it appears that Agongdian River water indeed exhibits evidence of untreated industrial wastewater. Furthermore, the volume of original industrial wastewater entering the river can be estimated from these results. This research offers a more precise and accurate approach to monitor potential industrial wastewater pollution in natural water bodies, contributing to the goal of environmental protection and sustainable development.

Emerging investigator series: Coprecipitation with glucuronic acid limits reductive dissolution and transformation of ferrihydrite in an anoxic soil.

Ferrihydrite, a poorly crystalline Fe(III)-oxyhydroxide, is abundant in soils and is often found associated with organic matter. Model studies consistently show that in the presence of aqueous Fe(II), organic carbon (OC)-associated ferrihydrite undergoes less transformation than OC-free ferrihydrite. Yet, these findings contrast microbial reductive dissolution studies in which the OC promotes the reductive dissolution of Fe(III) in ferrihydrite and leads to the release of associated OC. To shed light on these complex processes, we quantified the extent of reductive dissolution and transformation of native Fe minerals and added ferrihydrite in anoxic soil incubations where pure 57Fe-ferrihydrite (57Fh), pure 57Fe-ferrihydrite plus dissolved glucuronic acid (57Fh + GluCaq), a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate (57Fh13GluC), or only dissolved glucuronic acid (13GluCaq) were added. By tracking the transformation of the 57Fe-ferrihydrite in the solid phase with Mössbauer spectroscopy together with analysis of the iron isotope composition of the aqueous phase and chemical extractions with inductively coupled plasma-mass spectrometry, we show that the pure 57Fe-ferrihydrite underwent more reductive dissolution and transformation than the coprecipitated 57Fe-ferrihydrite when identical amounts of glucuronic acid were provided (57Fh + GluCaqversus57Fh13GluC treatments). In the absence of glucuronic acid, the pure 57Fe-ferrihydrite underwent the least reductive dissolution and transformation (57Fh). Comparing all treatments, the overall extent of Fe(III) reduction, including the added and native Fe minerals, determined with X-ray absorption spectroscopy, was highest in the 57Fh + GluCaq treatment. Collectively, our results suggest that the limited bioavailability of the coprecipitated OC restricts not only the reductive dissolution of the coprecipitated mineral, but it also limits the enhanced reduction of native soil Fe(III) minerals.

Paleo-marine redox environment fluctuation during the early Cambrian: Insight from iron isotope in the Tarim Basin, China

The Ediacaran to Cambrian period is generally considered to be the vital transition in the history of marine redox environment and life evolution on earth. The ocean oxygenation levels during this transition period are still debated. Since iron is widely involved in biogeochemical cycles and undergoes redox cycling both in the seawater and sediments, it has become a significant proxy to reconstruct paleo-marine environment. In order to constrain the paleo-marine redox state in the early Cambrian, the iron isotope composition of bulk rock (δ56FeT) is interpreted combining with iron-speciation, redox sensitive elements and pyrite sulfur isotope (δ34Spy) of Yuertusi Formation in Tarim Block. The δ56FeT values varies from −0.39 ‰ to 0.48 ‰, with an average of 0.07 ‰, mainly controlled by pyrite mineral facies in this study. Based on the mechanism of pyrite generation in different redox condition, it is proposed that the marine environment of the lower Cambrian in the Tarim basin is dominated by anoxic with intermittent euxinic state. The dynamic evolution of redox environment can be divided into three intervals. The gradual decrease of δ56Fe in Interval I indicates the paleo-marine environment changed from anoxic ferruginous to euxinic, and the paleo-marine sulfate reservoir decreased to a limited level, which might be attributed to abundant burial of organic matter and pyrite. For Interval II, δ56Fe values first increase to evident positive because of partial oxidization then decreased to that of seawater (about 0 ‰) due to complete oxidization. In Interval III, the continuous decrease of δ56Fe values infers a sustaining oxidization. In summary, the paleo-marine environment of the lower Cambrian Yuertusi Formation evolved from anoxic ferruginous to euxinic and then oxidized continuous. Iron isotope statistics from geological historical periods indicate that seawater was relatively oxidized after the NOE event but did not reach the oxidation levels of present-day seawater.

Heterogeneous mantle source compositions for boninite from Bonin and Troodos, evidence from iron isotope variations

Boninite has been widely accepted to be the melting product of highly refractory mantle with the addition of slab-derived fluids. Previous studies have found that boninite, like other subduction-related magmas (e.g., mature arc basalts), has generally consistent chondrite-like iron isotope composition (c. 0.03‰), which is on average lighter than those of oceanic basalts. However, it remains unclear whether the lighter iron isotope composition of subduction-related magmas is inherited from the depleted mantle source or caused by the addition of slab-derived oxidized fluids. Moreover, boninite has highly varied element patterns (e.g., Yb, Zr), reflecting varying degrees of mantle depletion and different contributions of slab-derived fluids. Given the limited data for iron isotope composition of boninite so far available, it is thus required to systematically study iron isotope composition of boninite with different compositions to further understand the petrogenesis of boninite and the iron isotope variation of subduction-related magmas. Here we report iron isotope compositions of boninite from two classic suites on Earth, i.e., those from Bonin islands and submarine forearc (low-Ca boninite; CaO/Al2O3 mainly <0.75) and Troodos ophiolite complex, Cyprus (high-Ca boninite: CaO/Al2O3 = c. 0.85). The δ56Fe values of the Bonin boninite vary from −0.046 ± 0.003‰ to 0.078 ± 0.031‰ (2SD, SD = standard deviation of 4 times repeated analysis; with an average of 0.03 ± 0.03‰, n = 14). The δ56Fe value of the Troodos boninite is relatively constant, i.e., from 0.046 ± 0.010‰ to 0.091 ± 0.016‰ with an average of 0.06 ± 0.01‰ (n = 7). Bonin forearc basalts (FABs) were also analyzed for comparison with δ56Fe of ~0.10‰, except for those FABs with higher Th/U ratio and Cu contents (δ56Fe = 0.01–0.05‰). Together with previously reported δ56Fe data for boninite from New Caledonia (low-Ca boninite with CaO/Al2O3 = ~ 0.4, δ56Fe = ~ +0.03‰), the lower δ56Fe values of the Bonin boninite than Bonin FAB and Troodos boninite reflect the lighter iron isotope composition of the more depleted mantle source. Based on relationships between different indicators for various fluids (e.g., Ba/La, Zr/Sm) and modeling, Bonin boninite is likely produced by partial melting of the refractory mantle with an addition of ~5–10% melts from sediments and oceanic crust, while the formation of Troodos boninite can be ascribed to the melting of less refractory mantle, which has been previously metasomatized by ~5% fluids derived from oceanic crust and sediments.

Silicon and iron isotopes in components of enstatite chondrites: Implications for metal–silicate–sulfide fractionation in the solar nebula

AbstractSilicon and iron isotope compositions of different physically separated components of enstatite chondrites (EC) were determined in this study to understand the role of nebular and planetary scale events in fractionating Si and Fe isotopes of the terrestrial planet‐forming region. We found that the metal–sulfide nodules of EC are strongly enriched in light Si isotopes (δ30Si ≥ −5.61 ± 0.12‰, 2SD), whereas the δ30Si values of angular metal grains, magnetic, slightly magnetic, and non‐magnetic fractions become progressively heavier, correlating with their Mg# (Mg/(Mg+Fe)). White mineral phases, composed primarily of SiO2 polymorphs, display the heaviest δ30Si of up to +0.23 ± 0.10‰. The data indicate a key role of metal–silicate partitioning on the Si isotope composition of EC. The overall lighter δ30Si of bulk EC compared to other planetary materials can be explained by the enrichment of light Si isotopes in EC metals along with the loss of isotopically heavier forsterite‐rich silicates from the EC‐forming region. In contrast to the large Si isotope heterogeneity, the average Fe isotope composition (δ56Fe) of EC components was found to vary from −0.30 ± 0.08‰ to +0.20 ± 0.04‰. A positive correlation between δ56Fe and Ni/S in the components suggests that the metals are enriched in heavy Fe isotopes whereas sulfides are the principal hosts of light Fe isotopes in the non‐magnetic fractions of EC. Our combined Si and Fe isotope data in different EC components reflect an inverse correlation between δ30Si and δ56Fe, which illustrates that partitioning of Si and Fe among metal, silicate, and sulfidic phases has significantly fractionated Si and Fe isotopes under reduced conditions. Such isotope partitioning must have occurred before the diverse components were mixed to form the EC parent body. Evaluation of diffusion coefficients of Si and Fe in the metal and non‐metallic phases suggests that the Si isotope compositions of the silicate fractions of EC largely preserve information of their nebular processing. On the other hand, the Fe isotopes might have undergone partial or complete re‐equilibration during parent body metamorphism. The relatively uniform δ56Fe among different types of bulk chondrites and the Earth, despite Fe isotope differences among their components, demonstrates that the chondrite parent bodies were not formed by random mixing of chondritic components from different locations in the disk. Instead, the chondrite components mostly originated in the same nebular reservoir and Si and Fe isotopes were fractionated either due to gas–solid interactions and associated changes in physicochemical environment of the nebular reservoir and/or during parent body processing. The heavier Si isotope composition of the bulk silicate Earth may require accretion of chondritic and/or isotopically heavier EC silicates along with cumulation of refractory forsterite‐rich heavier silicates lost from the EC‐forming region to form the silicate reservoir of the Earth.

Light stable Cr isotope compositions of mid-ocean ridge basalts: Implications for mantle source composition

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt.With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample’s 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample’s 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt’s Cr isotopic budget during partial melting.

Pyrite iron isotope compositions track local sedimentation conditions through the Smithian-Spathian transition (Early Triassic, Utah, USA)

Pyrite iron isotope compositions track local sedimentation conditions through the Smithian-Spathian transition (Early Triassic, Utah, USA)

Total Iron Absorbed from Iron-Biofortified Potatoes Is Higher than that from Nonbiofortified Potatoes: A Randomized Trial Using Stable Iron Isotopes in Women from the Peruvian Highlands

BackgroundYellow-fleshed potatoes biofortified with iron have been developed through conventional breeding, but the bioavailability of iron is unknown. ObjectivesOur objective was to measure iron absorption from an iron-biofortified yellow-fleshed potato clone in comparison with a nonbiofortified yellow-fleshed potato variety. MethodsWe conducted a single-blinded, randomized, crossover, multiple-meal intervention study. Women (n = 28; mean ± SD plasma ferritin 21.3 ± 3.3 μg/L) consumed 10 meals (460 g) of both potatoes, each meal extrinsically labeled with either 58Fe sulfate (biofortified) or 57Fe sulfate (nonfortified), on consecutive days. Iron absorption was estimated from iron isotopic composition in erythrocytes 14 d after administration of the final meal. ResultsMean ± SD iron, phytic acid, and ascorbic acid concentrations in iron-biofortified and the nonfortified potato meals (mg/per 100 mg) were 0.63 ± 0.01 and 0.31 ± 0.01, 39.34 ± 3.04 and 3.10 ± 1.72, and 7.65 ± 0.34 and 3.74 ± 0.39, respectively (P < 0.01), whereas chlorogenic acid concentrations were 15.14 ± 1.72 and 22.52 ± 3.98, respectively (P < 0.05). Geometric mean (95% CI) fractional iron absorption from the iron-biofortified clone and the nonbiofortified variety were 12.1% (10.3%–14.2%) and 16.6% (14.0%–19.6%), respectively (P < 0.001). Total iron absorption from the iron-biofortified clone and the nonbiofortified variety were 0.35 mg (0.30–0.41 mg) and 0.24 mg (0.20–0.28 mg) per 460 g meal, respectively (P < 0.001). ConclusionsTIA from iron-biofortified potato meals was 45.8% higher than that from nonbiofortified potato meals, suggesting that iron biofortification of potatoes through conventional breeding is a promising approach to improve iron intake in iron-deficient women.The study was registered at www.clinicaltrials.gov as Identifier number NCT05154500.

Formation pathways of Precambrian sedimentary pyrite: Insights from in situ Fe isotopes

Iron isotope compositions (expressed as δ56Fe) in sedimentary pyrite have been widely used as tracers of redox and chemical evolution of the ocean through geological time. Previous studies mostly built on the mechanical extraction of sulfides from bulk rock samples, and focused on visible macroscopic pyrites, which may introduce a sampling bias. In situ analyses of micropyrite grains can provide new insights into the processes of pyrite formation and their time evolution. Here, we compile ca. 2000 in situ iron isotope compositions of Archean to Paleoproterozoic sedimentary pyrite, from previous literature as well as new data. Contrasting with bulk analyses, micropyrite displays a large and constant range of δ56Fe values, from -4 to +4‰, through time. Micropyrite δ56Fe values are not significantly influenced by metamorphic grade. A bimodal distribution of positive versus negative δ56Fe values can be attributed to two different processes of pyrite formation, Fe (oxyhydr)oxide sulfidation, versus kinetic and possibly microbially mediated pyrite precipitation. These processes are tightly related to rock lithology and thus to sedimentary conditions, and have existed since 3.8 Ga.

S, Pb, and Fe isotope compositions of sulfides in middle and southern Okinawa Trough: Implying the complicated hydrothermal systems in back-arc spreading centers

Sulfur, lead, and iron isotopic systematics of sulfides from the Iheya North hydrothermal field in middle Okinawa Trough and the Yonaguni Knoll IV hydrothermal field in southern Okinawa Trough were investigated to understand the origin of ore-forming material and isotope fractionation during sulfide precipitation in back-arc spreading centers. The δ34S values (from 8.60‰ to 9.68‰) of sulfide minerals in Iheya North field suggest sulfur was derived from the leaching of basement rocks and the reduction of seawater sulfate. But an additional light sulfur source from the dissolution of biogenic sulfides in sediments is indispensable for the observed low δ34S values (between 4.78‰ and 4.97‰) of Yonaguni Knoll IV sulfides. Lead isotope compositions of all sulfides vary in a relatively wide range (206Pb/204Pb = 18.432 to 18.563, 207Pb/204Pb = 15.579 to 15.671, and 208Pb/204Pb = 38.539 to 38.979, respectively) which involves the mixing of mantle-derived and sediment-derived lead in varying proportions. Unusually high radiogenic lead isotope ratios of Yonaguni Knoll IV sulfides (207Pb/204Pb = 15.628 to 15.671, and 208Pb/204Pb = 38.854 to 38.979) imply the presence of old crustal fragments beneath southern OT. The hydrothermal fluids in OT are characterized by extremely low calculated δ56Fe values (−1.00‰ to −1.32‰) which is attributed to reducing sediments. The paired Fe-isotope compositions of pyrite and chalcopyrite revealed significantly different iron isotope compositions of hydrothermal fluids in study areas and extremely complicated ore-forming processes. Rapid precipitation of pyrite from FeS precursors accompanying iron isotopic disequilibrium moves from FeS-fluid equilibrium to pyrite-fluid equilibrium during pyrite recrystallization. The combination of sulfur, lead, and iron isotopic data suggests that hydrothermal systems in back-arc spreading centers related to plate subduction are generally affected by mantle, overlying sediments as well as potential old continental crust fragments, which results in various hydrothermal fluids and complicated mineralization processes.