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Iron isotope ratios in magnetite have been widely used to reveal critical geological and biological processes. Laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) is ideally suited for measurement of Fe isotope ratios, however the lack of suitable reference materials poses a significant challenge for in situ Fe isotopic measurements in magnetites. In this study, five high‐quality natural magnetite crystals were characterised for Fe isotope ratios using solution nebulisation (SN)‐MC‐ICP‐MS and LA‐MC‐ICP‐MS. The effects of LA‐MC‐ICP‐MS analytical conditions were investigated to obtain precise and accurate Fe isotope ratios. The yielded intermediate measurement precisions for the δ56Fe values in the five investigated magnetites were ± 0.05–0.06‰ (2s) using SN‐MC‐ICP‐MS and ± 0.08–0.14‰ (2s) using LA‐MC‐ICP‐MS. Magnetites with homogeneous Fe isotopic compositions in hand‐specimen measurements and microanalysis can serve as potential reference materials for in situ Fe isotopic measurement. Furthermore, the Fe isotope ratios in the magnetites from the Jinchuan Ni‐Cu‐PGE sulfide deposit were measured using LA‐MC‐ICP‐MS with natural magnetite as the bracketing calibrator. The increase in the Fe isotopic composition with magmatic sulfide evolution was primarily dominated by oxygen fugacity (fO2) and hydrothermal fluids. This finding implies that the Fe isotopic composition of magnetite can serve as a potential geochemical indicator of magmatic Ni‐Cu sulfide mineralisation. [ABSTRACT FROM AUTHOR]

The large variability in Fe isotope ratios of sedimentary rocks (particularly those from the Archean and Proterozoic) contrasts with that of igneous rocks, which display a much more limited range in values. Notably, among igneous rocks, those inferred to form via partial melting of siliciclastic sediments – strongly peraluminous granites (SPGs) – exhibit greater variability in their Fe isotope compositions, suggesting SPGs may capture isotopic variations in the sedimentary record. However, the extent and mechanisms of iron isotope fractionation between SPGs and their source remain poorly understood. Our study integrates iron isotope analyses with petrological modeling to investigate equilibrium isotopic fractionation during generation of SPG magmas. As a case study, we focus on the Neoarchean Ghost Lake Batholith and the adjacent metasedimentary rocks in Ontario, Canada. These units represent an internally differentiated SPG batholith and metamorphosed sedimentary rocks interpreted as the source of the batholith. We measured δ56Fe compositions of SPG samples, metasedimentary rocks, and a restitic rock. Sulfide grains were also measured in four metapelite samples and a granite sample. We find no correlation between the δ56Fe composition of metasedimentary rocks and their metamorphic grade, indicating iron isotopes behave as a closed system during metamorphism. Modeling results show that iron isotopes in SPGs from the Ghost Lake batholith are consistent with equilibrium fractionation during biotite dehydration melting, with predicted δ56Fe values for melts and restitic assemblages mainly controlled by the source composition. Our results predict negligible isotopic fractionation between the residue and the source, whereas ∼0.177–0.277 ‰ is expected between SPG melts and the residue, accounting for high δ56Fe values in granite samples. Lower δ56Fe values may indicate that some granites represent mixtures of melt and cumulus material or result from assimilation of restite or source/host rock. However, despite deviations from pure equilibrium fractionation, the variability in δ56Fe values for SPGs is about one order of magnitude smaller than that seen in the sedimentary record for the Archean and Proterozoic (∼0.2 ‰ vs. >2 ‰). We posit that this narrower range of isotopic variation in SPGs results from metamorphism and partial melting, which can homogenize large isotopic variations in sedimentary protoliths. Thus, SPGs represent reliable archives for the bulk iron isotope evolution of siliciclastic sedimentary rocks through time. [ABSTRACT FROM AUTHOR]

We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc basalts and their crystallization products. The Fe isotope composition of ultramafic cumulate xenoliths increases from dunite (δ56Fe = –0.09 to –0.02 ‰) to clinopyroxenite (δ56Fe = +0.06 to +0.09 ‰), consistent with higher modal proportions of clinopyroxene (δ56Fe = –0.05 to +0.11 ‰) relative to olivine (δ56Fe = –0.10 to +0.06 ‰) in the latter. Mid-crustal cumulate amphibole gabbro and hornblendite cumulates also record heavier Fe isotope compositions (δ56Fe = +0.04 to +0.08 ‰) due to the abundance of isotopically heavy amphibole (δ56Fe = +0.07 to +0.09 ‰) and magnetite (δ56Fe = +0.11 to +0.13 ‰) in these rocks. High inter-mineral fractionations observed in spinel-olivine and spinel-clinopyroxene pairs (Δ56Fe spl-ol = +0.12 to +0.28 and Δ56Fe spl-cpx = +0.06 to +0.19) suggest that spinel is not recording equilibrium crystallization conditions for the ultramafic assemblages, likely due to subsolidus Fe-Mg exchange. Our data also include Fe isotope measurements of one mantle dunite (δ56Fe = +0.03 ± 0.05 ‰). Five Adagdak lavas, spanning from basalts to andesites, yield a narrow range of δ56Fe between +0.03 and +0.06 ‰. Our results highlight the potential of amphibole in driving the Fe isotope depletion trends observed in many erupted arc lavas, as amphibole hosts 28–99 % of the FeO T budget in the amphibole gabbro and hornblendite cumulates. This is also supported by single-crystal synchrotron Mössbauer spectroscopy of two amphibole grains, the first from an amphibole gabbro and the second from a hornblendite, which yield Fe3+/ΣFe ratios of 0.55 ± 0.06 and 0.58 ± 0.02, respectively. Water content and hydrogen isotope compositions determined by secondary-ion mass spectrometry from the same amphibole grains indicate partial dehydrogenation. Using Rayleigh fractionation modeling to account for oxidation during post-crystallization dehydrogenation, we calculate magmatic Fe3+/ΣFe ratios of 0.41 ± 0.04 for the amphibole gabbro and 0.30 ± 0.05 for the hornblendite. These data are then used to estimate an appropriate Fe force constant for Adagdak amphibole and quantitatively evaluate the effects of amphibole fractionation. Through a fractional crystallization model, we show how arc melts may experience periods of increasing δ56Fe during olivine-dominated fractionation, followed by decreasing δ56Fe once magnetite and amphibole saturate as cumulate phases. Notably, this dichotomy between fractionation of isotopically light versus heavy cumulate assemblages and its effects on the Fe isotope evolution of arc magmas is not captured by the Adagdak lava record, highlighting the utility of cumulates in chronicling the early isotopic evolution of magmatic systems. [ABSTRACT FROM AUTHOR]

Modern Fe‐Si oxyhydroxide deposits occur in global marine hydrothermal vent sites. Despite their role as biogenic substrates and potential ore resources, much remains unknown about their formation processes. Here, we apply analyses of major and trace elements as well as Sr‐Nd‐Pb‐Fe isotopes combined with 238U‐230Th dating to Fe‐Si oxyhydroxides obtained from several hydrothermal fields along the Southwest Indian Ridge. These mineralized oxyhydroxides primarily consist of poorly crystalline two‐line ferrihydrite and amorphous opal‐A, with lesser amounts of nontronite and birnessite. The ubiquitous and characteristic Fe‐rich ultrastructures in the oxyhydroxides directly indicate microbial activity. The 238U‐230Th dating constrains their crystallization ages from ca. 11,873 to 384 years old. The seawater‐like 87Sr/86Sr and varying 143Nd/144Nd ratios underline a high proportion of seawater mixed with hydrothermal fluids. The radiogenic Pb isotopic patterns suggest a primary derivation of Pb leached from substrate basalts and to a lesser extent Pb from seawater. Stable iron isotopic compositions for different oxyhydroxides display a remarkable range between −1.47 and 0.82‰, which were interpreted as reflecting the fractionation processes during the formation of the deposits under evolving depositional redox conditions. The partial oxidation of Fe(II) and the subsurface removal of isotopically heavy Fe oxyhydroxides are suggested to play a vital role in shifting the Fe isotopic signature toward more negative values. Given that these Fe‐Si oxyhydroxide deposits exhibit features similar to certain ancient iron formations (IFs), Fe isotope systematics of these deposits may hold significant potential for fingerprinting the biological Fe oxidation processes that drove IF deposition on early Earth. Plain Language Summary: This study introduces a comprehensive examination of Fe‐Si oxyhydroxide deposits in marine hydrothermal vent sites along the Southwest Indian Ridge. Applying innovative techniques of major and trace element and Sr‐Nd‐Pb‐Fe isotope analysis combined with 238U‐230Th dating, the investigation uncovers new insights into the composition, age, and origin of microbial Fe‐Si oxyhydroxide ultrastructures. Notably, these deposits are found to be primarily composed of ferrihydrite and amorphous opal‐A, with ages ranging from ca. 11,873 to 384 years old. Our study provides evidence for their low‐temperature hydrothermal origin, with geochemical characteristics highlighting rare earth element patterns and radiogenic Sr‐Nd‐Pb isotopic compositions. Of particular significance is the discovery of a wide range of stable iron isotopic compositions, shedding new light on the depositional redox conditions. Our data, in concert with previous studies, reveal similarities between biogenic Fe‐Si oxyhydroxide deposits and ancient iron formations (IFs). This link indicates the potential use of Fe isotope systematics in tracing Fe oxidation processes and the biosignatures that may have shaped IF deposition on early Earth. These novel findings contribute fresh perspectives to the understanding of low‐temperature hydrothermal deposit geochemistry and the search for geological biosignatures. Key Points: We present a comprehensive Sr‐Nd‐Pb‐Fe isotope investigation of modern Fe‐Si oxyhydroxide deposits in marine hydrothermal vent sitesSr‐Nd‐Pb isotope variations in Fe‐Si oxyhydroxides were produced by varying degrees of seawater mixing with diffuse hydrothermal fluidsFe isotope systematics of Fe‐Si oxyhydroxides can provide key information for Fe oxidation processes during the formation of the deposits [ABSTRACT FROM AUTHOR]

Abstract Modern Fe‐Si oxyhydroxide deposits occur in global marine hydrothermal vent sites. Despite their role as biogenic substrates and potential ore resources, much remains unknown about their formation processes. Here, we apply analyses of major and trace elements as well as Sr‐Nd‐Pb‐Fe isotopes combined with 238U‐230Th dating to Fe‐Si oxyhydroxides obtained from several hydrothermal fields along the Southwest Indian Ridge. These mineralized oxyhydroxides primarily consist of poorly crystalline two‐line ferrihydrite and amorphous opal‐A, with lesser amounts of nontronite and birnessite. The ubiquitous and characteristic Fe‐rich ultrastructures in the oxyhydroxides directly indicate microbial activity. The 238U‐230Th dating constrains their crystallization ages from ca. 11,873 to 384 years old. The seawater‐like 87Sr/86Sr and varying 143Nd/144Nd ratios underline a high proportion of seawater mixed with hydrothermal fluids. The radiogenic Pb isotopic patterns suggest a primary derivation of Pb leached from substrate basalts and to a lesser extent Pb from seawater. Stable iron isotopic compositions for different oxyhydroxides display a remarkable range between −1.47 and 0.82‰, which were interpreted as reflecting the fractionation processes during the formation of the deposits under evolving depositional redox conditions. The partial oxidation of Fe(II) and the subsurface removal of isotopically heavy Fe oxyhydroxides are suggested to play a vital role in shifting the Fe isotopic signature toward more negative values. Given that these Fe‐Si oxyhydroxide deposits exhibit features similar to certain ancient iron formations (IFs), Fe isotope systematics of these deposits may hold significant potential for fingerprinting the biological Fe oxidation processes that drove IF deposition on early Earth.

Mantle oxidation plays an essential role in volatile cycling between Earth's interior and exterior. However, the conceivable oxidizer remains enigmatic and needs to be explored by geochemical tools. Here, we examine the potential of recycled carbonates in inducing redox changes of the deep mantle through combined isotopic studies of Mg (δ26Mg), Zn (δ66Zn), Fe (δ56Fe), and Cu (δ65Cu) on a suite of intraplate alkaline basalts proposed to have originated from the mantle transition zone (∼440–660 km). Compared with mid‐ocean ridge basalts (MORB), these basalts have variably lower δ26Mg and higher δ66Zn, the typical features of marine carbonates, indicating the presence of various amounts of deeply recycled carbonates in their sources. As the estimated amounts of recycled carbonates in sources increase (i.e., lower δ26Mg and higher δ66Zn), the basalts have higher δ56Fe and lower δ65Cu compared with MORB. These correlations cannot be explained alone by binary mixing between recycled carbonates and the pristine mantle because of the much lower Fe and Cu contents of marine carbonates than those of the mantle and instead require redox‐driven isotope fractionation induced by mantle carbonate metasomatism. During partial melting of an oxidized mantle source, isotopically heavy ferric iron and isotopically light sulfide were preferentially incorporated/dissolved into the melts that are enriched in heavy Fe and light Cu isotopes. These observations highlight that recycled carbonates serve as an important oxidizer for the deep mantle and facilitate the extraction of mantle sulfides, which can be effectively tracked by Fe and Cu isotopic systematics of intraplate basalts. Plain Language Summary: It is widely suggested that the shallow mantle can be oxidized by fluids released from subducted slabs at subarc depths, but the primary oxidizer for the deeper mantle remains enigmatic. We analyze Mg, Zn, Fe, and Cu isotopes for a suite of continental intraplate alkaline basalts proposed to be originated from the deep mantle, which have lower δ26Mg and δ65Cu but higher δ66Zn and δ56Fe values compared to mid‐ocean ridge basalts. These correlations indicate that the mantle oxidation induced by recycled carbonate facilitates the incorporation of ferric iron and dissolution of sulfide into the melts. Thus, recycled carbonates can serve as an important oxidizer for the deep mantle through the reduction of carbonate itself into diamond. The redox state of the mantle affects the efficiency of volatile extraction and provides an important link between oxygen–sulfur cycling and deep carbon–iron redox interactions. Key Points: Large Mg‐Zn‐Fe‐Cu isotopic variations are observed in a suite of Cenozoic intraplate alkaline basaltsBasalts with lower δ26Mg and higher δ66Zn have higher δ56Fe and lower δ65CuRedox‐driven Fe and Cu isotopic variations record oxidation of the deep mantle by recycled carbonate [ABSTRACT FROM AUTHOR]

Sediments are considered to be refractory materials with limited influences on dissolved iron (dFe) pool in the ocean. However, recent field observations and laboratory experiments suggest that iron released from resuspended sediment particles and transported from continental margins is prone to fertilize large areas of the world ocean. Here we conducted a dissolution experiment to quantify the amount of dFe released from two types of resuspended sediments (silicate and calcite-rich) to open ocean surface seawater under two temperatures (5 and 15 °C). We followed pH, dissolved oxygen (dO 2), phosphate, silicate, dissolved Fe and Mn concentrations (dFe and dMn), and bacterial abundance over 250 days. Extremely low and undetectable phosphate concentrations (<50 pmol kg−1) were measured throughout the duration of the experiment, causing limited bacteria growth and stable pH and dissolved O 2 concentrations under all conditions. Silicate and dFe concentrations increased through time and high temperature (15 °C) induced more iron dissolution from the two sediments than low temperature (5 °C). Temperature had no effect on the dissolution of Mn. Our results further show that Fe and Mn are not released concurrently from the sediment source and that their distribution can be very different. Scavenging of Fe likely caused a decrease of dFe observed during the experiment, which was probably linked to the formation of Mn oxides. We also observed elevated dissolved Fe isotope ratios after dissolution, around +0.16 to +0.27‰. Isotopically heavy Fe was released from sediments to the dissolved pool during the dissolution but no difference in Fe isotope ratios was observed between the two temperature conditions. The Fe isotope fractionation can likely be attributed to ligand complexation and scavenging of Fe. These two mechanisms can be important factors not only in controlling the amount of Fe released from sediments but also in fractionating Fe isotopes at the sediment–seawater boundary. [ABSTRACT FROM AUTHOR]

The early Cambrian Takab iron ore deposit is situated in the northern part of the Sanandaj-Sirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945–1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86‰) and O (+1.0 to +4.07‰) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15−200 ppm), V (≤100 ppm), Si and Mg (mostly ≤300 ppm) and variable Al. The ∂56Fe values vary from −0.2‰ to +1.12‰ but most values also support a magmatic/high-T hydrothermal origin. However, variable ∂18O (−2.52 to +1.22‰) values provide evidence of re-equilibration with lower-T fluid at ~200–300 °C. Nodular magnetite shows high Mn (≤1%), and mostly negative ∂56Fe values (average, −0.3‰) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher ∂18O (average, +6.30‰). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history. [ABSTRACT FROM AUTHOR]

Recycling of upper crustal sediments through slab subduction contributes to sub‐continental lithospheric refertilization and heterogeneity. However, the nature of recycled upper crustal components is unclear and direct evidence for sediment melt activity in the sub‐continental lithosphere is lacking. Here, we integrate major and trace elements, zircon U‐Pb dating, Sr‐Nd‐Zn‐Fe isotopic compositions of clinopyroxenites (crust‐mantle boundary) and a "glassy" xenolith from the North China Craton to relate their petrogenesis to the potential recycling of upper continental crust and provide direct insight into the sediment melt‐rock interaction. The clinopyroxenites have relatively uniform δ56Fe values (the permil deviation of the 56Fe/54Fe ratio from the IRMM014; −0.05‰–0.07‰, except for one outlier) and are not affected by melt metasomatism. The clinopyroxenites have highly variable whole‐rock δ66Zn values (the permil deviation of the 66Zn/64Zn ratio from the JMC‐Lyon standard) between 0.04‰ and 0.46‰, that closely correlate with Rb/La, K/U, Ba/Th, and Th/Nb ratios, and generate arrays that trend toward a composition similar to the "glassy" xenolith. The "glassy" xenolith has a high δ66Zn value (0.43‰ ± 0.05‰, 2SD) and a significantly low 143Nd/144Nd ratio (0.510991). This evidence implies that the "glassy" xenolith may represent a quenched sediment melt formed by the melting of carbonate‐bearing terrigenous sediments that may also be responsible for the metasomatism of clinopyroxenite xenoliths. The geochemical evidence from the "glassy" and clinopyroxenite xenoliths provides a direct evidence for the activity of sediment melt with upper continental crust components in the sub‐continental lithosphere. Plain Language Summary: Slab‐derived sediment melt activity in the overlying lithosphere is crucial for element cycling and lithospheric evolution. To understand the nature of recycled sediment and its impact on the overlying lithosphere, we analyzed the Zn‐Fe‐Sr‐Nd isotopic compositions of clinopyroxenites and a "glassy" xenolith from the North China Craton. The δ66Zn values of clinopyroxenites show correlations with proxies for hydrous melt metasomatism (Rb/La, K/U, Ba/Th, and Th/Nb ratios) and generate arrays that trend toward a composition similar to the "glassy" xenolith. The identification of "glassy" xenolith provides direct insight into the sediment melt‐rock interaction. Furthermore, this study proves the validity of using Zn isotopes as a powerful tool for tracing sediment melt activity in the sub‐continental lithosphere. Key Points: The Zn isotopic variations (0.04‰–0.46‰) of the clinopyroxenites are produced by melt metasomatismThe "glassy" xenolith may represent the preserved sediment melt with recycled carbonate‐bearing terrigenous componentsZn isotopes can be used to trace the activity of slab‐derived sediment melts in the sub‐continental lithosphere [ABSTRACT FROM AUTHOR]

Arc basalts typically have higher Fe3+/ΣFe ratios but lighter Fe isotopic compositions than mid-ocean ridge basalts (MORB); however, the reasons for these differences are unclear. Here, we report new Fe isotope data for fresh lavas from the Mariana subduction zone, along with available Fe-B-Sr-Nd isotope and B/Nb ratio data for global arc basalts, to constrain the role of serpentinite dehydration in sub-arc mantle oxidation. The basaltic lavas from the Mariana Arc, like other arc basalts from cold subduction zones in the west Pacific, have lighter Fe isotope compositions (δ56Fe = 0.02 ± 0.02‰; 1SD; n = 9) than the MORB-like δ56Fe values of the middle Mariana Trough basalts (δ56Fe = 0.09 ± 0.02‰; 1SD; n = 8). Notably, the significant negative correlations between the δ56Fe values and the proxy for the addition of serpentinite-derived fluids (i.e., B isotopes and B/Nb ratios) indicate that the serpentinite-derived fluid contribution is a first-order control on the across-arc Fe isotope variations. Dehydration of subducted slab serpentinites or dragged-down forearc serpentinites at sub-arc depths can release isotopically light Fe fluids in the form of Fe2+–SO x complexes. Sulfate-rich fluids containing adequate Fe to metasomatize the sub-arc mantle can cause light Fe isotope compositions and high Fe3+/ΣFe ratios in arc basalts. Alternatively, the low δ56Fe values and high Fe3+/ΣFe ratios of arc magmas may be generated by direct melting of the mélange, which includes dragged-down sulfate-rich forearc serpentinites. Comparatively, serpentinite-derived components are absent at back-arc depths because of the earlier breakdown of serpentine minerals. Accordingly, the back-arc basin basalts exhibit MORB-like δ56Fe-δ11B values and B/Nb–Fe3+/ΣFe ratios. Therefore, the across-arc Fe isotope variations and the decoupling of Fe3+/ΣFe and δ56Fe in global arc magmas can be explained by the different contributions of subducted serpentinites. [ABSTRACT FROM AUTHOR]

• Link between δ56Fe and redox variations in eclogitic meta-ophiolites. • Low Δ56Fe Mgt-Ol in meta-serpentinites testify of HP-HT equilibration. • Correlations between δ56Fe and Fe3+/∑Fe controlled by external fluids. • Samples equilibrated at high f O 2 displaying the most fractionated Fe isotope values. The isotopically light Fe signature of arc magmas relative to mid-oceanic ridge basalts (MORB) is expected to be related to the redox state of their source. This implies a link between Fe mobility and redox variations across subduction zones. Here we address slab chemical variations during prograde metamorphism through a comprehensive geochemical study, including major, trace elements and Fe stable isotope (δ56Fe) analyses, of meta-serpentinites composing the eclogitic meta-ophiolite of the Monviso massif (Western Alps, Italy). These rocks partly preserve mantle related geochemical heterogeneities, as supported by negative correlations between fluid immobile elements (High Field Strong Elements) and LREE/HREE (Light – Heavy Rare Earth Elements) ratios. They are however characterized by large Cs enrichments relative to U or alkali elements suggesting that abyssal fluid mobile elements signature was overprinted by subduction related processes. Meta-serpentinites display large δ56Fe variations, from -0.20 ± 0.05 ‰ to +0.09 ± 0.03 ‰. Mineral separate analyses of olivine bearing veins show low Δ56Fe between olivine and magnetite (∼ 0.4 ‰), which according to Fe isotope thermometry correspond to high temperature equilibration, estimated between 530 and 550 °C, compatible with Monviso metamorphic climax (520–570 °C and 2.6–2.7 GPa). These results, in combination with geochemical modelling, confirm a reset of δ56Fe signature of abyssal serpentinites during prograde metamorphism in subduction zones. The bulk rock δ56Fe values of Monviso meta-serpentinites are negatively correlated with their Fe3+/∑Fe and As concentrations (a redox-sensitive and fluid mobile element). These correlations can be attributed to a metasomatism by external fluids derived from slab serpentinites during subduction. It also concords with the oxygen fugacity (f O 2) record in meta-serpentinites, with meta-serpentinites equilibrated at high f O 2 displaying the lowest δ56Fe values while samples equilibrated at low f O 2 display mantle like δ56Fe signatures. These results highlight that oxidising conditions during serpentinite dehydration are likely to boost isotopically light Fe mobility in slab derived fluids. [ABSTRACT FROM AUTHOR]

Primitive ocean island basalts (OIB) display a large variability in stable iron (Fe) isotopes, beyond what can plausibly be explained by partial melting and crystal fractionation processes. This Fe isotopic heterogeneity is widely ascribed to inheritance from various exotic mantle components of enriched crustal origin, i.e., subducted and resurfacing oceanic or continental crust, its underlying lithosphere or overlying sediments. These enriched mantle components are characterised by variations in radiogenic Sr-Nd-Pb-Hf isotopes. The extent to which the inherited Fe isotopic signature of subducted and recycled material influences the Fe isotopic composition of OIB and which role secondary processes accompanied with partial melting play is, however, not well understood. The eastern Azores island of São Miguel displays a systematic change from a mantle source less radiogenic in Sr-Nd-Hf-Pb isotopes in the West to a highly enriched source in the East. These variations are among the largest reported in an OIB, indicating variable source component additions to the melt that offer a natural laboratory to elucidate sub-oceanic island processes. Among these, the eastern São Miguel component is unique amongst global OIB in that it has both, extremely radiogenic 206Pb/204Pb and 87Sr/86Sr isotopic ratios. The spatial distribution, complexity and uniqueness of these isotopic source characteristics are ideally suited to gain insights into the spatial distribution of mantle components and their potential controls on Fe isotope systematics. Comparing δ57Fe prim , which is the isotopic composition calculated to primitive lavas along a liquid line of descent, with 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb reveals that the depleted component of western São Miguel shows a heavy Fe isotopic composition ranging from δ57Fe prim = +0.09 to +0.18‰ (excluding one outlier at −0.02‰ δ57Fe prim), whereas the enriched component of eastern São Miguel shows lighter δ57Fe prim = +0.05 to +0.12‰. Both suites show a continuum of isotopic compositions between two apparent endmembers. The light Fe isotopic signature of eastern São Miguel lavas, coupled with elevated Rb/Sr and K 2 O, indicate K-rich mantle metasomatism in their source. The Fe isotopic composition of western São Miguel's lavas are likely related to a similar source, but have been overprinted by the active Terceira Rift. The latter adds low-degree mid-ocean ridge basalt (MORB)-type melts to the original OIB, likely in the form of a re-fertilised depleted mantle component that underpins the ultraslow spreading region. The two-component mix that contributes to western São Miguel lavas highlights that both enriched mantle components and shallow crustal features can add to the Fe isotopic signatures of erupting OIB, with spatial separation within single volcanic islands. This suggests that a careful evaluation of local geologic features, such as rifting, is required when OIB are probed for their mantle sources, and that heavy Fe isotopic signatures in OIB may not reflect deep, plume-source lithologies. In summary, São Miguel lavas are sourced by a mantle plume, but within a spatially controlled region accompanied by a plate-controlled melt associated with a rifting zone. This dichotomy adds another complication to the genesis of OIB, which however maybe elucidated through combined radiogenic-stable isotope systematics. [ABSTRACT FROM AUTHOR]

Separation of Fe–Ti oxides during magmatic differentiation plays an important role in controlling Fe isotopic evolution of residual magmas. Titanomagnetite (Tmag) is a common Fe–Ti oxide phenocryst phase in evolved basaltic lavas, but the effect of its separation on Fe isotopic evolution of residual melts remains poorly understood. Here we explore this issue with an Fe isotopic study on a suite of cogenetic alkaline volcanic rocks (range from picrobasalt to trachyandesite) and their titanomagnetite phenocrysts from St. Helena Island (South Atlantic). Results show that whole-rock δ57Fe values vary from 0.04 to 0.32‰ with decreasing MgO, and ulvöspinel-rich (X′usp ≈ 0.6) titanomagnetite phenocrysts have consistently lower δ57Fe than corresponding bulk samples with an average Δ57FeTmag−whole rock (δ57FeTmag − δ57Fewhole rock) value of − 0.05 ± 0.02‰ (2SD, N = 6). This value together with the speculated crystallization temperatures (~ 1100 ± 50 °C) of these titanomagnetite phenocrysts determine an equilibrium titanomagnetite-melt fractionation factor of Δ57FeTmag-melt = (− 0.094 ± 0.038) × 106/T2. Quantitative calculations involving this isotopic fractionation factor and previously suggested olivine-melt and clinopyroxene-melt isotopic fractionation factors can well reproduce the Fe isotopic evolution of St. Helena lavas. Specifically, the Fe isotopic variation before titanomagnetite saturation (MgO > 5 wt%) is dominated by fractional crystallization and accumulation of olivine and clinopyroxene, while that after titanomagnetite saturation is determined by fractional crystallization of multiphases including titanomagnetite, olivine and clinopyroxene. This study, combined with published mineral-melt fractionation factors for other Fe–Ti oxides, indicates that the removal of ulvöspinel-rich (X′usp > 0.5) titanomagnetite and near-pure ilmenite results in an increase of δ57Fe in evolved magmas, whereas separation of near-pure magnetite drives residual melt towards lighter Fe isotopic compositions. [ABSTRACT FROM AUTHOR]

Dissolved iron (Fe) isotopes in river water have a pivotal role in understanding the Fe cycle in the surficial environment. A total of 13 samples of river water were collected from the Mun River to analyze the Fe isotopes and their controlling factors in river water, such as dissolved organic carbon (DOC) and different supply sources. The results showed that dissolved Fe (DFe) concentrations ranged from 21.49 μg/L to 232.34 μg/L in the dry season and ranged from 10.48 μg/L to 135.27 μg/L in the wet season, which might be ascribed to the dilution effect. The δ56Fe of the dry season (−0.34 to 0.57‰, with an average 0.09‰) was lower than that of the wet season (−0.15 to 0.48‰, with an average 0.14‰). Combined with the δ56Fe and DFe/DAl ratios, the end-members of DFe were identified, including rock weathering (high δ56Fe and low DFe/DAl ratio), anthropogenic inputs (high δ56Fe and high DFe/DAl ratio) and groundwater inputs (low δ56Fe and low DFe/DAl ratio). The relationship between δ56Fe and DOC concentrations suggested that the chelation of organic matter with heavy Fe isotopes was one of the important sources of heavy Fe isotopes in river water. [ABSTRACT FROM AUTHOR]

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+/∑Fe melt (0.08 to 0.39) and Fe/Mn (> 65) relative to primary MORB (δ57Fe ∼0.10 ‰; Fe3+/∑Fe melt ∼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+/∑Fe source) 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. • Pyroxenite sources are a common origin of heavy Fe isotopes in intraplate basalts. • Basalts with higher δ57Fe have higher δ66Zn at similar proportions of pyroxenite melt. • Heavy Fe isotopes in intraplate basalts can record carbonate-induced mantle oxidation. [ABSTRACT FROM AUTHOR]

• The 1.89 Ga Frere granular Fe formation was deposited on a redox stratified shelf. • Trend in whole-rock and magnetite δ 56Fe data tracks changes in sea level. • The δ 56Fe of iron formations are a robust tracer for Paleoproterozoic ocean circulation. • (Ce/Ce*) PAAS holds a primary seawater signature in spite of terrestrial input. • Primary δ 18O values of minerals were overprinted by diagenesis. The Paleoproterozoic (ca.1.89 Ga) Frere Formation is a ca. 300-m-thick sedimentary succession of five stacked, Fe- and silicate-rich shallowing upwards cycles or parasequences that were deposited during marine transgression. Deposition in an oxygen-stratified water column is supported by the occurrence of magnetite-rich mudstones (anoxic and distal) beneath shallower, hematite- and chert-rich grainstones (suboxic and proximal). Minor (LREE/HREE) PAAS depletion and positive Ce anomalies in grainstones support this interpretation. Sequence stratigraphic relationships suggest that the Frere Formation accumulated on an unrimmed continental shelf associated with coastal upwelling of Fe. Large variations in oxygen isotope ratios (δ 18O) of magnetite (–0.6 ‰ ≤ δ 18O ≤ +2.8 ‰, n = 12) and quartz (+8.8 ‰ ≤ δ 18O ≤ +20.1 ‰, n = 11) from lithofacies composing the Frere Formation indicate that pristine δ 18O of minerals were overprinted by diagenesis. Highly variable δ 18O values calculated at 200 °C in equilibrium with quartz (–3.0 to + 8.3 ‰) and magnetite (+8.0 to + 12.7 ‰) suggest that fluids with heterogeneous δ 18O evolved through oxygen isotope exchange between minerals and porewater in accumulating sediment during diagenesis. The Fe isotope ratios (δ 56Fe) of bulk rock (–0.50 ‰ ≤ δ 56Fe ≤ +1.55 ‰, n = 20) and magnetite separates (–0.09 ‰ ≤ δ 56Fe ≤ +2.06 ‰, n = 11) from deeper anoxic and shallow suboxic paleoenvironments define a decreasing trend through stratigraphically stacked parasequences. The high δ 56Fe values of magnetite (+0.7 ‰ to + 2.06 ‰) characterizing the base of the Frere Formation is interpreted to reflect equilibrium isotopic fractionation between dissolved Fe2+ aq and freshly precipitated Fe-(oxyhydr)oxide (Fe(OH) 3) resulting from sluggish recharge of Fe2+ aq from deeper shelf environments. These high δ 56Fe values of magnetite from shallow paleoenvironments may also reflect microbial dissimilatory Fe reduction at the seafloor. As sea level rose, continued deepening and encroachment of a well-mixed water mass increased the oxidation of Fe2+ aq , which ultimately increased the influence of kinetic isotopic fractionation between Fe2+ aq and Fe(OH) 3. Such a process produced δ 56Fe compositions of magnetite that gradually decrease through the stratigraphic thickness of the Frere Formation (+2‰ to −0.1 ‰). Thus, Fe isotopic data framed in a sequence stratigraphic context provides greater insight into water column processes on Fe-rich, oxygen stratified Paleoproterozoic and Mesoproterozoic shelves, as well as geologically younger restricted basins with a redox gradient. [ABSTRACT FROM AUTHOR]

[Display omitted] • Variable Fe isotopic composition in metallic minerals controlled by multiple factors. • Magmas may be the main source providing iron to form the Beiya deposit. • Post-magmatism hydrothermal activities play a major role on leaching the metals out and transporting them to the mineralization area. The Beiya Au-base metal deposit in southwest China is characterised by a huge amount of iron associated with gold mineralization. The formation of the Beiya deposit is generally thought to be related to Tertiary potassic intrusion, as porphyry- and skarn-type mineralization are the two most important types of mineralization in the region. However, the lack of direct evidence to constrain the source of iron and gold as well as other metals makes it difficult to accurately conceptualise the ore genesis model. In this study, we report high-precision (0.03 ‰; 2sd) Fe isotope data on Fe-bearing minerals including magnetite, pyrite and chalcopyrite, and major igneous rocks exposed in the area, including monzogranite porphyry (MGP), mafic microgranular enclave (MME), lamprophyre and basalt. Magnetites have a wide range of δ Fe from 0.15 to 0.64 ‰, reflecting substantial isotopic fractionation at different mineralization stages. MMEs have heaviest Fe isotopic composition (0.35 ‰ to 0.88 ‰) compared to MGPs (0.19 ‰ to 0.48 ‰), lamprophyre (∼0.2 ‰) and basalt (∼0.0 ‰). A negative correlation between δ56Fe and TFe 2 O 3 is found for both MPGs and MMEs, which indicates the leaching of iron by hydrothermal fluids mobilizes light Fe and leaves behind isotopically heavy Fe in the remaining source rock. Fe isotopic compositions of MPG and MME overlap with those of magnetite, both of which are proposed to be the source of Fe in magnetite. The basalt has δ56Fe value of 0 ± 0.03 ‰, similar to those of global basalts, suggesting a negligible contribution to the formation of iron ores. Therefore, we propose that magmas parental to the MGP and MME may be the main source of iron to form the Beiya deposit; post-magmatism hydrothermal activities play a key role in leaching the metals out and transporting them to the mineralization area. [ABSTRACT FROM AUTHOR]

[Display omitted] • REY provenance and evolution of REY seawater chemistry at ∼2745 Ma ago. • Evidence for a non-steady state of the iron cycle during the late Neoarchean. • A depositional model has been proposed for the world-famous Carajás BIFs. • Emerging oxidizing conditions could have influenced the Carajás Basin before the GOE. One of the most important occurrences of banded iron formation (BIF) worldwide is situated in the Carajás Mineral Province, southeastern Amazonian Craton. The BIFs are jaspilites and are hosted in the Neoarchean (∼2.74 Ga) volcano-sedimentary sequence of the Grão-Pará Group. They are mostly composed of cm-thick intercalations of hematite, jasper, and chert. Their primary textures and structures are still preserved, which make them an ideal archive to evaluate the paleomarine environment. Low abundance of Al 2 O 3 (<1.0 wt%) and HFSE (<1 ppm) for most BIF samples indicate an essentially detritus-free depositional environment. Overall, the rare earth elements and yttrium (REY) patterns show a weak positive lanthanum (La) anomaly, and a pronounced positive europium (Eu) anomaly (Eu/Eu PAAS = 1.86 – 5.05), although the presence of true cerium (Ce) anomaly is not evident. Stratigraphic variations in iron isotope compositions, up to 0.80‰ (δ56Fe = +1.10 to +1.90‰) over tens to hundreds of meters of stratigraphic section, point to relative changes in the iron isotope composition of Carajás seawater over periods of a few million years. The jaspilites show heterogeneous distribution of Nd isotopic signature throughout the BIF sequence, and rocks from near the basaltic flows/jaspilite contact (type-II) have negative εNd (t) values (–4.97 to –0.90). In contrast, predominantly positive εNd(t) values (–0.84 to +5.40) are common in the remaining samples (type-I). The strongly positive δ56Fe values indicate a low degree of partial oxidation of Fe(II), which, combined with the pronounced positive Eu anomalies and the absence of Ce anomalies, hint towards that the deposition occurred mainly on a deep-sea environment with intense hydrothermal activity under anoxic and suboxic conditions, distal to continental landmasses. Locally, considerable oxygen was probably present in the ancient ocean's water masses, which led to the precipitation of BIFs. [ABSTRACT FROM AUTHOR]

Fe isotopes were used to determine the origin of iron bars from Les Saintes‐Maries‐de‐la‐Mer Roman shipwrecks, which was a major archaeological finding at the end of the 20th century in France. Their Fe isotope composition was measured by multi collector‐inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) after chemical Fe purification. The results allowed us to suggest provenances that were compared with those based on trace element analyses of slag inclusions. For most of the bars, we validate the provenance hypotheses previously proposed. Two bar groups originate from the Montagne Noire metallurgical district (south‐west France), whereas a third group comes from another source, not clearly identified so far. In this context of Roman iron production, we argue for a non‐spatially segmented production, where bars were manufactured close to smelting sites. Combined trace element and Fe isotope analyses on the same objects provide crucial information about the nature of their ore source. The elemental heterogeneity, positive Eu anomaly and Fe isotopic homogeneity of several bars were inherited from gossan‐type ores, whereas the negative Eu anomaly and variable Fe isotopes signature of others most likely correspond to sedimentary iron ores. This study demonstrates that combined trace element and Fe isotopes analyses in a well‐defined archaeological context is a promising approach for provenance studies of iron metals of archaeological interest. [ABSTRACT FROM AUTHOR]

Using differential scanning calorimetry, we have examined the effect of isotopic composition on the temperature and kinetics of the α → γ phase transition of isotopically natural (natFe), 56Fe-enriched, and 57Fe-enriched Fe samples and assessed kinetic phase transition parameters as functions of isotopic disorder parameter. A time–temperature–transition diagram is presented for the fraction of the γ-phase resulting from the α → γ phase transition. [ABSTRACT FROM AUTHOR]

The Ediacaran age (ca. 570 Ma) Shuram excursion, a ca. $12‰$ depletion in $\delta ^{13}\mathrm{C}_{\mathrm{carb}}$, may record a dramatic oxidation of marine sediments associated with a reorganization of Earth’s carbon cycle closely preceding the rise of large metazoans. However, several geochemical indicators suggest it may instead record secondary processes affecting the sediments such as post-depositional alteration. The stable isotopic composition of iron incorporated within carbonates $(\delta ^{56}\mathrm{Fe}_{\mathrm{carb}})$ reveals an anomalous $^{56}\mathrm{Fe}$-depletion (down to $-1.05‰$) in strata containing the Shuram excursion, while the underlying and overlying strata have crustal $\delta ^{56}\mathrm{Fe}_{\mathrm{carb}}$ values. These depleted $\delta ^{56}\mathrm{Fe}_{\mathrm{carb}}$ data during the Shuram excursion reflect incomplete reduction of iron oxides, limited by low ambient organic carbon contents. This elevated pulse of sedimentary iron oxides would consume the majority of the limited pool of organic carbon and therefore would give rise to very low net organic carbon burial during a time of enhanced detrital delivery of oxidized iron to the sediments. These results imply a syndepositional origin for the Shuram excursion, which represents a shift in the redox composition of Earth’s sedimentary shell toward more oxidizing conditions, perhaps removing a long-lived buffer on atmospheric oxygen.

Subducted oceanic crust can transform into eclogite in the upper mantle, and generate chemical heterogeneity of mantle plumes, as recorded by elemental and radiogenic isotopic variations in oceanic island basalts (OIBs). The secondary pyroxenite produced by the reaction between eclogite-derived melt and peridotite is increasingly regarded as a major source component of OIBs, as well as peridotite. However, it remains unclear whether eclogite can be a direct source component of OIBs. To test this possibility, we present high-precision whole-rock Fe isotopes and the chemical compositions of olivine phenocrysts from well-characterized EM1-type basalts from Pitcairn Island. The Pitcairn basalts are characterized by moderate 87Sr/86Sr, low 143Nd/144Nd and 206Pb/204Pb isotopic ratios, and lowest δ26Mg values among OIBs, suggesting a contribution from recycled ancient crustal components (oceanic crust plus sediment). For comparison, we also report the Fe isotope compositions of FOZO-type basalts (with low 87Sr/86Sr, moderately high 143Nd/144Nd and moderate 206Pb/204Pb ratios) from the Louisville hot spot track, which were suggested to be a typical peridotite-derived OIB. The Louisville basalts have MORB-like δ57Fe values (0.06‰–0.15‰), whereas the Pitcairn lavas have substantially heavier Fe isotopic compositions (δ57Fe = 0.17‰–0.31‰) than MORBs. Quantitative evaluations suggest that magmatic differentiation, partial melting, and elevated oxygen fugacity in the source are insufficient to generate the heavy Fe isotopic compositions of the Pitcairn basalts. A good correlation between δ57Fe and ε Nd(i) (or δ26Mg) values in the Pitcairn basalts indicates binary mixing, and the melts derived from the EM1 endmember have unusually heavy Fe and light Mg isotopic compositions. In order to explain the origin of the Fe and Mg isotopic compositions, we calculated the Fe, Mg, and Nd isotopic compositions of partial melts of eclogite, secondary pyroxenite and peridotite, respectively. The results indicate that eclogite is the only suitable candidate to generate melts with both heavy Fe and light Mg isotopes. This inference is strengthened by the major and minor elemental compositions of olivine phenocrysts from the Pitcairn basalts, which show low Fo (73.2–82.5) and Ni contents (620–1949 ppm), and low Mn/Fe (0.011–0.015) and Ni/(Mg/Fe) (524–1041) ratios. These characteristics are markedly different from those of olivines from MORBs and the Koolau lavas from Hawaii, which represent phenocrysts that equilibrated with peridotite-derived and secondary pyroxenite-derived melts, respectively. We therefore argue that eclogite is the source lithology of the EM1 endmember of the Pitcairn basalts. Binary mixing between our modelled eclogite- and peridotite-derived melts produced magmas with relatively low Mg# value, Mg/Fe ratios and moderate Ni contents characteristics, which are preserved in the low-Fo Pitcairn olivines. Our results highlight that, in addition to peridotite and secondary pyroxenite, eclogite may survive in mantle plumes at shallow depths and make a substantial contribution to the source of OIBs. [ABSTRACT FROM AUTHOR]

The alkalinity of ocean island basalts (OIB), which form by upwelling thermo-chemical instabilities in the mantle, is often associated with the degree of melting. Yet it remains to be tested if alkalinity and the degree of melting are systematically associated with enriched mantle components. The Tasmantid Seamounts, which are fossil remnants of the Tasmantid mantle plume, comprise a north-south age-progressive submarine volcanic chain in the Coral and Tasman Seas East of Australia. Dredged seafloor lavas from nine seamounts along the seamount chain, ranging in age from 50 to 6.5 Ma, show a dichotomy in alkalinity, similar to those observed in other hotspot areas such as Hawai'i or Pitcairn. In radiogenic Sr-Nd-Hf-Pb isotope systematics, Tasmantid Seamount lavas form a continuum between EM1 and PREMA and are thus part of the prevalent EM1-PREMA association in the Pacific. However, their radiogenic Pb isotope systematics mark the Tasmantid plume as an individuum with no resemblance to any other plume. In stable Fe isotopes (expressed as δ57Fe Prim , which is the Fe isotopic composition calculated to primitive lavas along a liquid line of descent) alkali basalts are, on average, isotopically heavier than tholeiitic lavas, ranging in δ57Fe Prim from +0.10 to +0.21‰, unrelated to radiogenic isotope systematics, but with co-variations of δ57Fe Prim and Ti* (which is the primitive melt Ti content). These systematics point towards residual garnet as a key factor and thus indicate a solely petrologic relation between degrees of melting and Fe isotopes. Tholeiitic lavas exhibit a near-depleted mantle like isotopic composition (ranging in δ57Fe Prim from −0.01 to +0.22‰, median at δ57Fe Prim + 0.04‰), yet with values of up to δ57Fe Prim + 0.22‰ that require an isotopically heavy source of unknown origin. The lack of systematics between alkalinity and radiogenic isotope signatures indicates that temperature is the likely driving force for variable melting degrees in some samples over others. Based on these observations, we surmise that tholeiitic lavas form in the plume centre whereas alkaline lavas form at the cooler rim of the conduit. Mixing between melts and associated enriched components in the Tasmantid mantle plume may occur but, at least for alkaline lavas, only on a small scale. [ABSTRACT FROM AUTHOR]

Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ56Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ56Fe values. In Statia, δ56Fe decreases with major and trace element indicators of differentiation (SiO2, Na2O + K2O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ56Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ56Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.

• Coesite firstly found in the granitic gneiss of the Dora Maira Massif provides direct evidence for the UHP metamorphism of country rocks. • Jadeite quartzite has a similar protolith of Permain granite to its host whiteschist. • The distinct Fe-O isotopes of jadeite quartzite are the result of complex fluid-rock interaction in the continental subduction zone. • Fe-O isotopes are powerful tracers for the fluid source and composition in the subduction zone. Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge. [ABSTRACT FROM AUTHOR]

The linear thermal expansion coefficients (LTECs) of isotopically natural Fe (natFe) and samples isotopically enriched in 56Fe (99.945 wt %) and 57Fe (90.954 wt %) have been determined as functions of temperature by X-ray diffraction dilatometry in the temperature range 173–433 K. The low-temperature LTEC of 57Fe has been shown to be larger. As the temperature is raised to the range 368–433 K, the difference between the LTECs of the samples differing in isotopic composition decreases. [ABSTRACT FROM AUTHOR]

Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74‐Ga‐old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of 13C‐depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin. [ABSTRACT FROM AUTHOR]

• Ultrapotassic lavas show lower δ41K and higher δ56Fe values than calc-alkaline lavas. • The low-δ41K values reflect the incorporation of deeply subducted sediments. • The high-δ56Fe signatures imply pyroxenite sources. • The interaction between sediment-derived melts and peridotites forms the pyroxenites. Ultrapotassic volcanism commonly occurs following calc-alkaline magmatism in continental collisional tectonic zones, but some key aspects of this transition remain ambiguous. In particular, there are uncertainties regarding the changing nature of the mantle source and the contribution of recycled continental material during the petrogenesis of ultrapotassic igneous rocks. Here, we show that the calc-alkaline to ultrapotassic transition in magmatism in Western Anatolia (Türkiye) during the past 55 Ma was associated with conspicuous shifts in the K and Fe isotope compositions of the bulk eruptive rocks. The ∼52–17 Ma transitional (tholeiitic to calc-alkaline) and calc-alkaline rocks show low δ56Fe (0.05 ± 0.07 ‰, 1SD) and high δ41K values (from –0.41 ‰ to 0.00 ‰), relative to the overall higher δ56Fe (0.16 ± 0.05 ‰, 1SD) and lower δ41K values (from –0.32 ‰ to –0.55 ‰) of the ∼19–15 Ma shoshonitic-ultrapotassic rocks in this area. The low δ41K values observed in the shoshonitic-ultrapotassic rocks are interpreted to reflect incorporation of potassium from deeply subducted recycled continental crust and the heavy Fe isotope signatures of these rocks imply a pyroxenite source. The rapid onset and short duration of eruption of the shoshonitic-ultrapotassic rocks is consistent with formation of phlogopite-pyroxenite veins within the peridotite mantle during slab rollback and/or breakoff. These veins were then rapidly and completely consumed by upwelling hot asthenospheric mantle. [ABSTRACT FROM AUTHOR]

The Zijinshan district, Fujian Province, southeastern China, is a globally renowned Cu Au orefield and hosts a wide variety of ore deposits within the Zijinshan granite complex and surrounding volcano-sedimentary rocks. We performed Fe S isotope analysis on pyrite separates from the Zijinshan high-sulfidation epithermal deposit, Yueyang intermediate-sulfidation epithermal deposit, Wuziqilong transition-type (from high-sulfidation epithermal to porphyry) deposit, and Jintonghu porphyry Cu Mo deposit, aiming to understand the formation process of Fe-sulfides and the evolution of ore-forming fluids in different deposits. Pyrites from the Jintonghu porphyry Cu Mo deposit show variations in δ56Fe and δ34S from −0.15 ‰ to 0. 45 ‰, and from 1.81 ‰ to 2.70 ‰, respectively. In contrast, pyrites from epithermal-type deposits (i.e., Zijinshan, Yueyang, and Wuziqilong) show a negatively shifted Fe isotopic composition from −1.74 ‰ to 0.45 ‰. The combination of Fe isotopic data from the Jintonghu porphyry Cu Mo deposit with published minerals-fluid fractionation factors and the use of a Rayleigh fractionation model allowed us to determine the δ56Fe range of regional ore-forming fluids (−1.00 ‰ to −0.40 ‰), which is consistent with the previous "light fluid" hypothesis, suggesting that pyrite of porphyry Cu Mo deposits has isotopic compositions reflecting the isotopic value of the hydrothermal fluids. For the epithermal deposits, pyrite Fe isotopic composition is the result of a complex interaction of Rayleigh fractionation, rapid precipitation and kinetic fractionation, and δ56Fe values of the initial fluids is difficult to estimate. Integrating the Fe and S isotope data for pyrite of different genesis, we found that porphyry Cu Mo deposits (δ56Fe: −0.9 ‰ to 0.46 ‰; δ34S: −4.06 ‰ to 3.9 ‰) can be distinguished from a non-magmatic/low-temperature region, while the transitional area of which may be a porphyry-related hydrothermal deposit (i.e., epithermal deposit). In general, we suggest that the Fe and S isotopic signatures of pyrite can effectively distinguish porphyry-type deposits from their associated hydrothermal deposits. [ABSTRACT FROM AUTHOR]

Dissolved iron (Fe) isotopes in river water have a pivotal role in understanding the Fe cycle in the surficial environment. A total of 13 samples of river water were collected from the Mun River to analyze the Fe isotopes and their controlling factors in river water, such as dissolved organic carbon (DOC) and different supply sources. The results showed that dissolved Fe (DFe) concentrations ranged from 21.49 μg/L to 232.34 μg/L in the dry season and ranged from 10.48 μg/L to 135.27 μg/L in the wet season, which might be ascribed to the dilution effect. The δ56Fe of the dry season (−0.34 to 0.57‰, with an average 0.09‰) was lower than that of the wet season (−0.15 to 0.48‰, with an average 0.14‰). Combined with the δ56Fe and DFe/DAl ratios, the end-members of DFe were identified, including rock weathering (high δ56Fe and low DFe/DAl ratio), anthropogenic inputs (high δ56Fe and high DFe/DAl ratio) and groundwater inputs (low δ56Fe and low DFe/DAl ratio). The relationship between δ56Fe and DOC concentrations suggested that the chelation of organic matter with heavy Fe isotopes was one of the important sources of heavy Fe isotopes in river water.

There is growing evidence that the earliest Paleozoic ocean experienced fluctuations in ocean redox chemistry, with anoxic ferruginous conditions being a prevalent feature at certain times. However, the general absence of early Paleozoic banded iron formations (BIFs) means that direct supporting evidence for ferruginous conditions inferred from geochemical proxy data is currently limited. Here, we describe an early-Paleozoic BIF (Tiande No.2) from the late Silurian Habahe Group in Altay, NW China. The BIF is hosted in meta-volcano-sedimentary rocks, and is characterized by alternating iron-rich and silica-rich laminae. In addition to abundant magnetite and quartz, Fe-silicates (garnet and amphibole) are dominant minerals in the BIF. Like most earlier Precambrian examples, the BIF horizons are dominated by high Si and Fe contents, but also contain significant Al, Ti and rare earth element concentrations, consistent with a dominant chemical origin, but with significant terrigenous contribution. We observe slightly positive Eu anomalies (1.17 to 1.57) and low ε Nd (t) values (−4.2 to −0.8) in BIF samples, indicating a mixed seawater and submarine hydrothermal source for the iron. The BIF also generally displays negative Ce anomalies, and homogeneous and slightly negative δ56Fe values. This suggests near-quantitative oxidation of dissolved Fe(II) from the water column, likely following upwelling into oxygenated surface waters as sea level decreased, in a region where the terrigenous input was low relative to the dissolved Fe2+ flux. The Tiande No.2 BIF is conformably bedded within schist, which also has high concentrations of Fe (up to 12.67 wt%), generally elevated Fe/Al ratios (>1.0), and no Ce anomaly, while magnetite in the schist has heavy and variable δ56Fe values. This suggests that the schist precursor, which likely comprised a significant contribution from deposited marine sediment, was also deposited from ferruginous waters, but with only partial oxygenation of dissolved Fe2+ in a dominantly anoxic water column. The occurrence of this late Silurian BIF and associated ferruginous rocks provides new constraints on the nature of ocean redox chemistry at this time, and supports continued dynamic Fe cycling and at least intermittent ferruginous deeper water conditions in this location during the late Silurian. [Display omitted] • Early-Paleozoic BIF has been identified in Altay, NW China • Tiande No.2 BIF was derived from mixed seawater and submarine hydrothermal fluids • A new depositional model has been proposed for early-Paleozoic BIFs • Dynamic Fe cycling and at least intermittent ferruginous deeper water conditions during the late Silurian [ABSTRACT FROM AUTHOR]

Iron isotope fractionation in hydrothermal systems is a useful diagnostic tool for tracing ore-forming processes. Here, we report on the Fe isotopic compositions of a suite of hydrothermal minerals from ores (pyrite, pyrrhotite, and quartz) from the Wulong gold deposit, Liaodong Peninsula, East China. Pyrites from quartz sulfide ores show a δ56Fe (56Fe/54Fe in the sample relative to IRMM-14) range from +0.11 ± 0.03‰ to +0.78 ± 0.03‰ (2SD), and pyrrhotites from the same vein are isotopically lighter than the pyrites, varying between –0.85 ± 0.01‰ and –0.07 ± 0.00‰. This result is consistent with theoretical predictions of equilibrium fractionation and published mineral compositions. For the first time, to our knowledge, we report the Fe isotopes of hydrothermal quartz that records the isotopic compositions of the ore-forming fluids. Two quartz separates in the quartz-sulfide vein yield δ56Fe values of –0.02 ± 0.02‰ and +0.07 ± 0.07‰. Our Fe isotope fractionation calculations show that pyrrhotite with light Fe isotopes crystallized first from the ore-forming fluids, which indicates a relatively reduced condition for the initial ore-forming fluids. Then the remaining fluids with heavy Fe isotopes precipitated pyrites with positive δ56Fe values, and their mineral crystallization sequence records an increase of oxygen fugacity during mineralization. Gold deposits in Wulong and Jiaodong share many similar geological characteristics. The pyrites from the Wulong deposit have higher δ34S values (+0.5 to +4.1‰) than the pyrrhotites (–1.2 to +1.2‰). Pyrites from the Jiaodong gold deposits show a wide range of both positive and negative δ56Fe values as well as high δ34S values, whereas those from the Wulong deposit have a relatively narrow range of positive δ56Fe values and near-zero δ34S values. The differences in Fe isotopes may be due to early precipitation of pyrrhotite with light Fe isotopes under a relatively low oxygen fugacity environment in the Wulong deposit, resulting in pyrite precipitated from the remaining fluids with heavy isotopes. The sulfur isotope variations between Wulong and Jiaodong gold deposits reflect differences in their source regions rather than oxygen fugacities. In addition, we have compiled Fe isotopic compositions of pyrites and pyrrhotites from different types of ore deposits to investigate their Fe isotopic behavior in magmatic-hydrothermal systems. Pyrite grains show a wide range of δ56Fe values. Positive pyrite δ56Fe values reflect an equilibrium isotope effect, whereas negative pyrite δ56Fe values may be due to kinetic isotope effects or to a mixture of sedimentary host rocks. Pyrrhotite grains show similar negative δ56Fe values, and they have a strong influence on the Fe isotope systematics in magmatic and hydrothermal systems. Our data show that Fe isotopes can be used to trace precipitation orders of pyrite and pyrrhotite and oxygen fugacity evolution in relatively reduced hydrothermal deposits. [ABSTRACT FROM AUTHOR]

• Fe isotopes were variable in ferruginous rocks during and after Kuroko formation. • Anoxic pools in Hokuroku Basin may have facilitated preservation of sulfide ores. • Prominent Eu anomalies in ferruginous rocks indicate association with VMS deposits. • Fe isotopes in Zn-rich black ores are negatively shifted due to a kinetic effect. • Duration of hydrothermal circulation is a primary control on ore grade and tonnage. 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. [ABSTRACT FROM AUTHOR]

• Shikebutai iron formations was chemical sediments of authigenic origin. • This Late Carboniferous-aged IFs intimate association with submarine volcanic activity. • Enhanced hydrothermal activity can shape the marine redox landscape and of the development ferruginous conditions. Iron formations (IFs) are iron-rich chemical sediments that were widespread throughout the Precambrian. The deposition of IFs has provided key evidence about the prevalence of anoxic and ferruginous marine conditions throughout Earth's history. Here, we describe the Shikebutai IF, hosted in a Late Carboniferous volcano-sedimentary sequence in the Western Tianshan region of northwest China. The IF is largely composed of hematite, jasper, and siderite. The chemical composition of the Shikebutai IF is dominated by Fe 2 O 3 and SiO 2 , and very low concentrations of Al 2 O 3 , TiO 2 , Th, Nb, and Sc were observed, indicative of an authigenic origin. High Fe/Ti, Fe/Al, Co/Zn, and Ni/Zn ratios, along with positive Eu anomalies and ε Nd (t) values point to a significant submarine volcanic and hydrothermal contribution. The absence of negative Ce anomalies and heterogeneous δ56Fe values suggest progressive oxidation of hydrothermally sourced Fe(II) and deposition in an anoxic to low oxygen basin. Low P/Fe ratios in the Shikebutai IF, comparable to those found in the Precambrian, are consistent with Si inhibition of P sorption and P drawdown with high hydrothermal inputs. The intimate association of this Late Carboniferous IF with submarine volcanic activity suggests an origin that is analogous to Precambrian Algoma-type IFs. Further, the Shikebutai IF provides evidence for an episode of basin-scale anoxic, ferruginous conditions coinciding with the generally well-oxygenated deep oceans of the Phanerozoic, with deposition driven by enhanced submarine volcanic and hydrothermal activity in the Western Tianshan region during the Late Carboniferous. This demonstrates how, even with near modern surface oxygen levels, enhanced hydrothermal activity can shape the marine redox landscape and cause the development of ferruginous conditions without quantitative sulfate drawdown. [ABSTRACT FROM AUTHOR]

Layered mafic-ultramafic intrusions (LMI) are among the largest igneous bodies on Earth, and represent aggregations of large volumes of mantle- and some crustal-derived melts. Melts are emplaced over time-intervals of less than 1 million years, predominantly through multiple pulses of injections into pre-existing melt-crystal slurries. The dynamic interaction of physical processes, including density-driven separation and mixing of different components, within a solidifying magma chamber leads to such extreme chemical diversity between cumulate rock units that no unified model currently explains all aspects of the genesis of these intrusions. Here we present whole-rock stable Fe isotope data (expressed in ‰ variations as δ57Fe relative to IRMM-014) for samples of drill core taken from the stratified paleo-magma chamber of the Upper Zone of the late-Archean Windimurra Igneous Complex, Western Australia. Variations from near chondritic (δ57Fe ∼ + 0‰) to heavy (δ57Fe∼ + 0.2‰) values show a co-variation with initial radiogenic Hf isotope data that is unique to the Windimurra Upper Zone. The systematic isotopic variations from the roof to the base of the Upper Zone are best explained by an intricate sequence of events that included fractional crystallization and physical mixing. We propose that melt freshly sourced from the mantle was injected into and inflated a pre-existing crystal-melt mush comprising the upper Middle Zone. Re-establishment of crystal layering after replenishment introduced a chemical stratification with the formation of what became the Upper Zone and a crystal-interstitial melt ratio decreasing from roof to base. Basal, vanadiferous magnetitite horizons crystallized from Fe out of the liquid-in-residence. Variable degrees of perturbation and chaotic stirring of crystals with imperfect mixing of new and old components was followed by rapid crystal settling and subsequent cumulate stratification. Such melt rejuvenation, proposed here to be the cause for a newly established Upper Zone, leaves no unique petrologic fingerprint and forges a range in hybrid magma compositions throughout the Upper Zone rather than one single parental melt. This incremental growth of the intrusion can explain not only the observed coupled Fe-Hf isotope systematics, but also mineral disequilibria and cryptic layering in layered intrusions. In the context of incremental pluton assembly, two-component mixing paired with source heterogeneity can, at least in parts, potentially explain zircon-hosted Hf isotope heterogeneity so often observed in larger magmatic bodies.

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. [ABSTRACT FROM AUTHOR]

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The Ediacaran age (ca. 570 Ma) Shuram excursion, a ca. 12‰ depletion in δ13Ccarb, may record a dramatic oxidation of marine sediments associated with a reorganization of Earth's carbon cycle closely preceding the rise of large metazoans. However, several geochemical indicators suggest it may instead record secondary processes affecting the sediments such as post-depositional alteration. The stable isotopic composition of iron incorporated within carbonates (δ56Fecarb) reveals an anomalous 56Fe-depletion (down to -1.05‰) in strata containing the Shuram excursion, while the underlying and overlying strata have crustal 56Fe-δ56Fecarb values. These depleted δ56Fecarb data during the Shuram excursion reflect incomplete reduction of iron oxides, limited by low ambient organic carbon contents. This elevated pulse of sedimentary iron oxides would consume the majority of the limited pool of organic carbon and therefore would give rise to very low net organic carbon burial during a time of enhanced detrital delivery of oxidized iron to the sediments. These results imply a syndepositional origin for the Shuram excursion, which represents a shift in the redox composition of Earth's sedimentary shell toward more oxidizing conditions, perhaps removing a long-lived buffer on atmospheric oxygen. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Iron isotopes have been widely applied to trace mineralization processes of magmatic-hydrothermal ore systems, yet the application of Fe isotopes in porphyry Cu deposits (PCDs) is still restricted by the lack of understanding of its behaviors during hydrothermal alteration and ore formation processes. In this study, Fe isotope systematics in whole-rocks and Fe-bearing minerals coupled with S isotope compositions of Fe-sulfides are investigated for the early Cretaceous Tongshankou porphyry Cu-Mo deposit from Eastern China. The results show that altered ore-hosting granodiorite porphyries display a small δ56Fe variation (0.04‰ to 0.17‰) with an average of 0.10 ± 0.08‰ (2SD, n = 10), indistinguishable from the values for global igneous rocks. The δ56Fe of magnetite (0.22‰) and biotite (-0.08‰ to 0.12‰) from the altered porphyries are also within the δ56Fe ranges of those from global igneous rocks. These results indicate that hydrothermal fluids have negligible effect on Fe isotope compositions of the porphyry host rocks, likely due to the insignificant contributions of fluid-derived iron relative to the total Fe budget of the host rocks. The Fe-sulfides in the porphyry system record geochemical and iron isotopic characteristics of the ore-forming fluids. Specifically, chalcopyrites show large Fe isotope variations with δ56Fe ranging from −0.60‰ to 0.61‰. The calculated Fe isotope compositions of ore-forming fluids based on the chalcopyrite δ56Fe are thus not uniform and fall in the range from −0.69 ± 0.18‰ to 0.52 ± 0.18‰ (2SD). On the other hand, pyrites exhibit a δ56Fe variation between −0.48‰ and 0.40‰ and show distinct Fe isotopic characteristics for disseminated and vein-type pyrite. The δ56Fe for disseminated pyrites are consistently higher than whole-rock values, ranging from 0.14‰ to 0.40‰, likely reflecting equilibrium Fe isotopic exchange between pyrite and dissolved Fe in fluid. By contrast, δ56Fe for pyrites in veins are highly variable (−0.48‰ to 0.34‰) and show an inverse correlation with the chalcopyrite δ56Fe. These observations, in concert with S isotope data, have been interpreted to reflect pyrite formation through the FeS pathway, whereas the wide range in δ56Fe for pyrite may record isotopic change from initial FeS-fluid equilibrium towards pyrite-fluid equilibrium due to different extent of reaction upon precipitation. Additionally, when compared to sulfides formed by non-magmatic, low-temperature processes, the sulfides in our study exhibit a relatively narrow range of δ56Fe and δ34S, implying a high-temperature magmatic origin for both elements. Overall, our study demonstrates that the coupling of Fe and S isotopes could be a useful tool for tracking the processes of sulfide mineralization in porphyry-related deposits. [ABSTRACT FROM AUTHOR]

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations. [ABSTRACT FROM AUTHOR]

Fluids released by subducting slabs are important to the geochemical evolution of Earth's crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (f O 2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO 2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe 2 O 3 and FeO t contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the f O 2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas. [ABSTRACT FROM AUTHOR]

The microbial origin of Precambrian iron formations is debated due to the lack of direct fossil evidence. In order to reveal the genesis of ironstones under low-oxygen levels, integrative studies of sedimentology, petrography, mineralogy, and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal Paleoproterozoic Chuanlinggou Formation (ca. 1.65–1.64 Ga) of North China, using microscopy, SEM, EDS, ICP-OES, ICP-MS and MC-ICP-MS techniques. Mineralogical study shows that the Fe-rich mineral is predominantly hematite that resulted from dehydration of amorphous Fe-oxyhydroxide during diagenesis. Petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus. Basinward increases of the ironstone abundance, Eu anomalies (from 1.39 to 1.56) and δ56Fe values of the ironstones (from +0.5‰ to +1.0‰) suggest that the iron was mainly sourced from seafloor hydrothermal fluids, and partially oxidized and precipitated in shallow subtidal to intertidal environments. The common existence of Fe-oxide coated sheaths, spiral stalks, residual extracellular polymer substances (EPS) and other biogenic fabrics indicates that microaerophilic iron-oxidizing bacteria (FeOB) may have played an important role in precipitating the Chuanlinggou ironstones. The extremely low oxygen concentrations implied by the proliferation of microaerophilic FeOB in the shallow waters, the weak positive Ce anomalies (0.94–1.12) and low Mn concentrations in the ironstones are broadly consistent with the previous result of a Cr isotope study. Thus the establishment of a microaerophilic FeOB genetic model for the widespread Chuanlinggou ironstones in North China provides new insight into the origin of Precambrian iron formations and the redox evolution of ocean-atmosphere systems during the "Boring Billion". Unlabelled Image • The Chuanlinggou ironstones (∼1.64 Ga) were largely formed by microaerophilic FeOB. • Iron composition of the ironstones mainly sourced from hydrothermal fluids • Oxygen concentration in the shallow seawaters was probably at <3 to 15 μM. [ABSTRACT FROM AUTHOR]

The mobility of redox sensitive elements such as iron, carbon and sulfur in subduction zone fluids is important for redox transfer between the subducting slab and mantle wedge. However, the speciation of Fe during its mobility in subduction zone fluids at forearc depths is still unclear. Here, we studied the Fe isotope compositions of high-pressure (HP) fluid-metasomatized rocks (Mg-rich leucophyllite and gneiss, which is mineralogically transitional between leucophyllite and metagranite) from the Eastern Alps and found evidence for two distinct types of Fe species during Fe mobility in subduction zone fluids. The protolith of both types of rocks is metagranite, which shows δ56Fe values of 0.12–0.25‰. Compared to the metagranite, the transitional gneiss shows slightly higher Fe 2 O 3t contents, lower Fe3+/∑Fe ratios and significantly lower δ56Fe values of −0.03–0.16‰, whereas the leucophyllite exhibits much lower Fe 2 O 3t contents, comparable or slightly lower Fe3+/∑Fe ratios and generally higher δ56Fe values of up to 0.55‰. These observations indicate that the granitic protolith has experienced two types of fluid metasomatism. Combined with previous results, we infer the mobility of Fe in subduction zone fluids at forearc depths via two kinds of Fe species, Fe2+-(CO 3), and Fe2+-(HS) or Fe2+-Cl. The Fe mobility was dominant in the form of the Fe2+-(CO 3) complex in a carbonate-rich fluid during the formation of transitional gneiss, consistent with reported radiogenic Sr and isotopically light Mg isotope compositions, whereas Fe loss in the form of Fe2+-(HS) or Fe2+-Cl complexes via dissolution of biotite and phlogopite can account for the formation of leucophyllites. According to the tectonic evolution of the Eastern Alps, both types of fluids possibly originated from the dehydration of the sediment-serpentinite mélange produced by the subducting oceanic slab of the Alpine Penninic unit. The diversity of ferrous iron species in the subduction zone fluids at forearc depths probably reflects the variation in the redox properties of fluids derived from carbonate-rich sediment and serpentinite under such conditions. Our results provide an excellent example of tracing Fe mobility with distinct Fe species in subduction zone fluids. [ABSTRACT FROM AUTHOR]

• Large Fe isotopic variation for continental intraplate basalts from SE China. • A two-stage melting process explains both heavy and light Fe isotope data. • Early-stage low-degree melt preserves heavy Fe isotopes of carbonated pyroxenite. • Further decompression and mixing with SCLM melt causes light Fe isotope signature. • Fe isotopes provide insights into generation of continental intraplate basalts. Iron isotope studies on ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) have disclosed the contribution of pyroxenite lithologies in the generation of basaltic magmas. Whether Fe isotopic compositions of continental intraplate basalts can be applied to trace lithological heterogeneity within the mantle source regions remains poorly investigated. To explore the systematics of stable Fe isotopes as a potential probe of lithological heterogeneity in the source of continental intraplate basalts, we present twenty-four Fe isotope data on a suite of well-characterized Cenozoic basalts from SE China. The samples show a large range of Fe isotope values (δ56Fe = +0.09‰ to +0.20‰), which correlate with SiO 2 , CaO/Al 2 O 3 , Ti/Eu, Hf/Hf*, Zr/Nb, Dy/Yb, La/Yb, Nb/Y, K/La, Sr/Ce, δ66Zn and estimated equilibrium pressures. The samples with the highest δ56Fe values represent early-stage low-silica basalts with moderately enriched Sr-Nd isotope ratios and high δ66Zn values, while the late-stage high-silica basalts display a broad δ56Fe decrease with increasing 87Sr/86Sr and with decreasing ε Nd and δ66Zn. We demonstrate that the heaviest Fe isotope signatures cannot be derived from a pure peridotite source and require a pyroxenite component in the source. Combined with other geochemical proxies, we show that the heaviest basalt compositions are consistent with low-degree melts produced by the adiabatic decompression of a carbonated pyroxenite-bearing asthenospheric mantle. Mixing of these hybrid melts with melt produced from in-situ melting of the subduction-modified sub-continental lithospheric mantle (SCLM) reproduces the Fe-isotope variability of the late-stage basalts. Our results demonstrate the significance of lithological heterogeneity in the mantle source of continental intraplate basalts and highlight the subsequent imprint of mantle lithosphere in the evolution of their Fe isotope compositions. Finally, this study exemplifies the potential of the Fe-Zn stable isotope pair for mantle geochemistry; coupled with traditional radiogenic isotopes and major and trace element concentrations, they formed an efficient tool to trace the nature and contribution of the mantle source components in the formation of intraplate basalts. [ABSTRACT FROM AUTHOR]

There is growing evidence from Mo and Fe stable isotope tracers that shallow-water sediments of the 2.95 Ga Mozaan Group (Pongola Supergroup) in South Africa were deposited under oxidizing conditions, while mineralogical, elemental and S isotopic evidence favors the contemporaneous atmosphere to have remained reducing. In contrast, the first reported Cr-stable isotope data for White Mfolozi riverbed outcrop samples of the Ijzermijn iron formation (IF), Mozaan Group, and drill core samples of the underlying paleosol of the Nsuze Group were interpreted to require oxidative weathering conditions during the time of deposition. More recent Cr-isotope data of at least the Ijzermijn IF samples of the same White Mfolozi riverbed outcrop, however, suggested that modern weathering effects can superimpose oxidative signatures on paleo-redox proxies. This study compares (234U/238U) activity ratios along with stable Mo and Fe isotope data of corresponding drill core and White Mfolozi riverbed outcrop samples in order (i) to test (234U/238U) disequilibria as a potential tool to identify samples affected by modern weathering and (ii) to further investigate weathering effects on outcrop samples, particularly with regard to the stable Mo and Fe isotope systematics. All drill core samples from the Ijzermijn IF have (234U/238U) activity ratios that are within uncertainties of secular equilibrium (i.e. a value of 1), and thus were not significantly affected by modern weathering-related U gain or loss or other sub-recent rock-water interaction. By contrast, the outcrop IF and shale samples from the White Mfolozi riverbed show very strong deviations from the secular equilibrium (234U/238U) activity ratio. Furthermore, these deviations are accompanied by the observations that (i) outcrop samples have on average heavier δ56/54Fe values (−0.96‰) than drill cores (−1.59‰), and (ii) positive correlations in drill core samples between log[Fe/Mn] and δ98/95Mo or δ56/54Fe are disrupted by inclusion of the outcrop samples, as are negative correlations between log[MnO] and δ98/95Mo. However, δ56/54Fe and δ98/95Mo of our outcrop samples do not correlate with the extent of (234U/238U) disequilibria. As such, (234U/238U) activity ratios fail to act as a direct identification tool for modern weathering effects on stable Fe and Mo isotopes in outcrop samples. Overall, this study confirms the adsorption of isotopically light Mo onto MnO 2 particles in oxic surface water conditions in the epicontinental Pongola Sea. Reduction of the Mn-oxides together with microbial Fe reduction in the reduced sediment pile of the Pongola basin led to the formation of Mn- and Fe-carbonates during early diagenesis storing the observed stable Fe and Mo isotopic signatures. We thus extend the so-called MnO 2 -shuttle first proposed for the Neoarchean 2.46 Ga old Koegas IF to late Mesoarchean shallow-marine depositional settings. [ABSTRACT FROM AUTHOR]

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated materials that provide crucial information about the isotopic reservoirs present in the early Solar System. For a variety of elements, CAIs have isotope compositions that are uniform yet distinct from later formed solid material. However, despite being the most abundant metal in the Solar System, the isotopic composition of Fe in CAIs is not well constrained. In an attempt to determine the Fe isotopic compositions of CAIs, we combine extensive work from a previously studied CAI sample set with new isotopic work characterizing mass-dependent and mass-independent (nucleosynthetic) signatures in Mg, Ca, and Fe. This investigation includes work on three mineral separates of the Allende CAI Egg 2. For all isotope systems investigated, we find that in general, fine-grained CAIs exhibit light mass-dependent isotopic signatures relative to terrestrial standards, whereas igneous CAIs have heavier isotopic compositions relative to the fine-grained CAIs. Importantly, the mass-dependent Fe isotope signatures of bulk CAIs show a range of both light (fine-grained CAIs) and heavy (igneous CAIs) isotopic signatures relative to bulk chondrites, suggesting that Fe isotope signatures in CAIs largely derive from mass fractionation events such as condensation and evaporation occurring in the nebula. Such signatures show that a significant portion of the secondary alteration experienced by CAIs, particularly prevalent in fine-grained inclusions, occurred in the nebula prior to accretion into their respective parent bodies. Regarding nucleosynthetic Fe isotope signatures, we do not observe any variation outside of analytical uncertainty in bulk CAIs compared to terrestrial standards. In contrast, all three Egg 2 mineral separates display resolved mass-independent excesses in 56Fe compared to terrestrial standards. Furthermore, we find that the combined mass-dependent and nucleosynthetic Fe isotopic compositions of the Egg 2 mineral separates are well correlated, likely indicating that Fe indigenous to the CAI is mixed with less anomalous Fe, presumably from the solar nebula. Thus, these reported nucleosynthetic anomalies may point in the direction of the original Fe isotope composition of the CAI-forming region, but they likely only provide a minimum isotopic difference between the original mass-independent Fe isotopic composition of CAIs and that of later formed solids. [ABSTRACT FROM AUTHOR]

We report the first measurements of Fe isotope fractionation between coexisting pyrrhotite and chalcopyrite from the Lengshuiqing Ni-Cu magmatic sulfide deposit, Southwest China. The δ56Fe values of chalcopyrite (Cp) range from 0.24‰ to 1.25‰, much higher than those in the coexisting pyrrhotite (Po, −1.25‰ to −0.13‰). The δ56Fe values of pyroxene separates from the Lengshuiqing intrusions are in a range from −0.03‰ to 0.18‰. Pyrrhotite in sulfide-mineralized rocks with greater than ∼2 wt.% S has δ56Fe values of −0.3 ± 0.2‰, indicating Δ56Fe Po-sil (pyrrhotite – silicate melt) in the range of −0.4 ± 0.2‰ with parental silicate magma δ56Fe of 0.1‰ as calculated from the least evolved pyroxene δ56Fe value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Δ56Fe Po-sil of approximately −0.4‰ represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than ∼2 wt.% S show δ56Fe values as low as −1.25‰. The low δ56Fe values can be modeled by a non-equilibrium process, where the fractionation between immiscible sulfide liquid and silicate magma decreases from −0.4 to −1.4‰ as the R-factor (mass ratio of silicate melt to coexisting sulfide liquid) increases from 1 to 1000. We suggest that the relative proportion of externally introduced Fe from sulfidic sedimentary country rocks may be higher in low-S samples, controlling the low δ56Fe values of the sulfide assemblage in the mineralized rocks. The trend of Fe isotope enrichment at Lengshuiqing is consistent with Fe-S bond strengths, but the difference between coexisting pyrrhotite and chalcopyrite is not consistent with equilibration at high temperatures. The Δ56Fe Cp-Po values at Lengshuiqing (1.06–1.77‰) are similar to those measured for samples from the Voisey's Bay and Eagle Ni-Cu deposits, and suggest that in a cooling sulfide liquid Fe isotope equilibration may continue to temperatures less than 200 °C. [ABSTRACT FROM AUTHOR]

• First stable iron isotope data from Pitcairn island, the type-locality of EM1 ocean island basalts. • Stable iron isotopes in primitive plume-related melts show a strong covariation with radiogenic Sr, Nd, Pb isotopes. • Mixing of primitive and enriched plume components occurs during melting at mantle depth. • Radiogenic isotope trajectories in OIB are mixing arrays between enriched components and plume matrix. 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. [ABSTRACT FROM AUTHOR]

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 H 2 S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe 3 S 4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fe pyrite - δ56Fe FeSx , where FeS x 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 <−2‰ isotopic fractionation determined in previous experiments (Guilbaud et al., 2011b). Numerical models indicate that isotopic exchange between pyrite and Fe(II) aq is necessary to explain the experimental data. The inferred rate of isotopic exchange decreases with time in our experiments, likely as a function of particle size, but shows no clear correlation with temperature across different studies. In the presence of isotopic exchange, modeling results indicate that pyrite precipitated from Fe(II) aq may theoretically have δ56Fe values ranging from −3 to + 4‰, which spans nearly the whole δ56Fe range observed in nature. Negative values reflect the expression of the KIE when isotopic exchange is slow (relative to net precipitation rate) while positive values reflect the expression of the equilibrium isotope effect (EIE) when isotopic exchange is relatively fast. We therefore propose that the variation in sedimentary pyrite δ56Fe can be explained in terms of varying expression of the KIE and the EIE, either during different stages of precipitation or as controlled by the availability of Fe(II), sulfide, and oxidants throughout Earth's history. The predominantly negative (but highly variable) pyrite δ56Fe values in modern marine sediments suggest a higher expression of the KIE in low temperature systems, but do not rule out the importance of isotopic exchange. The isotopic exchange rate is currently underconstrained in low temperature systems with an uncertainty range that spans 8 orders of magnitude. Our work suggests that isotopic exchange has the potential to affect sedimentary pyrite δ56Fe unless the current upper limit for isotopic exchange rate is overestimated by 5 orders of magnitude. [ABSTRACT FROM AUTHOR]