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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]

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]

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]

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]

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]

• 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]

• 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]

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]

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]

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]

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]

Combined Fe isotopic and olivine fabric analyses have been conducted on the nodular chromitites from the Luobusa ophiolite, Tibet, to study the physico-chemical effects of plastic flow in modifying chromitites and their associated mantle peridotites below spreading centers. The Luobusa nodular chromitites are generally composed of fresh dunitic matrices and well-aligned chromite nodules. Olivine grains in both the dunitic matrices and chromite nodules underwent pressure solution, and those in the dunitic matrices have prevalent Fe-rich stripes parallel to their kink bands and sometimes show lattice dislocation-induced preferred orientations of (1 0 0). Chromite grains in the chromite nodules have Cr# ∼ 75–80 and δ56Fe values from −0.122 to 0.067. The Fo and δ56Fe values of olivine in the dunitic matrices vary from 96 to 99 and 0.165 to 0.294, respectively, distinguishably higher than those of olivine in ordinary ophiolitic dunites (mostly 90–95 and 0.050–0.150). The calculated Mg-Fe exchange temperatures between olivine in the dunitic matrices and chromite in the chromite nodules range from 700 to 900 °C, overall higher than those for ordinary dunites and chromitites (∼600–800 °C). Combined with numerical modeling of diffusion of Fe in olivine, our study reveals that the abnormally high Fo and δ56Fe values of olivine in the dunitic matrices of nodular chromitites were not controlled by their parental magmas but caused by significant loss of Fe from olivine under subsolidus conditions. Intensive plastic deformation in nodular chromitites largely promoted the migration of Fe from olivine and facilitated the Mg-Fe exchange between the chromite nodules and dunitic matrices, maximizing the Fo and δ56Fe values of olivine. However, such an effect is expected to be negligible in chromite-barren dunites and harzburgites because such rocks do not have enough chromite to generate effective chemical-potential gradients for Fe migration. The high Fo values of olivine in nodular and massive chromitites require fast cooling rates (∼0.05–0.1 °C/yr) in the surrounding mantle, consistent with settings below intermediate-fast spreading ridges. Boudinization of small chromitite bodies in less competent dunites under 900–1200 °C is a potential mechanism for generating the nodular texture, also accounting for the en-echelon or bead-like occurrences of chromitite bodies in ophiolites. Effective mantle flow below spreading centers plays an important role in shaping the textures and compositions of mantle rocks, explaining the absence of deformation-related nodular chromitites in layered mafic-ultramafic intrusions. [ABSTRACT FROM AUTHOR]

Abstract Colloids have been recognized as key vectors of pollutants in aqueous environment. Amongst them, those formed by iron (Fe) and organic matter (OM) are of major importance due to their ubiquity in the surface environment and strong affinity for metals. In the recent years, Fe stable isotopes have been increasingly used to elucidate the sources and biogeochemical cycling of Fe in Earth's surface environments. In this study, we aim to elucidate (i) the possible Fe isotopic signature resulting from the Fe/OM colloid formation and (ii) the mechanisms involved in the development of such isotopic signature. For this purpose, Fe-OM associations were synthesized through binding and titration experiments. Various pH levels were used in order to study the isotope behavior of Fe occurring as free species at pH 1, as Fe-OM complexes at pH 2 and as mixed Fe-oxyhydroxide/OM nanoaggregates or particles at pH 6.5. Organic matter-free, Fe-free and OM membrane-deposition experiments were also performed. These suspensions were (ultra)filtered at 0.2 µm, 30 kDa and 5 kDa to evidence the possible Fe isotope fractionation between fractions. This protocol allowed also testing the potential of (ultra)filtration techniques to generate isotope fractionation. The results provided evidence that abiotic Fe precipitation, (ultra)filtration techniques and OM deposition were not able to produce significant Fe isotope fractionation under the experimental conditions. However, at circum-neutral pH, the Fe-OM binding and titration experiments displayed a significant enrichment of heavy Fe isotopes in the <30 kDa fractions relative to the total Fe pool δ56Fe = 0.35 ± 0.05‰ and 0.26 ± 0.05‰ (95% confidence interval, 2σ and relative to international standard IRMM-14), respectively. Mass balance and error propagation calculation showed Fe isotope fractionation in binding and titration experiments between the >30 kDa and <30 kDa fractions for −0.35 ± 0.05‰ and −0.27 ± 0.05‰, respectively. This Fe isotope fractionation could be due to the complexation of Fe by OM in the <30 kDa fractions. At pH 2, the OM-free experiment, the <30 kDa fraction showed Fe isotope ratio δ56Fe = 0.75 ± 0.03‰ with an enrichment in heavy Fe isotopes of δ56Fe' = 0.14 ± 0.04‰ relative the total Fe pool (δ56Fe' is δ56Fe value which was corrected by δ56Fe of total fraction). This enrichment in heavy Fe isotopes induced an isotopic fractionation factor of −0.87 ± 0.26‰ between the >30 kDa and <30 kDa fractions produced by the complexation between the heavy Fe isotopes and OH− ligands in the <30 kDa fraction. The natural Fe-OM associations were further investigated through oxidation experiments of a reduced wetland soil solution. The oxidized soil solution was (ultra)filtered at 5 µm, 3 µm, 0.2 µm, 30 kDa and 5 kDa. The highest δ56Fe was obtained in the smallest size fraction, i.e. <5 kDa fraction, yielding a negative isotopic fractionation Δ56Fe >5kDa − <5kDa = −0.23 ± 0.08‰ suggesting that Fe heavy isotopes are preferentially bound to small humic OM molecules in the form of Fe monomers or small clusters. This study highlights the importance of organic matter for metals' isotopic systems. [ABSTRACT FROM AUTHOR]

The origin of heavy Fe isotope compositions in high-silica (>70 wt% SiO 2 ) igneous rocks remains a highly controversial topic. Considering that fluid exsolution in eruptive rocks is more straight-forward to constrain than in plutonic rocks, this study addresses the problem of Fe isotope fractionation in high-silica igneous rocks by measuring Fe isotope compositions of representative rhyolitic samples from the Neoproterozoic volcanic-sedimentary basins in southern China and the Triassic Tu Le Basin in northern Vietnam. The samples show remarkably varied δ 56 Fe IRMM014 values ranging from 0.05 ± 0.05‰ to 0.55 ± 0.05‰, which is among the highest values reported from felsic rocks. The extensional tectonic setting and short melt residence time in magma chambers for the studied rhyolites rule out Soret diffusion and thermal migration processes as causes of the high δ 56 Fe values. Effects of volcanic degassing and fluid exsolution on bulk rock δ 56 Fe values for the rhyolites are also assessed using bulk rock geochemical indicators and Rayleigh fractionation models, and these processes are found to be insufficient to produce resolvable changes in Fe isotope compositions of the residual melt. The most probable mechanism accounting for heavy Fe isotope compositions in the high-silica rhyolites is narrowed down to fractional crystallization processes in the magma before rhyolite eruption. Removal of isotopically light Fe-bearing minerals (i.e. ulvöspinel-rich titanomagnetite, ilmenite and biotite) is proposed as the main cause of Fe isotope variation in silicic melts during magmatic evolution. This study implies that crystal fractionation is the dominant mechanism that controls Fe isotope fractionation in eruptive rocks and Fe isotopes could be used to study magmatic differentiation of high-silica magmas. [ABSTRACT FROM AUTHOR]

Incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions, which has implications for Fe export from soils. This study uses Fe isotopes to assess the processes controlling Fe export in a range of Icelandic soils including peat soils derived from the same parent basalt, where Fe isotope variations principally reflect differences in weathering and drainage. In poorly weathered, well-drained soils (non-peat soils), the limited Fe isotope fractionation in soil solutions relative to the bulk soil (Δ 57 Fe solution-soil = −0.11 ± 0.12‰) is attributed to proton-promoted mineral dissolution. In the more weathered poorly drained soils (peat soils), the soil solutions are usually lighter than the bulk soil (Δ 57 Fe solution-soil = −0.41 ± 0.32‰), which indicates that Fe has been mobilised by reductive mineral dissolution and/or ligand-controlled dissolution. The results highlight the presence of Fe-organic complexes in solution in anoxic conditions. An additional constraint on soil weathering is provided by Si isotopes. The Si isotope composition of the soil solutions relative to the soil (Δ 30 Si solution-soil = 0.92 ± 0.26‰) generally reflects the incorporation of light Si isotopes in secondary aluminosilicates. Under anoxic conditions in peat soils, the largest Si isotope fractionation in soil solutions relative to the bulk soil is observed (Δ 30 Si solution-soil = 1.63 ± 0.40‰) and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation. Si supersaturation in solution with respect to amorphous silica is reached upon freezing when Al availability to form aluminosilicates is limited by the affinity of Al for metal–organic complexes. Therefore, the precipitation of amorphous silica in peat soils indirectly supports the formation of metal–organic complexes in poorly drained soils. These observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands, Fe export from soils as Fe-organic complexes will increase, which in turn has implications for Fe transport in rivers, and ultimately the delivery of Fe to the oceans. [ABSTRACT FROM AUTHOR]

The behavior of Fe during serpentinization largely controls the potential for oxidation-reduction reactions and energy budget for serpentinite-hosted microbial communities. We present Fe isotope data for mineral separates from a partially serpentinized dunite from New Caledonia to understand the behavior of Fe during serpentinization processes. Our new Fe isotope data in mineral separates is compared to existing data from whole rock studies of serpentinites, which have generally concluded that Fe mobility during serpentinization is restricted to the highest temperatures of serpentinization in subduction zones. Measurements of mineral separates from New Caledonia show significant Fe isotope fractionations, with serpentine-brucite mixtures having the lowest δ 56 Fe ∼ −0.35‰ and magnetite having the highest δ 56 Fe ∼ +0.75‰. Olivine, orthopyroxene, and the whole rock composition are all within error of δ 56 Fe = 0.00‰. Fe isotope thermometry between mineral phases reveals two distinct temperatures of equilibration, one for the mantle olivine and pyroxene (∼1325 °C), and a second, much lower temperature (∼335 °C) for the serpentinite assemblage. The combined isotopic, mineralogical and geochemical data indicate that during the magnetite-forming stage of serpentinization, a pore fluid in equilibrium with the mineralogical assemblage evolves to higher Fe concentrations as serpentinization proceeds. When this pore fluid is removed from the serpentinizing environment, the total abundance of Fe removed from the rock in the pore fluid is much less than the bulk rock Fe and has a minimal effect on the overall rock composition. [ABSTRACT FROM AUTHOR]

This study presents Fe–Zn isotope data for a suite of well–characterized bimodal volcanic rocks from Hailar Basin, northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks. The samples range from basaltic trachyandesites to trachytes–rhyodacites, and rhyolites. The δ 56 Fe values increase with increasing SiO 2 contents with the rhyolites having the highest δ 56 Fe (up to 0.64 ± 0.02‰) among the previously reported data for igneous rocks at a similar SiO 2 . The lack of correlation between δ 56 Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes. The δ 56 Fe do not show a clear correlation with δ 66 Zn and radiogenic isotopes, suggesting that thermal diffusion or crustal contamination cannot produce the high δ 56 Fe in Hailar volcanic rocks. Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism. During the first step, partial melting of basaltic trachyandesites with an average δ 56 Fe of 0.09 ± 0.14‰ produced trachytes–rhyodacites with an average δ 56 Fe of 0.24 ± 0.27‰. Modelling using rhyolite–MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe 3+ and Fe 2+ between the solid residue and partial melt. The second step involves formation of rhyolites with significantly high δ 56 Fe through partial melting or extensive crystallization of crust materials, during which isotopically heavy Fe preferentially partition into the rhyolitic melt. Therefore, fractionation of Fe isotopes between melts and minerals can result in high δ 56 Fe in SiO 2 -rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust. [ABSTRACT FROM AUTHOR]

Equilibrium Fe isotope fractionation between chalcopyrite and dissolved Fe was determined in acidic chloride-bearing fluid at 350 °C and 500 bars. The study utilized deformable gold-cell technology, which allowed time-series sampling of solution during chalcopyrite recrystallization and isotope exchange. A key element of the experimental design involved the addition of anomalous dissolved 57 Fe to an on-going experiment as a means of determining the degree and rate of isotope exchange. Taking explicit account of imposed chemical and isotopic mass balance constraints of Fe in fluid and mineral (chalcopyrite) reservoirs, these data indicate that no more than 1000 h is required for the isotopically anomalous dissolved Fe reservoir to exchange completely with the coexisting chalcopyrite. The experimental calibration of the rate of Fe isotope exchange for the δ 57 Fe-spiked experiment provides critical insight for the time necessary to achieve Fe isotope exchange in two non-spiked, but otherwise identical experiments. The Fe isotope data indicate that the equilibrium fractionation between chalcopyrite and dissolved Fe, Δ 56 Fe Cpy-Fe (aq) , at 350 °C is small, 0.09 ± 0.17‰ (2 σ ), and is in good agreement with recent theoretical equilibrium predictions. Owing to the apparent rate of Fe isotope exchange at 350 °C, it is likely that chalcopyrite formed at high temperature deep-sea vents (black smoker systems) achieves isotopic equilibrium, and effectively records the Fe isotopic composition of the coexisting end-member hydrothermal fluid. Comparison of the experimental mineral– fluid equilibrium fractionation factors with conjugate chalcopyrite and dissolved Fe pairs sampled from high temperature hydrothermal vent systems at Axial Caldera and Main Endeavour Field (Juan de Fuca Ridge) are in agreement with this inference. The experimental data were further used to determine the mineral– mineral equilibrium Fe isotope fractionation between pyrite-chalcopyrite, Δ 56 Fe Pyr-Cpy , at 350 °C by combining previously determined pyrite-Fe 2+ (aq) equilibrium fractionation data with chalcopyrite-Fe 2+ (aq) from this study. The empirically determined Δ 56 Fe Pyr-Cpy value, 0.90 ± 0.34‰ (2 σ ), is consistent with theoretical predictions, and when coupled with mineral– fluid Fe isotope fractionation systematics and experimentally determined exchange rates, helps to delineate processes of sulfide mineralization in hydrothermal systems. [ABSTRACT FROM AUTHOR]

This study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four Dabie I-type granitoids to evaluate the factors that control inter-mineral Fe isotopic fractionation in granitoids. The order of heavy iron isotope enrichment is feldspar > pyrite > magnetite > biotite ≈ hornblende. Feldspar has heavier iron isotopic compositions than its co-existing magnetite (Δ 56 Fe plagioclase–magnetite = +0.376‰ to +1.084‰, Δ 56 Fe alkali-feldspar–magnetite = +0.516‰ to +0.846‰), which can be attributed to its high Fe 3+ /Fe tot ratio and low coordination number (tetrahedrally-coordinated) of Fe 3+ . Δ 56 Fe magnetite–biotite of coexisting magnetite and biotite ranges from 0.090‰ to 0.246‰. Based on homogeneous major and iron isotopic compositions of mineral replicates, the inter-mineral fractionation in this study should reflect equilibrium fractionation. The large variations of inter-mineral fractionation among feldspar, magnetite and biotite cannot be simply explained by temperature variation, but strongly depend on mineral compositions. The Δ 56 Fe plagioclase–magnetite and Δ 56 Fe alkali-feldspar–magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali-feldspar, respectively. This could be explained by different Fe–O bond strength in feldspar due to different Fe 3+ /∑Fe or different crystal parameters. The Δ 56 Fe magnetite–biotite increases with decreasing Fe 3+ /∑Fe biotite and increasing mole (Na + K)/Mg biotite , indicating a decrease of β factor in low Fe 3+ /∑Fe and high (Na + K)/Mg biotite. High-silica leucosomes from Dabie migmatites with a feldspar accumulation petrogenesis have higher δ 56 Fe values (δ 56 Fe = 0.42–0.567‰) than leucosome that represents pristine partial melt (δ 56 Fe = 0.117 ± 0.016‰), indicating that accumulation of feldspar could account for high δ 56 Fe values of these rocks. High δ 56 Fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals, e.g., anorthosites. Feldspar accumulation, however, cannot explain high δ 56 Fe values of most high-silica granitoids reported in the literature, based on their low Sr, Ba contents and negative Eu anomalies. [ABSTRACT FROM AUTHOR]