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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 parts per thousand (delta Fe-56) 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. Zincrich black ores in both the Matsumine and Fukazawa deposits have lower delta Fe-56 values, due to rapid precipitation of pyrite triggered by the mixing of hydrothermal fluids with seawater. Positive shifts in delta Fe-56 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 delta Fe-56 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.

Fe isotopes have been applied to the petrogenesis of ore deposits. However, the behavior of iron isotopes in the mineralization of porphyry-skarn deposits is still poorly understood. In this study, we report the Fe isotopes of ore mineral separations (magnetite, pyrite, chalcopyrite and pyrrhotite) from two different skarn deposits, i.e., the Tonglvshan Cu-Fe skarn deposit developed in an oxidized hydrothermal system and the Anqing Cu skarn deposit developed in a reduced hydrothermal system. In both deposits, the Fe isotopes of calculated equilibrium fluids are lighter than those of the intrusions responsible for the skarn and porphyry mineralization, corroborating the “light-Fe fluid” hypothesis. Interestingly, chalcopyrite in the oxidized-Tonglvshan skarn deposit has lighter Fe than chalcopyrite in the reduced-Anqing skarn deposit, which is best understood as the result of the prior precipitation of magnetite (heavy Fe) from the ore fluid in the oxidized-Tonglvshan systems and the prior precipitation of pyrrhotite (light Fe) from the ore fluid in the reduced-Anqing system. The δ56Fe for pyrite shows an inverse correlation with δ56Fe of magnetite in the Tonglvshan. In both deposits, the Fe isotope fractionation between chalcopyrite and pyrite is offset from the equilibrium line at 350 °C and lies between the FeS-chalcopyrite equilibrium line and pyrite-chalcopyrite equilibrium line at 350 °C. These observations are consistent with the FeS pathway towards pyrite formation. That is, Fe isotopes fractionation during pyrite formation depends on a path from the initial FeS-fluid equilibrium towards the pyrite-fluid equilibrium due to the increasing extent of Fe isotopic exchange with fluids. This finding, together with the data from other deposits, allows us to propose that the pathway effect of pyrite formation in the Porphyry-skarn deposit mineralization is the dominant mechanism that controls Fe isotope characteristics.

Weathering processes on Ni-rich ultramafic rocks in tropical areas produce laterites that become exploitable for Ni mining. To better understand these processes, samples were collected stratigraphically with water samples from several Ni mines in the Philippines and studied for Ni and Fe isotope compositions. This study found that Ni isotope fractionation takes place during the formation of Ni-enriched minerals (goethite), when light Ni isotopes are preferentially incorporated into new minerals formed, leaving heavy Ni isotopes in groundwater. The Δ60NiLimonite-Bedrock is up to -0.19 ± 0.32‰. Even though Fe is partitioned with redox state change during these reactions, Fe isotope fractionation was not detected within our analytical precision. In complement, iron meteorites were analyzed for their δ57Fe to shed light on the origin of Lovina ataxite. This study supports other reports that Lovina has instead a terrestrial origin.

Chondrules are a major constituent of primitive meteorites. The formation of chondrules is one of the most elusive problems in cosmochemistry. We use Fe isotope compositions of chondrules and bulk chondrites to constrain the conditions of chondrule formation. Iron isotope compositions of bulk chondrules are so far only known from few studies on CV and some ordinary chondrites. We studied 37 chondrules from the CM chondrite Murchison. This is particularly challenging, as CM chondrites contain the smallest chondrules of all chondrite groups, except for CH chondrites. Bulk chondrules have δ 56 Fe between −0.62 and +0.24‰ relative to the IRMM-014 standard. Bulk Murchison has as all chondrites a δ 56 Fe of 0.00‰ within error. The δ 56 Fe distribution of the Murchison chondrule population is continuous and close to normal. The width of the δ 56 Fe distribution is narrower than that of the Allende chondrule population. Opaque modal abundances in Murchison chondrules is in about 67% of the chondrules close to 0 vol.%, and in 33% typically up to 6.5 vol.%. Chondrule Al/Mg and Fe/Mg ratios are sub-chondritic, while bulk Murchison has chondritic ratios. We suggest that the variable bulk chondrule Fe isotope compositions were established during evaporation and recondensation prior to accretion in the Murchison parent body. This range in isotope composition was likely reduced during aqueous alteration on the parent body. Murchison has a chondritic Fe isotope composition and a number of chondritic element ratios. Chondrules, however, have variable Fe isotope compositions and chondrules and matrix have complementary Al/Mg and Fe/Mg ratios. In combination, this supports the idea that chondrules and matrix formed from a single reservoir and were then accreted in the parent body. The formation in a single region also explains the compositional distribution of the chondrule population in Murchison.

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δ57Fe ≈ 0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δ57Fe≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is δ57Fe = +0.04 ± 0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δ57Fe(+0.05 ± 0.03‰), whereas MgO-rich rocks are lighter (δ57Fe≈−0.01 ±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δ57Fe SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.

Multiple sulfur isotope variability in Archean sedimentary rocks provides constraints on the composition of the Earth’s earliest atmosphere. The magnitude and sign of mass-independent anomalies reflect not only atmospheric processes, but also transformations due to the Archean marine sulfur cycle prior to preservation into sedimentary pyrite. The processes affecting the Archean marine sulfur cycle and the role of microbial or abiotic redox reactions during pyrite formation remain unclear. Here we combine iron (Fe) and multiple sulfur (S) isotope data in individual pyrite grains with petrographic information and a one-dimensional reactive transport model, to investigate the sources of Fe and S in pyrite formed in a Paleoarchean sedimentary basin. Pyrites were selected from mudstones, sandstones and chert obtained from a drill core in the ca. 3.2 Ga Mapepe and Mendon Formations of the Fig Tree and Onverwacht Groups, respectively, in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa. Pyrite textures and δ 56 Fe distinguish early-diagenetic pyrite formed with pore-water ferrous iron (disseminated grains with average δ 56 Fe pyrite = 0‰) from late-diagenetic pyrite formed through sulfidation of iron oxide minerals (layered and aggregate forms with average δ 56 Fe pyrite = + 1‰). Mass dependent S isotope variability in pyrite was small (δ 34 S pyrite ranged from − 1.1 to + 3.3‰) with a correspondingly minor spread in Δ 33 S pyrite (ranging from + 0.3 to + 2.1‰) and Δ 36 S pyrite (ranging from − 3.08 to + 0.27‰) that indicates a lack of post-depositional re-working with other distinct sulfur sources. Our combined Fe and S isotope data are most readily explained with pyrite sulfide derived from microbial-reworking of solid elemental S. Iron oxide minerals were necessary to buffer sulfide concentrations and provide favorable conditions for microbial sulfur disproportionation to proceed. The lack of a negative Δ 33 S signal indicates that pyrite from relatively deep marine diagenetic environments only partially records the products of atmospheric photolysis, consistent with low sulfate concentrations in the Paleoarchean ocean.

Concentrically laminated pyrite is a relatively common, although volumetrically minor, component of auriferous conglomerates in the Archaean (ca. 3.0–2.7 Ga) Witwatersrand Basin of South Africa. This type of pyrite contains high amounts (several tens of ppm) of Au, but the origin of the pyrite is debated, and the timing of Au deposition in these grains is not known. In order to constrain the formation of pyrite, we have studied concentrically laminated pyrite and other coexisting types of pyrite (inclusion-rich, massive pyrite) by analysing the contents and distribution of Au and other trace elements by laser ablation ICP-MS, the S and Fe isotope composition by SIMS, and the mineral inclusions by scanning electron microscope and laser Raman spectroscopy. Trace element maps indicate that concentrically laminated pyrite is enriched in Sb, Mn, Au, Ag, Tl, Cu, Mo, Mn, and contains two types of gold: finely dispersed Au (“invisible gold”, with Au/Ag ~ 0.1 and likely of primary origin) and Au inclusions with Au/Ag ~ 10 of secondary origin. The study of mineral inclusions revealed the presence of muscovite, chlorite, fine-grained carbonaceous matter, monazite, Ti-oxides, and quartz. Iron and multiple S isotopes suggest that concentrically laminated pyrite and inclusion-rich pyrite were formed from two separate pools of S and Fe with different isotope characteristics. Sulfur was derived from atmospheric S that had undergone mass-independent isotope fractionation to form SO 4 2− with negative Δ 33 S that constituted concentrically laminated pyrite, and elemental S with positive Δ 33 S that formed inclusion-rich pyrite. Iron pools were derived from partial oxidation of Fe 2 + , so that concentrically laminated pyrite formed from a low-δ 56 Fe residual Fe 2 + (average + 0.2‰) and inclusion-rich pyrite formed from a high-δ 56 Fe Fe 3 + pool (average + 2.7‰). Biological activity may have been involved in the reduction of SO 4 2− , causing a wide spread of δ 34 S values (~ 25‰, S reducing microorganisms), as well as in the partial oxidation of Fe 2 + (anaerobic photosynthetic Fe reducers or photosynthetic O 2 producers), and in the formation of pyrite from Fe 3 + (dissimilatory Fe reducers). We propose that concurrent biogenically-mediated pyrite formation and Au trapping suggest that microbial activity was responsible for the accumulation of Au and other trace elements (e.g. Sb, Mn, Ag, Tl, Cu, Mo, Mn) which are commonly enriched in organic matter-rich sediments.

The source and cycling of Fe in groundwaters from the phreatic aquifer which characterizes a site impacted by metallurgical activity in a coastal area of northern Italy (Adriatic Sea) have been investigated by Fe-isotope analyses, H-O-Sr stable-isotope systematics and major and trace element chemistry. Waters are characterized by circum-neutral to alkaline pH and dissolved O-2 zoning, and range from a low-salinity Ca-Mg-HCO3 type to a brackish Na-Cl type resulting from seawater intrusion. The O-H isotopic data indicate that the Ca-Mg-HCO3 waters originate from meteoric precipitation that infiltrated at elevated altitudes, and that a variable seawater fraction, in some cases exceeding 90%, characterizes the Na-Cl type waters. The Fe content ranges from 0.48 to 9.99 mg/L and from 2.50 to 43.8 mg/L in low-salinity and brackish waters, respectively. The delta Fe-56 value varies over the wide range from +0.87 parts per thousand to -5.29 parts per thousand in low-salinity waters and between +2.15 parts per thousand and -2.34 parts per thousand in brackish waters. The isotopically lighter compositions are interpreted as reflecting isotopic fractionation during repeated cycling of Fe precipitation. Positive delta Fe-56 values might indicate either a higher solubility of oxyhydroxides, which during diagenesis preferentially incorporated the isotopically heavier fraction of Fe, or the leaching of the foundry landfill disposal which characterizes the site.

The genesis of hydrothermal ore deposits is of crucial economic importance. This study investigates the extent, causes and consequences of zinc and iron isotope fractionation in a large hydrothermal system at the world-class Navan Zn–Pb orebody, Ireland. Large variations in Zn, Fe and S isotope compositions have been measured in microdrilled sphalerite (ZnS) at the millimetre scale. d66Zn and d56Fe display a well-defined positive correlation and both also correlate with d34S. These relationships represent the combined effects of kinetic Zn and Fe isotope fractionation during sphalerite precipitation, and S isotope variation through mixing of hot, metal-rich hydrothermal fluids and cool, bacteriogenic sulfide-bearing brines. Combined with S isotope data, d56Fe and d66Zn data on mine concentrates confirm that hydrothermal sulfide is a minor component of the overall deposit signature. Our data suggest that incoming pulses of metal-rich hydrothermal fluid triggered sulfide mineralisation, and that rapid precipitation of sphalerite from hydrothermal fluids will lead to strong kinetic fractionation of Zn and Fe isotopes at very short time and length scales, thereby limiting the use of Fe and Zn isotopes as exploration tools within deposits, but revealing the possibility of detecting new deposits from isotopically heavy Zn–Fe geochemical halos. Science Foundation Ireland DG 16/10/2012

International audience; Iron concentrations and isotopic compositions were measured in spherical hematite and goethite concretions, together with associated red (Fe-oxide coated) and white (bleached) sandstones from the Jurassic Navajo formation, Utah (USA). Earlier studies showed that, in the Navajo Sandstone, reducing fluids (presumably rich in hydrocarbons) mobilized Fe present as Fe-oxide coatings on detrital quartz grains. Dissolved Fe then precipitated as spherical concretions by interaction with oxidizing groundwater. Despite being depleted in Fe by not, vert, similar 50%, the bleached sandstones have Fe isotopic compositions similar to adjacent red sandstones (not, vert, similar 0‰/amu relative to IRMM-014). This shows that dissolution of Fe-oxide did not produce significant isotope fractionation, in agreement with previous experimental studies of abiotic Fe-oxide dissolution. In contrast, the concretions are depleted in the heavy isotopes of iron by − 0.07 to − 0.68‰/amu. This is opposite to the expected fractionation for partial Fe oxidation, which tends to enrich the precipitate in the heavy isotopes. Several scenarios are considered for explaining the measured Fe isotopic compositions. Although diffusion might be an important process in controlling the growth of spherical concretions, the associated isotopic fractionation is negligible compared to the observed variations. Kinetic isotope fractionation during precipitation can be ruled out as well because no isotopic zonation is seen within indurated concretions and Fe isotope evidence supports the occurrence of dissolution–reprecipitation reactions consistent with equilibrium growth conditions. The Fe isotopic compositions of the concretions are best explained by evolution of the fluid composition through precipitation and/or adsorption of isotopically heavy Fe during fluid flow through the sandstone. This scenario is supported by a regional trend in the isotopic composition of Fe, showing that this element was transported in fluids over several kilometres along major tectonic structures. These results demonstrate for the first time the virtue of Fe isotopes for tracing the directions and scales of paleofluid flows in porous media.

International audience; Assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer. In recent studies, we concluded that most igneous rocks from the continental crust were homogeneous to < +/- 0.05 parts per thousand in delta Fe-57 [Poitrasson, F., Halliday, AX, Lee, D.C., Levasseur, S., Teutsch, N., 2004. Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms. Earth Planet. Sci. Lett., 223, 253-266.; Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.]. This conclusion was previously reached by Beard et al. [Beard, B.L., Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol., 195, 87-117.], though defined by a broader range (+/- 0.08 parts per thousand). The main purpose of the Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on "Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS" by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] comment is to emphasize again this conclusion reached by the two groups: the degree of the iron isotope variability observed in most bulk rocks from the continental crust is small. However, controversy lies with high silica magmatic rocks (> similar to 71 wt.% SiO2) that we concluded were significantly heavier than less silicic rocks from the crust on the basis of Fe isotopic analyses of seven bulk rock samples from different geological settings. Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on "Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS" by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] challenge this finding on the basis of previously "published" data. It is shown here that this controversy stems from the confusion between whole-rock and mineral iron isotope analyses, since this isotopic tracer, like many others, is more heterogeneous at the mineral scale than the bulk rock scale. Thus, minerals cannot be used in place of bulk rock Fe isotope compositions. The controversy also arises from different levels of analytical uncertainties. If only the best Fe isotope probes of planetary reservoirs-bulk rocks - are considered, then the conclusion that igneous rocks with SiO2 > 71 wt% are significantly heavier than the mean delta Fe-57 of the continental crust [Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.] remains valid. Nonetheless, given that granites represent only a minor fraction of the continental crust, this finding does not change the bulk igneous Fe isotope composition estimate previously defined for the Earth.

Temporal variations in Fe isotope compositions at three locations in the Pacific Ocean over the last 10 Ma are inferred from high-resolution analyses of three hydrogenetic ferromanganese crusts. Iron pathways to the central deep Pacific Ocean appear to have remained constant over the past 10 Ma, reflected by a remarkably constant Fe isotope composition, despite large changes in the Fe delivery rates to the surface ocean via dust. These results suggest that the Fe cycle in the deep ocean is decoupled from that in surface waters. By contrast, one ferromanganese crust from the Izu-Bonin (IB) back-arc/marginal basin of the W. Pacific exhibits large delta Fe-56 variations. In that crust, decreases in delta Fe-56 values correlate with increases in Mn, Mg, Ni, Cu, Zn, Me, and V contents, and consistent with periods of intense hydrothermal input and increased growth rates. A second crust located within 100 km of the first IB sample does not record any of these periods of enhanced hydrothermal input. This probably reflects the isolated pathways by which hydrothermally sourced Fe may have migrated in the back arc, highlighting the high degree of provinciality that Fe isotopes may have in the modem (oxic) oceans. Our results demonstrate that despite efficient removal at the source, hydrothermal Fe injected into the deep ocean could account for a significant fraction of the dissolved Fe pool in the deep ocean, and that hydrothermally sourced Fe fluxes to the open ocean may have lower delta(56) Fe values than those measured so far in situ at hydrothermal vents. Correlation between 656 Fe values and elements enriched in hydrothermal fluids may provide a means for distinguishing hydrothermal Fe from other low-delta Fe-56 sources to the oceans such as dissolved riverine Fe or porewaters in continental shelf sediments. (c) 2006 Elsevier B.V. All rights reserved.

Iron isotopic compositions of olivine, clinopyroxene and titanomagnetite of the Baima Fe–Ti oxide-bearing layered mafic intrusion, SW China, are used to investigate Fe isotopic fractionation during the formation of the Fe–Ti oxide ore bodies and to constrain the origin of silicate minerals and Fe–Ti oxides. The Baima intrusion comprises the Lower Zone of interlayered troctolite, clinopyroxenite and oxide ores and the Upper Zone of gabbros. Significant differences of Fe isotope values between olivine ( δ Fe 56 = − 0.01 to + 0.11 ‰ ), clinopyroxene ( δ Fe 56 = + 0.11 to + 0.22 ‰ ), and titanomagnetite ( δ Fe 56 = + 0.20 to + 0.31 ‰ ) were observed in rocks and ores of the Lower Zone. Iron isotopic fractionation between olivine and clinopyroxene, olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0.06‰ to 0.22‰, 0.12‰ to 0.27‰, and 0.00‰ to 0.20‰, respectively, suggesting disequilibrium fractionation. This disequilibrium cannot be explained by thermal or chemical gradient-induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite. Instead, it could be attributed to crystallization of silicates and titanomagnetite from two immiscible Si-rich and Fe-rich liquids. Continuous segregation of the Fe-rich liquid changed the Fe isotopic composition of the Si-rich liquid and thus the Fe isotopic compositions of olivine and clinopyroxene crystallized from it, resulting in disequilibrium Fe isotopic fractionation between them. The effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the Fe-rich melts, on the other hand, gave rise to the disequilibrium Fe isotope fractionation between titanomagnetite and olivine/clinopyroxene.

International audience; High mass resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) was assessed for iron isotope measurement of natural samples after matrix separation by anion exchange chromatography. No remaining interferences were observed on the plateaus used for the mass spectrometric measurements. The approach developed and the instrument used permitted analyses in the static mode. Various mass bias corrections using Ni doping were tested, and even the assumption of similar fractionation factors for Fe and Ni did not produce significantly inaccurate data. However, the daily regression method between In Fe-57/(54) Fe and 1n(61)Ni/Ni-60 on the standard reference material IRMM-14 to characterize the instrumental mass bias appeared to give the best precision. The reproducibility observed over four months is about 0.013 parts per thousand/amu, 2 SD, on both delta(57)Fe/Fe-54 and delta(56)Fe/Fe-54 values, provided that each sample is analyzed at least six times. Accuracy, as estimated on interlaboratory comparison of natural samples that included geostandards, lies within this uncertainty. Among the bulk granitic rocks analysed, those with MgO below 0.6 wt.% and SiO2 above 71 wt.% have delta(57) Fe/Fe-54 values significantly heavier than the bulk mafic Earth. This shows that the iron isotope composition of terrestrial igneous rocks is more scattered than previously thought. There are good correlations between the Fe isotope composition and the MgO and SiO2 contents of the granitoids. These correlations are interpreted as reflecting the exsolution of late magmatic aqueous fluids from the granitic melt that preferentially removed the lighter isotopes of iron and enriched the residual magma in the heavier isotopes.