Search

Iron isotopic compositions potentially provide a powerful new tracer of planetary formation and differentiation processes and of secular and spatial changes in mantle oxidation state. However, the processes governing iron isotope fractionation in igneous rocks remain poorly understood. Here we show that there are significant variations in the iron isotope compositions (δ57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰, clinopyroxenes 0.97permil; and orthopyroxenes 0.8‰), with spinels showing the greatest total variation of 1.7‰. Positive linear functional relationships with slopes that are, within error, equal to unity are found between the δ57/54Fe values of coexisting orthopyroxene, clinopyroxene and olivine, strongly suggesting that the δ57/54Fe values of these minerals reflect intra-sample mineral-mineral isotopic equilibrium. Positive correlations between the δ57/54Fe values of silicate minerals and spinels also exist, although they are more scattered, which could be caused by late disturbance of mineral-spinel isotopic equilibrium. Bulk-rock, clinopyroxene and spinel δ57/54Fe values correlate with chemical indices of both melt extraction and oxidation. Iron isotope fractionation during spinel-facies partial melting is investigated using simple models, which demonstrate that the maximum expected fractionation between melt and residue will be ∼0.5‰, with the residue becoming isotopically light relative to the melt and to the initial source region. Hence melt extraction, in combination with significant changes in mantle oxidation state, may be an explanation for Fe isotopic variations in mantle peridotites. Metasomatism of the sub-arc mantle by iron-rich silicate melts originating from the subducting slab may also explain the light bulk-sample δ57/54Fe values of some arc peridotites (-0.2‰ to -0.6‰), but mass-balance calculations require these metasomatic agents to have extreme δ57/54Fe values (e.g. -3.0‰). The large differences in the δ57/54Fe values of garnet and spinel facies rocks are likely to be caused by the contrasting behaviour of Fe3+ during melting in the spinel and garnet facies. However, there is little difference in the δ57/54Fe values of MORB and OIB, despite the fact that OIB are considered, on the basis of incompatible element abundances, to arise dominantly by melting in the garnet stability field. Given that iron is a relatively compatible element, the similarities in the δ57/54Fe values of MORB and OIB provide strong evidence that MORB and OIB are both dominated by melting in the spinel facies. © 2005 Elsevier B.V. All rights reserved.

Three ferromanganese crusts from the northeast, northwest and central Atlantic were re-dated using osmium (Os) isotope stratigraphy and yield ages from middle Miocene to the present. The three Os isotope records do not show evidence for growth hiatuses. The reconstructed Os isotope-based growth rates for the sections older than 10 Ma are higher than those determined previously by the combined beryllium isotope (10Be/9Be) and cobalt (Co) constant-flux methods, which results in a decrease in the maximum age of each crust. This re-dating does not lead to significant changes to the interpretation of previously determined radiogenic isotope neodymium, lead (Nd, Pb) time series because the variability of these isotopes was very small in the records of the three crusts prior to 10 Ma. The Os isotope record of the central Atlantic crust shows a pronounced minimum during the middle Miocene between 15 and 12 Ma, similar to a minimum previously observed in two ferromanganese crusts from the central Pacific. For the other two Atlantic crusts, the Os isotope records and their calibration to the global seawater curve for the middle Miocene are either more uncertain or too short and thus do not allow for a reliable identification of an isotopic minimum. Similar to pronounced minima reported previously for the Cretaceous/Tertiary and Eocene/Oligocene boundaries, possible interpretations for the newly identified middle Miocene Os isotope minimum include changes in weathering intensity and/or a meteorite impact coinciding with the formation of the Nördlinger Ries Crater. It is suggested that the eruption and weathering of the Columbia River flood basalts provided a significant amount of the unradiogenic Os required to produce the middle Miocene minimum. © 2008 Elsevier B.V.

Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe-Mn crusts. © 2005 Elsevier B.V. All rights reserved.

Seven bulk chondrites, with δ 57 Fe/ 54 Fe values between −0.1‰ and 0‰ relative to IRMM-14, tend to be slightly lighter than 11 bulk iron meteorites, which have δ 57 Fe/ 54 Fe values ranging from 0.04‰ to 0.2‰. At the mineral scale, taenite from two iron meteorites, Cranbourne and Toluca, shows δ 57 Fe/ 54 Fe values heavier by up to 0.3‰ than their kamacite counterpart, thus calling into question the significance of bulk iron meteorite data. On three pallasites (Esquel, Marjalahti and Springwater) we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites (Eagle Station and Imilac), but the range of δ 57 Fe/ 54 Fe differences (from 0.32‰ to 0.07‰) is larger than that originally found. Troilite from two pallasites appears to be even heavier than the metal fraction, whereas schreibersite is lighter than its olivine counterpart. There is thus a general tendency for minerals within a given rock to show a heavier Fe isotope composition as the coordination number of Fe increases, although troilite is an exception to this rule. Iron meteorites are classically considered as remnants of asteroid cores and pallasites as core–mantle interfaces. The simultaneous finding that the metal fractions of pallasites have a higher δ 57 Fe/ 54 Fe signature than the coexisting olivines, and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core–mantle differentiation is accompanied by sizeable iron isotope fractionation. In this hypothesis, mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material. That is not observed, however, since all planetary mantles analyzed so far have δ 57 Fe/ 54 Fe values equivalent to or heavier than those of chondrites. It thus appears that the moderate temperature and pressure metal–silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as Fe isotopes are concerned. Instead, these mantle signatures could reflect equilibrium fractionation at a higher temperature, or the lack of a global core–mantle equilibration at the planetary scale. Overall, these new results show that the mass-dependent isotopic scatter observed among inner solar system bodies from the bulk-rock to the planetary scale (∼0.3‰ δ 57 Fe/ 54 Fe) is more restricted than previously thought. This likely confirms a homogenization process that occurred in the protoplanetary accretion disk, between refractory inclusion condensation and chondrule formation.

The iron (Fe) isotope compositions of 37 hydrogenetic ferromanganese deposits from various oceans have been analysed by MC-ICPMS; they permit the construction of a global map of Fe isotopic values. The isotopic compositions range between −1.2 and −0.1‰ in δ57FeIRMM14. Averages for the Atlantic and the Pacific are −0.41 and −0.88‰, but their standard deviations are identical (0.27, 1σ) and the data very largely overlap. No correlation is found with Pb or Nd isotope compositions and there is no evidence that the observed oceanic Fe isotopic heterogeneity is directly controlled by variations in continental sources. The small quantities of Fe that can be introduced from hydrothermal sources render as unlikely the possibility that the isotopic variations reflect variable proportions of continental and hydrothermal Fe, as recently proposed. The more likely explanation is that the variations are induced locally within the ocean. The exact sources of fractionation remain unclear. Likely possibilities are the dissolution and reprecipitation processes that liberate Fe from sediments during anoxic events, dissolution in surface waters or processes occurring during growth of the crusts

The behavior of dissolved osmium at the river/ocean interface was studied in the Lena River estuary. Dissolved osmium removal is observed at very low salinities. The loss is estimated to be 28% of the dissolved concentration of the river. The removal cannot be related to the flocculation of iron oxide–organic matter colloids, but occurs simultaneously with the loss of aluminum. The proposed mechanism is the adsorption of dissolved osmium on suspended particles in the maximum turbidity zone. If this is correct, then the removal of osmium in estuaries is probably a common phenomenon. No contribution of osmium from the sediments is detected but neither the samples nor the stratified nature of the estuary are favorable to the study of a flux from sediments. If this finding can be generalized, the estimated global riverine flux of osmium to the ocean has to be recalculated and the osmium residence time in the ocean would change from 3.3×104 yr to 4.6×104 yr.

The osmium concentration ([Os]) and isotopic composition were determined in a set of 17 of the largest rivers of the world. [Os] varies between 4.6 and 52.1 pg/kg and the 187Os/188Os ratio varies between 0.64 and 2.94. Measurement of rainwater samples shows that there is no input of oceanic Os to the continent through rain. Assuming a negligible anthropogenic Os input in the dissolved load, the natural average river water has an Os concentration of 7.9 pg/kg and a mean 187Os/188Os ratio of 1.54. The total riverine flux of Os to the ocean is estimated to be 295 kg/yr. The dissolved Os flux from island arcs and oceanic islands represents less than 5% of the total riverine flux and is not further considered. The continental Os flux to the ocean is then represented by the riverine flux, as dissolved Os from eolian dust and glacial sediments is negligible. Assuming steady state, it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg/yr (cosmic dust or mantle-derived) and an oceanic residence time between 2.5×104 and 5.4×104 with a mean of 3.5×104 year. The estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater Os would be ∼14% of the contribution of the unradiogenic component, which means that the mantle-derived flux should contribute a major part. The first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible, which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and/or of the ultramafic exposures contributes dominantly to the input of unradiogenic Os to the seawater. We show that it would be necessary to leach 1.3% of the Os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution. Another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic Os to the ocean in which case the riverine input represents 94% of the total dissolved flux to the ocean instead of 70%. The modern global dissolved Os flux to the ocean would then have a 187Os/188Os ratio of 1.44 instead of 1.06 and the system would be far from steady state.

It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re-Os, Hf-W, and Mn-Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous-Tertiary (K-T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni-rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.

Magmatic iron meteorites are considered to be remnants of the metallic cores of differentiated asteroids, and may be used as analogues of planetary core formation. The Fe isotope compositions (δ 57/54Fe) of metal fractions separated from magmatic and non-magmatic iron meteorites span a total range of 0.39‰, with the δ 57/54Fe values of metal fractions separated from the IIAB irons (δ 57/54Fe 0.12 to 0.32‰) being significantly heavier than those from the IIIAB (δ 57/54Fe 0.01 to 0.15‰), IVA (δ 57/54Fe - 0.07 to 0.17‰) and IVB groups (δ 57/54Fe 0.06 to 0.14‰). The δ 57/54Fe values of troilites (FeS) separated from magmatic and non-magmatic irons range from - 0.60 to - 0.12‰, and are isotopically lighter than coexisting metal phases. No systematic relationships exist between metal-sulphide fractionation factor (Δ 57/54Fe M-FeS = δ 57/54Fe metal - δ 57/54Fe FeS) metal composition or meteorite group, however the greatest Δ 57/54Fe M-FeS values recorded for each group are strikingly similar: 0.79, 0.63, 0.76 and 0.74‰ for the IIAB, IIIAB, IAB and IIICD irons, respectively. Δ 57/54Fe M-FeS values display a positive correlation with kamacite bandwidth, i.e. the most slowly-cooled meteorites, which should be closest to diffusive equilibrium, have the greatest Δ 57/54Fe M-FeS values. These observations provide suggestive evidence that Fe isotopic fractionation between metal and troilite is dominated by equilibrium processes and that the maximum Δ 57/54Fe M-FeS value recorded (0.79 ± 0.09‰) is the best estimate of the equilibrium metal-sulphide Fe isotope fractionation factor. Mass balance models using this fractionation factor in conjunction with metal δ 57/54Fe values and published Fe isotope data for pallasites can explain the relatively heavy δ 57/54Fe values of IIAB metals as a function of large amounts of S in the core of the IIAB parent body, in agreement with published experimental work. However, sequestering of isotopically light Fe into the S-bearing parts of planetary cores cannot explain published differences in the average δ 57/54Fe values of mafic rocks and meteorites derived from the Earth, Moon and Mars and 4-Vesta. The heavy δ 57/54Fe value of the Earth's mantle relative to that of Mars and 4-Vesta may reflect isotopic fractionation due to disproportionation of ferrous iron present in the proto-Earth mantle into isotopically heavy ferric iron hosted in perovskite, which is released into the magma ocean, and isotopically light native iron, which partitions into the core. This process cannot take place at significant levels on smaller planets, such as Mars, as perovskite is only stable at pressures > 23 GPa. Interestingly, the average δ 57/54Fe values of mafic terrestrial and lunar samples are very similar if the High-Ti mare basalts are excluded from the latter. If the Moon's mantle is largely derived from the impactor planet then the isotopically heavy signature of the Moon's mantle requires that the impacting planet also had a mantle with a δ 57/54Fe value heavier than that of Mars or 4-Vesta, which then implies that the impactor planet must have been greater in size than Mars. © 2006 Elsevier B.V. All rights reserved.

The iron isotope compositions of Shergotty–Nakhla–Chassigny (SNC) meteorites thought to come from Mars, eucrites and diogenites assumed to sample asteroid 4 Vesta, and rocks from the Moon and Earth have been measured using high precision plasma source mass spectrometry. The means of eight samples from Mars and nine samples from Vesta are within error identical despite a range of rock types. They are lighter by ∼0.1‰ in δ57Fe/54Fe compared to the average of 13 terrestrial mantle-derived rocks. The latter value is identical within uncertainty with a previously published mean of 46 igneous rocks from the Earth. The average for 14 lunar basalts and highland plutonic rocks covering a broad spectrum of major element composition is heavier by ∼0.1‰ in δ57Fe/54Fe relative to our estimate for the Earth's mantle, and therefore ∼0.2‰ heavier than the eucrites, diogenites and SNC meteorites. However, the data scatter somewhat and the Apollo 15 green glass and Apollo 17 orange glass are identical to samples from Mars and Vesta. There is no clear relationship between petrological characteristics and Fe isotope composition despite a wide spectrum of samples. Instead, contrasted planetary isotopic signatures are clearly resolved statistically. After evaluating alternative scenario, it appears that the most plausible explanation for the heavier Fe in the Earth and Moon is that both objects grew via processes that involved partial vaporisation leading to kinetic iron isotope fractionation followed by minor loss. This is consistent with the theory in which the Moon is thought to have originated from a giant impact between the proto-Earth and another planet. Combined with numerical simulations, Fe isotope data can offer the potential to provide constraints on the processes that occurred in planetary accretion.