Tungsten in iron meteorites

Tungsten concentrations have been determined by instrumental neutron activation in 104 iron meteorites, and range from 0.07 to 5 μg/g. In individual groups, concentrations vary by factors of between 1.5 and 8, but there are negative W-Ni correlations in 8 groups: IAB, IC, IIAB, IID, IIE, IIIAB, IIICD, and IIIF. The lowest W concentrations are found in groups IAB and IIICD, which also have the smallest slopes on a W-Ni plot. Eighteen anomalous irons have W concentrations between 5 μg/g (Butler) and 0.11 μg/g (Rafrüti). The distribution of W in irons shows similarities to that of other refractory siderophilic elements (except Mo), but is closest to the distribution of Ru and Pt.Assuming that chemical trends in group IIIAB were produced by fractional crystallization, a value of 1.6 can be deduced for the distribution coefficient of W between solid and liquid metal, cf. 0.89 for Mo. Experimental evidence in support of these values is tenuous.     

Total nitrogen in iron meteorites

Total nitrogen contents have been measured by RNA of 30 iron and 4 stony-iron meteorites. Wide variations in N concentrations are noted (0.5 ppm to about 200 ppm). As a group, the IA irons have the highest nitrogen. This element is positively correlated with C, Ga, and Ge. Samples of meteorites that have been exposed to shock do not show any abnormal N distribution.     

Al andBe production in iron meteorites

We have measured by accelerator mass spectrometry the²⁶Al contents of 20 and the¹⁰Be contents of 14 iron meteorites. The²⁶Al contents are typically 30% or more lower than values obtained by counting techniques; the¹⁰Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the⁴He/²¹Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP₂₆(Fe) as 3–4 dpm/kg andP₁₀(Fe) as 4–5 dpm/kg. The average P/₁₀P₂₆ activity ratio is close to 1.5. Exposure ages calculated from²¹Ne/²⁶Al ratios cannot be calibrated so as to agree with both⁴⁰KK/ ages and ages based on the shorter-lived nuclides³⁹Ar and³⁶Cl. If agreement with the latter is forced, then the disagreement with⁴⁰KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10⁷ a.     

Non-cosmogenic lithium-6 in iron meteorites

A neutron activation method is used to measure⁶Li via the reaction⁶Li(n, α)³H in iron meteorites. It is found that most⁶Li occurs in non-metallic inclusions which can be separated by dissolution of the metal in 4 M H₂SO₄. The non-magnetic portion of such a residue has as high as 0.9 ppm⁶Li, while in the bulk sample⁶Li ranges from 0.02 to 10 ppb. Average⁶Li contents (ppb) for various classes are IA, 1.8; IIAB, 1.6; and IIIA, 0.4. Cosmic-ray-produced⁶Li is generally much smaller than the native⁶Li.     

Condensation and the composition of iron meteorites

The pressure-temperature conditions in the primordial nebula which could produce the observed Ni, Ga and Ge abundances in the major iron meteorite groups have been calculated assuming equilibrium condensation. Included in these calculations are the effect on the metal composition of Fe oxidation and sulphide formation during accretion, GeS and GaCl in the nebula gases and pressure variations in the nebula. It was found that the IIAB irons had their abundances of these elements fixed at the low-pressure extreme of the range which gives the IAB irons, but at 50 ± 10K higher temperatures. IIIAB and IVA formed over the same temperature range as IAB (600–670_?40⁺⁶⁰ K) in regions where the pressure was lower by a factor of 20 and 10⁴ respectively. Group IVB accreted soon after condensation of the metal and at pressures of less than 10^?3 atmosphere. The distribution of sulphur and carbon are consistent with this. The abundance of carbon in group IAB suggests that this and group IIAB accreted at about 10^?4 atmosphere, so that IIIAB and IVA accreted where the pressure was 5 × 10^?6 and 10^?8 atmosphere, respectively.     

Cooling rates of group IVA iron meteorites

Kamacite Ni profiles in low-Ni and high-Ni IVA irons are distinctly different, and cannot be fit with the same α/(α + γ) boundary in the low-temperature Fe-Ni phase diagram. This is attributed to an expansion of the α field to higher Ni contents resulting from the substantially higher P contents of the high-Ni irons. New α/(α + γ) boundaries are derived for P contents of 0.03 and 0.16%.Cooling rates of six group IVA iron meteorites were estimated by a taenite central Ni concentration-taenite half-width method similar to that of Wood [1]. Narrow (<20 μm) taenite lamellae were used to minimize uncertainties resulting from differences in nucleation temperatures. The calculated cooling rates range between 13 and 25°C/Myr, with an average of 20°C/Myr. No correlation between cooling rate and bulk Ni content is observed, and the data appear to be consistent with a uniform cooling rate as expected from an igneous core origin. This result differs from previous studies reporting a wide range in cooling rates that were strongly correlated with bulk Ni contents. The differences mainly result from differences in the phase diagram and the selected diffusion coefficients.Cooling rates inferred from taenite Ni concentrations at the interface with kamacite are consistent with those based on taenite central Ni contents.     

Genetic relations between iron and stony meteorites

The main group pallasites and the mesosiderites fall within the oxygen isotope group previously determined for the calcium-rich achondrites (eucrites, howardites and diogenites), consistent with derivation from a common source material, and perhaps a common parent body. The group IIE iron meteorites were derived from the same source material as H-group ordinary chondrites. The chondrite-like silicate inclusions in group IAB iron meteorites are not related to the ordinary chondrites, but may be related to the enstatite chondrites. Several meteorites previously considered “anomalous” fall into these groups: Pontlyfni and Winona with the IAB irons, and Netschaëvo possibly with the H chondrites and IIE irons. The unusual pallasites Eagle Station and Itzawisis have remarkable oxygen isotopic compositions, and have more of the ¹⁶O-rich component than any other meteorite. Bencubbin and Weatherford are also unusual in their isotopic compositions, and may bear some relationship to the C2 carbonaceous chondrites. Lodran and Enon are isotopically similar to one another and are close to the achondrite-mesosiderite-pallasite group.     

Spalogenic rare gases in iron meteorites with anomalous silver

He, Ne, and Ar have been measured in seven iron meteorites for which anomalous Ag isotopic compositions were reported, in order to determine if¹⁰⁷Ag excesses could be related to galactic cosmic-ray bombardment of these meteorites. Our results show that no correlation exists between¹⁰⁷Ag excess and either the fluence or the energy spectrum of the particles producing spallogenic rare gases. Cosmic-ray-produced¹⁰⁷Ag estimated from³⁸Ar concentrations can account for only about 1% of the observed excess. Elimination of cosmic-ray bombardment as a production mechanism for¹⁰⁷Ag excess strengthens the conclusion that the excess¹⁰⁷Ag is the decay product of short-lived¹⁰⁷Pd (τ_1/2=6.5Myr). The iron meteorite Pin?on is shown to contain trapped rare gases with⁴He/²⁰Ne～600.     

Cooling rate variations of group IVA iron meteorites

Cooling rates of eight group IVA iron meteorites were determined using a modification of the Wood method where cooling rate curves are calculated as a function of central taenite Ni content and taenite half-width. The major modification of the Wood method was to include the effect of P on the Ni solubility limits and Ni diffusion coefficients in the kamacite phase for each meteorite studied. The Borg and Lai binary kamacite Ni diffusivities were judged to be the best quality data available for the calculations. The calculated cooling rates range between 3 and 65°C/Myr. A correlation between decreasing cooling rate and increasing Ni content within the group IVA irons is observed. This cooling rate variation agrees closely with the 6–70°C/Myr range calculated by the independent bulk Ni-kamacite bandwidth method. Such a large variation in cooling rate within the group IVA argues against formation within the core of a single parent body.Willis and Wasson in the preceding paper found only a factor of 2 variation in cooling rate for the six IVA irons studied. The differences between the results of Willis and Wasson and this study are due mainly to the choice of the Ni diffusion coefficients in α and to the choice of the expressions for the effect of P on both the diffusion coefficients and the Ni solubility in kamacite. The Hirano et al. diffusivities used by Willis and Wasson were judged to be incorrect particularly because they are significantly higher than diffusivities in kamacite and taenite measured by other investigators. The assumption of Willis and Wasson that the same P content (0.03 wt.%) can be used for the low-Ni IVA's and that the same P content (0.16 wt.%) can be used for the high-Ni IVA's was judged to be a serious oversimplification.     

The pre-terrestrial history of group IV A iron meteorites

A number of group IVA iron meteorites show metallographic features suggesting that a shock annealing event intervened during the formation of the Widmanstatten structure. This could complicate the estimation of cooling rates based on diffusion profile methods but would have less influence on methods based upon macroscopic measurements of kamacite band widths.     

Cooling rates of group IVA iron meteorites determined from a ternary Fe-Ni-P model

Cooling rates have been determined for twelve group IVA iron meteorites using a ternary (Fe-Ni-P) model that simulates the growth of the Widmanstätten pattern. The new ternary model is governed by a set of differential diffusion equations that are coupled through the phase growth velocity and elemental concentration profiles. Measured ternary diffusivities and phase diagram solubilities were extrapolated below 500°C for use in the model. The model is more sophisticated than previous ones in that P as well as Ni gradients are calculated, ternary α-γ tie lines are allowed to vary, and ternary diffusivities are used.Output from the simulation is used to create a family of cooling rate curves on plots of central taenite Ni vs. log taenite half-width for each meteorite. A comparison of measured data to the cooling rate curves yields unique meteorite cooling rates. The measured cooling rates for the twelve IVA irons vary inversely with Ni content by over an order of magnitude (4–200°C/Myr). It is proposed that the group IVA irons were accommodated at various depths in an asteroidal-sized body.     

Fe isotope fractionation in iron meteorites: New insights into metal-sulphide segregation and planetary accretion

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.     

Investigations of cosmic-ray-produced nuclides in iron meteorites, 4. Identification of noble gas abundance anomalies

The literature on cosmic-ray-produced nuclides in iron meteorites occasionally reports unusual (“anomalous”) abundance proportions for the associated noble gases. The anomalies are in some cases ascribed to excesses of⁴He, caused by the presence of primordial or radiogenic components; in other cases to abundance deficiencies of³He, caused by partial loss of cosmogenic tritium. The arguments and data used previously for the recognition, identification and determination of anomalies are, however, imperfect or incorrect. New procedures are here proposed. They are based on more reliable data on the abundance patterns of the cosmogenic component and on a novel system of test correlations which describes these patterns. In most cases in which anomalies are recognised, the system allows an unequivocal identification of the nuclide which is the cause of the anomaly. This is a prerequisite for the quantitative determination of excesses and deficiencies. The procedures are applied to evaluate anomalous noble gas data reported in the literature for about 15 samples of various iron meteorites. In some cases, previous identifications of³He deficiencies and of⁴He excesses prove to be correct. However, guesses that⁴He excesses were present in certain specimens from Arispe, Cranbourne, El Taco, Hoba, Pin?on, Pitts and Sandia Mountains are invalidated by the present investigation.⁴He excesses are more exceptional than heretofore believed.     

26Al in iron meteorites and the constancy of cosmic ray intensity in the past

Cosmic-ray-produced²⁶Al in iron meteorites has been measured by low-level γγ-coincidence counting. The²⁶Al activities, in dpm/kg, are: Aroos3.0 ± 1.0, Braunau2.6 ± 0.5. Kayakent4.6 ± 1.5, N'Goureyma4.4 ± 1.1, Okahandja3.6 ± 0.9, Treysa4.0 ± 0.5. Exposure ages based on²⁶Al/²¹Ne are in agreement, within experimental error(±20%), with those based on³⁶Cl/³⁶Ar and³⁹Ar/³⁸Ar but the ages based on⁴⁰K/⁴¹K are higher by about 50%. The difference in exposure ages is probably caused by a real change of the cosmic ray intensity in the inner solar system.     

A core origin for group IVA iron meteorites: a reply to Moren and Goldstein

Because uncertainties in experimental data are large, one has considerable latitude in choosing the input parameters needed to calculate iron meteorite cooling rates. The best way to test input parameters is by examining their ability to yield the observed properties of the meteorites. Our phase diagram yields fits to kamacite profiles that are superior to those based on the Moren-Goldstein phase diagram. Our method of allowing for the effect of P on the Ni diffusion coefficient takes into account the enhancement in this effect with decreasing temperature; Moren and Goldstein use a relationship derived for a temperature of 1100°C, well outside the 700–350°C range where kamacite growth occurs. Use of our input parameters yields cooling rates in IVA irons that are independent of composition, consistent with a core origin. Since the fractionation of siderophiles in group IVA also indicates a core origin, we conclude that this is the correct model for this group.     

Nickel isotopic studies in meteorites

We have analyzed the nickel isotopic composition of meteoritic materials by high-precision mass spectrometry. The samples analyzed include almost all meteorite types for which large isotopic anomalies have been reported for oxygen, silver, magnesium and titanium. These samples are C₁, C₃, L, LL, H and E chondrites, IVB irons, Eagle Station pallasite and inclusion, matrix and “whole rock” samples of the Allende meteorite. The result is that we have not found any anomaly for nickel isotopic compositions within our accuracy of 0.7‰ for⁶¹Ni/⁶⁰Ni, 0.4-0.08‰ for⁶²Ni/⁶⁰Ni and 1–1.5‰ for⁶⁴Ni/⁶⁰Ni.     

Zirconium and hafnium in meteorites

The abundances of zirconium and hafnium have been determined in nine stony meteorites by a new, precise neutron-activation technique. The Zr/Hf abundance ratios for the chondrites vary in a rather narrow range, consistent with previously published observations from our group. Replicate analyses of new, carefully selected clean interior samples of the C1 chondrite Orgueil yield mean zirconium and hafnium abundances of 5.2 and 0.10 ppm, respectively. These abundances are lower than we reported earlier for two C1 chondrite samples which we now suspect may have suffered contamination. The new C1 zirconium and hafnium abundances are in closer agreement with predictions based on theories of nucleosynthesis than the earlier data.     

Organic matter in ancient meteorites

Fragments of ancient meteorites are cosmic time capsules, holding records of the first chemical steps towards life. Mark A Sephton explains what they have revealed so far and discusses what secrets they may yet still hold.