Ruthenium endemic isotope effects in chondrites and differentiated meteorites

We report on the abundances of Ru isotopes in (1) iron meteorites, (2) stony-iron meteorites (pallasites), (3) ordinary and carbonaceous chondrites, and (4) in refractory inclusions from the carbonaceous meteorite Allende. We have developed improved Multiple-Collector, Negative-ion Thermal Ionization Mass Spectrometric (MC-NTIMS) techniques for Ru, with high ionization efficiency of 4% and with chemical separation techniques for Ru, which reduce mass interferences to the ppm level, so that no mass interference corrections needed to be applied. Our data were normalized to ⁹⁹Ru/¹⁰¹Ru to correct for mass-dependent fractionation. We find no Ru isotopic effects in the ordinary chondrites and group IAB iron meteorites we have measured. There are significant effects (deficits) in the pure s-process nuclide ¹⁰⁰Ru, in the Allende whole-rock and in refractory inclusions of up to 1.7 parts in 10,000 (εu). There are also endemic deficits in ¹⁰⁰Ru in iron meteorites and in pallasites of up to 1.1 εu. The Ru data suggest a wide spread and large scale heterogeneity in p-, s-, and r-process components resulting in a deficit in s-process nuclides or enhancements in both p- and r-process nuclides, in refractory siderophiles condensing in the early solar nebula. In contrast, the data on bulk Murchison suggest an excess in ¹⁰⁰Ru and in ¹⁰⁴Ru, which are distinct from the rest of the measured patterns. Our results establish the presence of significant isotopic heterogeneity for Ru in the early solar nebula. The observation of endemic Ru effects in planetary differentiates, such as iron meteorites and pallasites, must reflect the siderophile nature of Ru and the preservation in condensing FeNi metal of refractory metal condensate grains formed in the early solar nebula. Once incorporated in the metal phase, the refractory siderophiles remained in the metal phase through the melting and differentiation of planetesimals to form FeNi cores and silicate mantles and crusts.     

U-Pb systematics in iron meteorites: Uniformity of primordial lead

Pb isotopic compositions and U-Pb abundances were determined in the metal phase of six iron meteorites: Canyon Diablo IA, Toluca IA, Odessa IA, Youndegin IA, Deport IA and Mundrabilla An. Prior to complete dissolution, samples were subjected to a series of leachings and partial dissolutions. Isotopic compositions and abundances of the etched Pb indicate a contamination by terrestrial Pb which is attributable to previous cutting of the meteorite. Pb isotopic compositions measured in the decontaminated samples are identical within 0.2% and essentially confirm the primordial Pb value defined by Tatsumotoet al. (1973). These data invalidate more radiogenic Pb isotopic compositions published for iron meteorites, which are the result of terrestrial Pb contamination introduced mainly by analytical procedure. Our results support the idea of a solar nebula which was isotopically homogeneous for Pb 4.55 Ga ago. The new upper limit for U-abundance in iron meteorites, 0.001 ppb, is in agreement with its expected thermodynamic solubility in the metal phase.     

Comparative stable isotope geochemistry of Ni, Cu, Zn, and Fe in chondrites and iron meteorites

High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes.The observed variations of the ⁶²Ni/⁵⁸Ni, ⁶¹Ni/⁵⁸Ni, and ⁶⁰Ni/⁵⁸Ni ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no ⁶⁰Ni anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45‰ per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation.     

Geochemical affinities of cobalt and germanium toward metal,silicate, and sulfide phases at high temperature

Hydrothermal studies indicate that Co and Ge are strongly siderophile when metallic iron is in equilibrium with olivine at 900°C and 500 bars. If the metal is replaced by troilite (FeS), Ge is strongly lithophile whereas Co tends to concentrate in the sulfide phase. If iron meteorites were formed in a core derived from the sulfide phase, they would be depleted in Ge but retain Co.     

Stony-iron meteorites: History of the metal phase according to tungsten isotopes

The global composition of the early solar system is thought to be roughly chondritic in terms of refractory components, and this means that metal and silicate should be present together in early planetesimals. To fully understand the metal-silicate differentiation process within the eucrite parent body (EPB), it is important to try and identify the metal reservoir that is complementary to the silicate part. The isotope 182 of tungsten (W), a siderophile element, is partly formed from the decay of ¹⁸²Hf, and W isotopes are useful for examining metal-silicate segregation. The W isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites. However, mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities. Therefore, we undertook the analysis of the metal phase of these stony-irons. Here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body (bodies) and to assess possible relationships with eucrites.All stony-iron metals are depleted in radiogenic tungsten by −1.3 to −4.2 ε units, relative to the terrestrial standard, while chondrites, for comparison, are depleted by −1.9 ε units. In addition to W isotopic heterogeneity from one stony-iron to another, there is also W isotopic heterogeneity within individual meteorites. A formation model is tentatively proposed, where we show that mesosiderites, pallasites, and eucrites could possibly come from the same parent body. Several hypotheses are discussed to explain the isotopic heterogeneity: the production of cosmogenic tungsten, the in situ decay of hafnium present in inclusions, and tungsten diffusion processes after metal-silicate mixing during the cooling of the meteorites. The two latter hypotheses provide the best explanation of our data.     

Pressures of formation of iron meteorites from sphalerite compositions

The FeS content of sphalerite, a minor phase in some meteorites, is strongly dependent on pressure when the sphalerite is in equilibrium with troilite. We have determined FeS contents for sphalerite in Bogou, Gladstone, Sardis and Odessa ; these, together with published data on Odessa and Campo del Cielo, have been used to calculate pressures of formation of meteorites, assuming that FeS-diffusion in sphalerite ceases at 350°C. Calculated pressures range from 0.2 to 3.1 kbar, corresponding to formation at centres of chondritic objects from 140 to 410 km in radius, or metallic objects of from 50 to 200 km radius. Formation at shallower depths would require the objects to have been correspondingly larger.All meteorites in this study are members of Ga-Ge group I. Inverse correlation between Ge content and pressure of formation suggests formation at various depths in a compositionally zoned (fractionated?) object. Comparison between our pressure estimates and radii estimated from cooling rates (Frickeret al., 1970, Geochim. Cosmochim. Acta34, 475–492) suggests that Odessa, Bogou and possibly other Group I meteorites formed in a single object with a radius between 400 and 180 km and an overall composition richer in metal than average chondrites.     

Mercury abundances and isotopic compositions in the Murchison (CM) and Allende (CV) carbonaceous chondrites

The abundance and isotopic composition of Hg was determined in bulk samples of both the Murchison (CM) and Allende (CV) carbonaceous chondrites using single- and multi-collector inductively coupled plasma mass spectrometry (ICP-MS). The bulk abundances of Hg are 294 ± 15 ng/g in Murchison and 30.0 ± 1.5 ng/g in Allende. These values are within the range of previous measurements of bulk Hg abundances by neutron activation analysis (NAA). Prior studies suggested that both meteorites contain isotopically anomalous Hg, with δ^196/202Hg values for the anomalous, thermal-release components from bulk samples ranging from −260 ‰ to +440 ‰ in Murchison and from −620 ‰ to +540 ‰ in Allende Jovanovic and Reed 1976a, Jovanovic and Reed 1976b, Kumar and Goel 1992. Our multi-collector ICP-MS measurements suggest that the relative abundances of all seven stable Hg isotopes in both meteorites are identical to terrestrial values within 0.2 to 0.5 ‰.On-line thermal-release experiments were performed by coupling a programmable oven with the single-collector ICP-MS. Powdered aliquots of each meteorite were linearly heated from room temperature to 900°C over twenty-five minutes under an Ar atmosphere to measure the isotopic composition of Hg released from the meteorites as a function of temperature. In separate experiments, the release profiles of S and Se were determined simultaneously with Hg to constrain the Hg distribution within the meteorites and to evaluate the possibility of Se interferences in previous NAA studies. The Hg-release patterns differ between Allende and Murchison. The Hg-release profile for Allende contains two distinct peaks, at 225° and 343°C, whereas the profile for Murchison has only one peak, at 344°C. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 ‰, depending on the isotope ratio. In both meteorites the Hg peak at ∼340°C correlates with a peak in the S-release profile. This correlation suggests that Hg is associated with S-bearing phases and, thus, that HgS is a major Hg-bearing phase in both meteorites. The Hg peak at 225°C for Allende is similar to release patterns of physically adsorbed Hg on silicate and metal grains. Prior studies suggested that the isotopic anomalies reported from NAA resulted from interference between ²⁰³Hg and ⁷⁵Se. However, the amount of Se released from both meteorites, relative to Hg, is insufficient to produce all of the observed anomalies.     

Chemical Analysis of Iron Meteorites Using a Hand‐Held X‐Ray Fluorescence Spectrometer

We evaluate the performance of a hand‐held XRF (HHXRF) spectrometer for the bulk analysis of iron meteorites. Analytical precision and accuracy were tested on metal alloy certified reference materials and iron meteorites of known chemical composition. With minimal sample preparation (i.e., flat or roughly polished surfaces) HHXRF allowed the precise and accurate determination of most elements heavier than Mg, with concentrations > 0.01% m/m in metal alloy CRMs, and of major elements Fe and Ni and minor elements Co, P and S (generally ranging from 0.1 to 1% m/m) in iron meteorites. In addition, multiple HHXRF spot analyses could be used to determine the bulk chemical composition of iron meteorites, which are often characterised by sulfide and phosphide accessory minerals. In particular, it was possible to estimate the P and S bulk contents, which are of critical importance for the petrogenesis and evolution of Fe‐Ni‐rich liquids and iron meteorites. This study thus validates HHXRF as a valuable tool for use in meteoritics, allowing the rapid, non‐destructive (a) identification of the extraterrestrial origin of metallic objects (i.e., archaeological artefacts); (b) preliminary chemical classification of iron meteorites; (c) identification of mislabelled/unlabelled specimens in museums and private collections and (d) bulk analysis of iron meteorites.     

Siderophile elements in Martian meteorites and implications for core formation in Mars

Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess ¹⁸²W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10⁻²–10⁻³ × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars.     

Chromium and phosphorus enrichment in the metal of type II (C2) carbonaceous chondrites

Electron microprobe analyses of metal grains in nine C2 meteorites show consistently high Cr and P contents, with large grain to grain variations within individual meteorites. Cr ranges from 0.16 to 1.0 wt.% (average 0.6 per cent for 43 analyses). P ranges from 0.00 to 3.2 wt.% (average 0.42 per cent). In addition, metal grains in seven C3 meteorites show a lesser enrichment of Cr, 0.00 to 0.7 per cent (average 0.13 per cent for 59 analyses), with no P present. Both of these elements, Cr and P, are generally below detection in the metal of other chondrite groups, equilibrated and unequilibrated. C2 metal grains occur typically as spherical to ovate blebs contained within single, euhedral forsterite crystals or crystal fragments that are either isolated in the black C2 matrix or are in clusters making up white inclusions. Metal is rare within true chondrules. Calculations of the expected Cr contents in metal condensing directly from a solar nebular gas agree remarkably well with the observed values. Unfortunately, no calculation is possible for P because of insufficient data at the present time. The composition and the textural relationships indicate this Cr, P rich metal, and the enclosing forsterite are direct condensates from a cooling solar nebula. Cr and P, in the quantities reported here, characterize C2 metal.     

GEOCHEMICAL AND MINEROLOGIC STUDY OF A STONY METEORITE FROM THE VICINITY OF THE VILLAGE OF SEVRYUKOVO

In studying the conditions of formation of stony meteorites, we assume that 1) they are fragments of asteroids fallen to the surface of the earth. During their flight through the atmosphere, the meteorites develop a melted surface layer but their texture and mineralogic composition remain unchanged. 2) According to V. M. Goldschmidt, stone meteorites crystallize in a lesser gravity field than that of the earth, which is the reason for their chondritic texture and high porosity (about 4%). 3) Meteorites were formed in a medium with a deficiency of free oxygen. As a result, part of their iron and nickel was segregated as native metal; in addition, lawrencite and oldhamite, sulfides typical of meteorites, were formed.

We identify three stages of meteorite formation: magmatic, pneumatolytic, and hydrothermal. The interval 1450-850°C. corresponds to the magmatic stage at which a silicate phase and native iron with nickel were formed. As a result of thermal dissociation of water and because of the deficiency of oxygen required for a complete oxidation of metals and carbon, in the gaseous phase, free oxygen and H₂O were absent and the phase consisted probably of H₂, CH₄, CO₂, and CO.

The temperature interval 750-500°C. corresponds to the pneumatolytic phase. Here, H₂S, CH₄, CO₂, and CO were the principal agents of the gaseous phase. CH₄ was formed in a high temperature reaction between hydrogen and elementary carbon. As the temperature dropped to 750°C., electrolytic dissociation of H₂O rendered possible the formation of sulfides, especially of troilite.

Mineralization at the hydrothermal stage with a temperature interval of 400 to 300°C. has been observed only in carbon meteorites with a considerable graphitic carbon content. Here, a small portion of the ferrous iron is oxidized to the ferric, in the presence of CO₂ and at a temperature of 450° to 500°C.; the iron sulfide so formed is represented by pyrrhotite. Simultaneously, colored silicates are chloritized, with a separation of CaCO₃.—Auth. English summ.     

The IIG iron meteorites: Probable formation in the IIAB core

The addition of two meteorites to the iron meteorite grouplet originally known as the Bellsbank trio brings the population to five, the minimum number for group status. With Ga and Ge contents in the general “II” range, the new group has been designated IIG. The members of this group have low-Ni contents in the metal and large amounts of coarse schreibersite ((Fe,NI)₃P); their bulk P contents are 17-21 mg/g, the highest known in iron meteorites. Their S contents are exceptionally low, ranging from 0.2 to 2 mg/g. We report neutron-activation-analysis data for metal samples; the data generally show smooth trends on element-Au diagrams. The low Ir and high Au contents suggest formation during the late crystallization of a magma.Because on element-Au or element-Ni diagrams the IIG fields of the important taxonomic elements Ni, Ga, Ge and As are offset from those of the IIAB irons, past researchers have concluded that the IIG irons could not have formed from the same magma, and thus that the two groups originated on separate parent bodies. However, on most element-Au diagrams the IIG fields plot close to extensions of IIAB trends to higher Au concentrations.There is general agreement that immiscibility led to the formation of an upper S-rich and a lower P-rich magma in the IIAB core. We suggest that the IIG irons formed from the P-rich magma, and that schreibersite was a liquidus phase during the final stages of crystallization. The offsets in Ni and As (and possibly other elements) may result from solid-state elemental redistribution between metal and schreibersite during slow cooling. For example, it is well established that the equilibrium Ni content is >2× higher in late-formed relative to early-formed schreibersite. It is plausible that As substitutes nearly ideally for P in schreibersite at eutectic temperatures but becomes incompatible at low temperatures.[Wasson J. T., Huber, H. and Malvin, D. J. (2007) Formation of IIAB iron meteorites. Geochim. Cosmochim. Acta71, 760-781] argued that, in the most evolved IIAB irons, the amount of trapped melt was high. The high P contents of IIG irons also require high contents of trapped melt but the local geometry seems to have allowed the S-rich immiscible melt to escape as it formed. The escaping melt may have selectively depleted elements such as Au and Ge.     

Formation of metal and silicate globules in Gujba: a new Bencubbin-like meteorite fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ¹⁵N values (∼+685‰). The ³He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.     

Studies of an artificially shock-loaded H group chondrite

Five samples of the naturally unshocked Kernouve (H6) meteorite were artificially shock-loaded to pressures of 70, 165, 270, and 390 kbar and the silicates and metal examined optically, by scanning and transmission electron microscopy and by thermoluminescence (TL). Olivine deformation is closely comparable to that in naturally shocked meteorites, producing dislocations with Burgers vector [001]. At pressures of ?165 kbar, these are formed in well-defined slip planes. At 270 kbar, olivine develops optical mosaicism, has high dislocation densities throughout and is also highly fractured. Recovery, due to heating is minimal. In orthopyroxene, the deformation mechanism changes, from the clino-inversion to unit-dislocation slip, between 70 and 165 kbar. In diopside, (001) and (100) twinning was produced. Plagioclase is inferred to have been progressively converted to maskelynite, but some is still present in 270 kbar sample.The microhardness of the kamacite in the samples increases with shock pressure. The α → ? transformation pressure in the kamacite is 30–40 kbar higher than observed for iron meteorites. Annealed kamacite displays incipient polycrystallinity and α-martensite and taenite sometimes contains slip lines. Troilite acquired cracks, undulose extinction, twins, polycrystallinity and finally melted as the shock pressure increased.At pressures over 200 kbar there was a systematic decrease in the natural TL and the TL sensitivity. Detailed considerations of changes in the natural $\text{TL}\text{TL}$ sensitivity ratio for various regions of the TL glow curve suggest that two processes were effective during shock; thermal drainage of electron traps and a reduction in the effective trap density. It is suggested that the latter process associated with the vitrification of feldspar, the TL phosphor.An additional sample was subjected to a shock pulse which was “spiked” instead of square. Very distinctive changes were apparent; thermal effects are conspicuous and with widespread annealing (~600–800°C) of metal and sulfide. Glassy, opaque veins were produced which are analogous to the black veins in shock-lithified gas-rich meteorites. Anomalous low-temperature TL was induced, suggesting that a new or modified phase or mineral has become the dominant TL phosphor.     

Chemical classification of iron meteorites—X. Multielement studies of 43 irons,resolution of group IIIE from IIIAB,and evaluation of Cu as a taxonomic parameter

We report structural and compositional data leading to the classification of 41 iron meteorites, increasing the number of classified independent iron meteorites to 576. We also obtained data on a new metal-rich mesosiderite and on two new iron masses that are paired with previously studied irons. For the first time in this series we also report concentrations of Cr, Co, Cu, As, Sb, W, Re and Au in each of these 44 meteorites. We determined 7 of these elements (all except Sb) in 30 previously studied ungrouped or unusual irons, and obtained Cu data on 104 irons, 21 pallasites, and 3 meteorite phases previously studied by E. Scott. We show that Cu possesses characteristics well suited to a taxonomic element: a siderophile nature, a large range among all irons, and a low range within magmatic groups. For the first time we report the partial resolution of the C-rich group IIIE from its populous twin group IIIAB on element-Ni diagrams other than Ir-Ni. Cachiyuyal previously classified ungrouped and Armanty (Xinjiang) previously classified IIIAB are reclassified IIIE. Despite the addition of 3 new irons and the reanalysis of 3 previously studied irons the members of the set of 15 ungrouped irons having very low Ga (<3 μg/g) and Ge (<0.7 μg/g) contents remain individualists. The same is generally true for irons having $\text{100 ≤ Ni ≤ 180}\text{mg/g}$ and compositional similarities to IIICD, but A80104 increases the Garden Head trio to a quartet. Algoma is reclassified from ungrouped to IIICD-an and Hassi-Jekna and Magnesia from IIICD to IIICD-an. The metal of Horse Creek and Mount Egerton is compositionally closely related to metal from EH chondrites. We suggest that the P-rich Bellsbank trio irons formed in the IIAB core in topographic lows filled with an immiscible, P-rich second liquid.     

Comments on the paper ‘Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif,Ligurian Alps,NW Italy’ by G. B. Piccardo

Analyses were made of samples of the several classes of iron meteorites: (hexahedrites, octahedrites, ataxites, and troilite inclusions) in further study of the isotopic composition of primordial lead and toward establishing correlation between the distribution of lead among the mineral inclusions and the nickel-iron mass of the meteorite. Two groups of iron meteorites can be distinguished on the basis of isotopic composition lead suggesting two ages for the parent bodies of common iron meteorites. The distribution of lead in iron meteorites ranges markedly but no relation could be found between isotopic composition of lead and the several structures and compositions. The content of lead in troilites are one or two orders of magnitude higher than in the nickel-iron phase.-- M. Russell.     

Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates

Petrographic, mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from ‘ hot’ deserts and Antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments. Terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of Fe,Ni metal, producing Fe-oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins. Troilite weathers readily in ‘ hot’ desert environments but undergoes very little alteration under Antarctic conditions. Most of the primary porosity of ordinary chondrites has been occluded by the time that ∼ 15 to 25% of the initial Fe⁰ and Fe²⁺ has been oxidised to Fe³⁺ in both environments. Results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products. Dilation of primary pores and brecciation, which has been observed in parts of some meteorites, will only occur if the meteorite is especially metal-rich, or has a low primary porosity. These weathering products are absent from recent falls but have formed in a fall after ∼ 100 yr of museum storage.Cl-bearing akaganéite and hibbingite are common weathering products in Antarctic finds but occur in abundance in only one ‘ hot’ desert meteorite, Daraj 014. The majority of Fe-rich weathering products in meteorites from both environments contain low, but variable concentrations of Si, Mg and Ca. In most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a ∼ 1.0-nm lattice fringe spacing; where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene. In the heavily-weathered Antarctic finds ALHA 78045 and 77002, Si is concentrated in cronstedtite, a Fe-rich phyllosilicate. An unidentified hydrous Si-Fe-Ni-Mg mineral or gel has also partially replaced taenite in ALHA 78045. In addition to Fe-rich weathering products, ‘ hot’ desert meteorites contain sulphates, Ca-carbonate and silica, whereas such minerals are largely absent from Antarctic finds. The abundance of silicate weathering products in Antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments. As preterrestrial cronstedtite is abundant in CM2 carbonaceous chondrites, the Antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites.     

Petrographic variations among carbonaceous chondrites of the Vigarano type

The Vigarano subtype is a petrographically complex class of meteorites. Oxidized and reduced groups can be distinguished on the basis of metal vs magnetite abundances and Ni contents of sulfide minerals. These meteorites also differ in the proportions of matrix and chondrules and in polymict character. Slight bulk chemical differences correlate with the recognized petrologic groupings. It is likely that the Vigarano subtype includes several previously unrecognized subgroups. Metamorphism has affected Coolidge, Mulga (West) and, to a lesser extent, Allende, as evidenced by ferromagnesian mineral equilibration, Fe-enrichment of fine-grained inclusions, and loss of some volatile gases. Because of the metamorphic effects in the Allende chondrite (the only meteorite of the group that has been intensively studied) and the petrographie differences among all meteorites of the Vigarano subtype, it is suggested that Allende alone may not adequately reflect the wide spectrum of properties in this important class of meteorites.     

The distribution of total nitrogen in iron meteorites

Total nitrogen abundances in 123 iron meteorites have been determined by inert carrier-gas fusion extraction-gas chromatography. The median value for the iron meteorites was found to be 18 ppm N. The N contents of Sulfide inclusions are greater, in nine cases out of ten, than the corresponding metallic phase. The N content of the iron meteorites is positively correlated with germanium content. The effects of terrestrial weathering and heat treatment by man are discussed in relation to the N contents measured for certain specimens. A correlation between N and cooling rates was found, with lower cooling rates associated with greater N abundances.